Revert #1142 - Remove unboxed objects

- accounting for removal of watch()/unwatch()

48 files changed, 3400 insertions, 75 deletions

diff --git a/js/src/jit-test/tests/basic/unboxed-object-clear-new-script.js b/js/src/jit-test/tests/basic/unboxed-object-clear-new-script.js
new file mode 100644
index 0000000000..f55456222d
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-clear-new-script.js
@@ -0,0 +1,49 @@
+
+function Foo(a, b) {
+    this.a = a;
+    this.b = b;
+}
+
+function invalidate_foo() {
+    var a = [];
+    var counter = 0;
+    for (var i = 0; i < 50; i++)
+        a.push(new Foo(i, i + 1));
+    Object.defineProperty(Foo.prototype, "a", {configurable: true, set: function() { counter++; }});
+    for (var i = 0; i < 50; i++)
+        a.push(new Foo(i, i + 1));
+    delete Foo.prototype.a;
+    var total = 0;
+    for (var i = 0; i < a.length; i++) {
+        assertEq('a' in a[i], i < 50);
+        total += a[i].b;
+    }
+    assertEq(total, 2550);
+    assertEq(counter, 50);
+}
+invalidate_foo();
+
+function Bar(a, b, fn) {
+    this.a = a;
+    if (b == 30)
+        Object.defineProperty(Bar.prototype, "b", {configurable: true, set: fn});
+    this.b = b;
+}
+
+function invalidate_bar() {
+    var a = [];
+    var counter = 0;
+    function fn() { counter++; }
+    for (var i = 0; i < 50; i++)
+        a.push(new Bar(i, i + 1, fn));
+    delete Bar.prototype.b;
+    var total = 0;
+    for (var i = 0; i < a.length; i++) {
+        assertEq('a' in a[i], true);
+        assertEq('b' in a[i], i < 29);
+        total += a[i].a;
+    }
+    assertEq(total, 1225);
+    assertEq(counter, 21);
+}
+invalidate_bar();
diff --git a/js/src/jit-test/tests/basic/unboxed-object-convert-to-native.js b/js/src/jit-test/tests/basic/unboxed-object-convert-to-native.js
new file mode 100644
index 0000000000..691fe166c3
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-convert-to-native.js
@@ -0,0 +1,47 @@
+
+// Test various ways of converting an unboxed object to native.
+
+function Foo(a, b) {
+    this.a = a;
+    this.b = b;
+}
+
+var proxyObj = {
+  get: function(recipient, name) {
+    return recipient[name] + 2;
+  }
+};
+
+function f() {
+    var a = [];
+    for (var i = 0; i < 50; i++)
+        a.push(new Foo(i, i + 1));
+
+    var prop = "a";
+
+    i = 0;
+    for (; i < 5; i++)
+        a[i].c = i;
+    for (; i < 10; i++)
+        Object.defineProperty(a[i], 'c', {value: i});
+    for (; i < 15; i++)
+        a[i] = new Proxy(a[i], proxyObj);
+    for (; i < 20; i++)
+        a[i].a = 3.5;
+    for (; i < 25; i++)
+        delete a[i].b;
+    for (; i < 30; i++)
+        a[prop] = 4;
+
+    var total = 0;
+    for (i = 0; i < a.length; i++) {
+        if ('a' in a[i])
+            total += a[i].a;
+        if ('b' in a[i])
+            total += a[i].b;
+        if ('c' in a[i])
+            total += a[i].c;
+    }
+    assertEq(total, 2382.5);
+}
+f();
diff --git a/js/src/jit-test/tests/basic/unboxed-object-getelem.js b/js/src/jit-test/tests/basic/unboxed-object-getelem.js
new file mode 100644
index 0000000000..b30b8325a0
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-getelem.js
@@ -0,0 +1,20 @@
+
+function f() {
+    var propNames = ["a","b","c","d","e","f","g","h","i","j","x","y"];
+    var arr = [];
+    for (var i=0; i<64; i++)
+	arr.push({x:1, y:2});
+    for (var i=0; i<64; i++) {
+        // Make sure there are expandos with dynamic slots for each object.
+        for (var j = 0; j < propNames.length; j++)
+            arr[i][propNames[j]] = j;
+    }
+    var res = 0;
+    for (var i=0; i<100000; i++) {
+	var o = arr[i % 64];
+	var p = propNames[i % propNames.length];
+	res += o[p];
+    }
+    assertEq(res, 549984);
+}
+f();
diff --git a/js/src/jit-test/tests/basic/unboxed-object-set-property.js b/js/src/jit-test/tests/basic/unboxed-object-set-property.js
new file mode 100644
index 0000000000..fdcfcf6b22
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-set-property.js
@@ -0,0 +1,31 @@
+
+// Use the correct receiver when non-native objects are prototypes of other objects.
+
+function Thing(a, b) {
+    this.a = a;
+    this.b = b;
+}
+
+function foo() {
+    var array = [];
+    for (var i = 0; i < 10000; i++)
+        array.push(new Thing(i, i + 1));
+
+    var proto = new Thing(1, 2);
+    var obj = Object.create(proto);
+
+    Object.defineProperty(Thing.prototype, "c", {set:function() { this.d = 0; }});
+    obj.c = 3;
+    assertEq(obj.c, undefined);
+    assertEq(obj.d, 0);
+    assertEq(obj.hasOwnProperty("d"), true);
+    assertEq(proto.d, undefined);
+    assertEq(proto.hasOwnProperty("d"), false);
+
+    obj.a = 3;
+    assertEq(obj.a, 3);
+    assertEq(proto.a, 1);
+    assertEq(obj.hasOwnProperty("a"), true);
+}
+
+foo();
diff --git a/js/src/jit-test/tests/basic/unboxed-property-enumeration.js b/js/src/jit-test/tests/basic/unboxed-property-enumeration.js
new file mode 100644
index 0000000000..142d932dd3
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-property-enumeration.js
@@ -0,0 +1,24 @@
+function O() {
+    this.x = 1;
+    this.y = 2;
+}
+function testUnboxed() {
+    var arr = [];
+    for (var i=0; i<100; i++)
+	arr.push(new O);
+
+    var o = arr[arr.length-1];
+    o[0] = 0;
+    o[2] = 2;
+    var sym = Symbol();
+    o[sym] = 1;
+    o.z = 3;
+    Object.defineProperty(o, '3', {value:1,enumerable:false,configurable:false,writable:false});
+    o[4] = 4;
+
+    var props = Reflect.ownKeys(o);
+    assertEq(props[props.length-1], sym);
+
+    assertEq(Object.getOwnPropertyNames(o).join(""), "0234xyz");
+}
+testUnboxed();
diff --git a/js/src/jit-test/tests/ion/unboxed-objects-invalidate.js b/js/src/jit-test/tests/ion/unboxed-objects-invalidate.js
new file mode 100644
index 0000000000..02e27614fe
--- /dev/null
+++ b/js/src/jit-test/tests/ion/unboxed-objects-invalidate.js
@@ -0,0 +1,16 @@
+
+var a = [];
+for (var i = 0; i < 2000; i++)
+    a.push({f:i});
+
+function f() {
+    var total = 0;
+    for (var i = 0; i < a.length; i++)
+        total += a[i].f;
+    return total;
+}
+assertEq(f(), 1999000);
+
+var sub = Object.create(a[0]);
+
+assertEq(f(), 1999000);
diff --git a/js/src/jit/BaselineCacheIR.cpp b/js/src/jit/BaselineCacheIR.cpp
index 67c80473b4..7fb5868115 100644
--- a/js/src/jit/BaselineCacheIR.cpp
+++ b/js/src/jit/BaselineCacheIR.cpp
@@ -16,7 +16,7 @@ using namespace js;
 using namespace js::jit;
 
 // OperandLocation represents the location of an OperandId. The operand is
-// either in a register or on the stack.
+// either in a register or on the stack, and is either boxed or unboxed.
 class OperandLocation
 {
   public:
@@ -815,6 +815,36 @@ BaselineCacheIRCompiler::emitGuardSpecificObject()
 }
 
 bool
+BaselineCacheIRCompiler::emitGuardNoUnboxedExpando()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+
+    FailurePath* failure;
+    if (!addFailurePath(&failure))
+        return false;
+
+    Address expandoAddr(obj, UnboxedPlainObject::offsetOfExpando());
+    masm.branchPtr(Assembler::NotEqual, expandoAddr, ImmWord(0), failure->label());
+    return true;
+}
+
+bool
+BaselineCacheIRCompiler::emitGuardAndLoadUnboxedExpando()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    Register output = allocator.defineRegister(masm, reader.objOperandId());
+
+    FailurePath* failure;
+    if (!addFailurePath(&failure))
+        return false;
+
+    Address expandoAddr(obj, UnboxedPlainObject::offsetOfExpando());
+    masm.loadPtr(expandoAddr, output);
+    masm.branchTestPtr(Assembler::Zero, output, output, failure->label());
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitLoadFixedSlotResult()
 {
     Register obj = allocator.useRegister(masm, reader.objOperandId());
@@ -841,6 +871,26 @@ BaselineCacheIRCompiler::emitLoadDynamicSlotResult()
 }
 
 bool
+BaselineCacheIRCompiler::emitLoadUnboxedPropertyResult()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    AutoScratchRegister scratch(allocator, masm);
+
+    JSValueType fieldType = reader.valueType();
+
+    Address fieldOffset(stubAddress(reader.stubOffset()));
+    masm.load32(fieldOffset, scratch);
+    masm.loadUnboxedProperty(BaseIndex(obj, scratch, TimesOne), fieldType, R0);
+
+    if (fieldType == JSVAL_TYPE_OBJECT)
+        emitEnterTypeMonitorIC();
+    else
+        emitReturnFromIC();
+
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitGuardNoDetachedTypedObjects()
 {
     FailurePath* failure;
diff --git a/js/src/jit/BaselineIC.cpp b/js/src/jit/BaselineIC.cpp
index e65f10aaca..1b98325b79 100644
--- a/js/src/jit/BaselineIC.cpp
+++ b/js/src/jit/BaselineIC.cpp
@@ -44,8 +44,8 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/EnvironmentObject-inl.h"
 #include "vm/Interpreter-inl.h"
-#include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using mozilla::DebugOnly;
 
@@ -741,6 +741,11 @@ LastPropertyForSetProp(JSObject* obj)
     if (obj->isNative())
         return obj->as<NativeObject>().lastProperty();
 
+    if (obj->is<UnboxedPlainObject>()) {
+        UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+        return expando ? expando->lastProperty() : nullptr;
+    }
+
     return nullptr;
 }
 
@@ -1157,6 +1162,56 @@ TryAttachNativeOrUnboxedGetValueElemStub(JSContext* cx, HandleScript script, jsb
 
     ICStub* monitorStub = stub->fallbackMonitorStub()->firstMonitorStub();
 
+    if (obj->is<UnboxedPlainObject>() && holder == obj) {
+        const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+
+        // Once unboxed objects support symbol-keys, we need to change the following accordingly
+        MOZ_ASSERT_IF(!keyVal.isString(), !property);
+
+        if (property) {
+            if (!cx->runtime()->jitSupportsFloatingPoint)
+                return true;
+
+            RootedPropertyName name(cx, JSID_TO_ATOM(id)->asPropertyName());
+            ICGetElemNativeCompiler<PropertyName*> compiler(cx, ICStub::GetElem_UnboxedPropertyName,
+                                                            monitorStub, obj, holder,
+                                                            name,
+                                                            ICGetElemNativeStub::UnboxedProperty,
+                                                            needsAtomize, property->offset +
+                                                            UnboxedPlainObject::offsetOfData(),
+                                                            property->type);
+            ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+            if (!newStub)
+                return false;
+
+            stub->addNewStub(newStub);
+            *attached = true;
+            return true;
+        }
+
+        Shape* shape = obj->as<UnboxedPlainObject>().maybeExpando()->lookup(cx, id);
+        if (!shape->hasDefaultGetter() || !shape->hasSlot())
+            return true;
+
+        bool isFixedSlot;
+        uint32_t offset;
+        GetFixedOrDynamicSlotOffset(shape, &isFixedSlot, &offset);
+
+        ICGetElemNativeStub::AccessType acctype =
+            isFixedSlot ? ICGetElemNativeStub::FixedSlot
+                        : ICGetElemNativeStub::DynamicSlot;
+        ICGetElemNativeCompiler<T> compiler(cx, getGetElemStubKind<T>(ICStub::GetElem_NativeSlotName),
+                                            monitorStub, obj, holder, key,
+                                            acctype, needsAtomize, offset);
+        ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+        if (!newStub)
+            return false;
+
+        stub->addNewStub(newStub);
+        *attached = true;
+        return true;
+    }
+
     if (!holder->isNative())
         return true;
 
@@ -1404,7 +1459,7 @@ TryAttachGetElemStub(JSContext* cx, JSScript* script, jsbytecode* pc, ICGetElem_
     }
 
     // Check for NativeObject[id] and UnboxedPlainObject[id] shape-optimizable accesses.
-    if (obj->isNative()) {
+    if (obj->isNative() || obj->is<UnboxedPlainObject>()) {
         RootedScript rootedScript(cx, script);
         if (rhs.isString()) {
             if (!TryAttachNativeOrUnboxedGetValueElemStub<PropertyName*>(cx, rootedScript, pc, stub,
@@ -1816,6 +1871,14 @@ ICGetElemNativeCompiler<T>::generateStubCode(MacroAssembler& masm)
     Register holderReg;
     if (obj_ == holder_) {
         holderReg = objReg;
+
+        if (obj_->is<UnboxedPlainObject>() && acctype_ != ICGetElemNativeStub::UnboxedProperty) {
+            // The property will be loaded off the unboxed expando.
+            masm.push(R1.scratchReg());
+            popR1 = true;
+            holderReg = R1.scratchReg();
+            masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+        }
     } else {
         // Shape guard holder.
         if (regs.empty()) {
@@ -1866,6 +1929,13 @@ ICGetElemNativeCompiler<T>::generateStubCode(MacroAssembler& masm)
         if (popR1)
             masm.addToStackPtr(ImmWord(sizeof(size_t)));
 
+    } else if (acctype_ == ICGetElemNativeStub::UnboxedProperty) {
+        masm.load32(Address(ICStubReg, ICGetElemNativeSlotStub<T>::offsetOfOffset()),
+                    scratchReg);
+        masm.loadUnboxedProperty(BaseIndex(objReg, scratchReg, TimesOne), unboxedType_,
+                                 TypedOrValueRegister(R0));
+        if (popR1)
+            masm.addToStackPtr(ImmWord(sizeof(size_t)));
     } else {
         MOZ_ASSERT(acctype_ == ICGetElemNativeStub::NativeGetter ||
                    acctype_ == ICGetElemNativeStub::ScriptedGetter);
@@ -2618,6 +2688,18 @@ BaselineScript::noteArrayWriteHole(uint32_t pcOffset)
 // SetElem_DenseOrUnboxedArray
 //
 
+template <typename T>
+void
+EmitUnboxedPreBarrierForBaseline(MacroAssembler &masm, T address, JSValueType type)
+{
+    if (type == JSVAL_TYPE_OBJECT)
+        EmitPreBarrier(masm, address, MIRType::Object);
+    else if (type == JSVAL_TYPE_STRING)
+        EmitPreBarrier(masm, address, MIRType::String);
+    else
+        MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(type));
+}
+
 bool
 ICSetElem_DenseOrUnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
 {
@@ -4061,7 +4143,18 @@ TryAttachSetValuePropStub(JSContext* cx, HandleScript script, jsbytecode* pc, IC
         return true;
 
     if (!obj->isNative()) {
-        return true;
+        if (obj->is<UnboxedPlainObject>()) {
+            UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+            if (expando) {
+                shape = expando->lookup(cx, name);
+                if (!shape)
+                    return true;
+            } else {
+                return true;
+            }
+        } else {
+            return true;
+        }
     }
 
     size_t chainDepth;
@@ -4209,6 +4302,40 @@ TryAttachSetAccessorPropStub(JSContext* cx, HandleScript script, jsbytecode* pc,
 }
 
 static bool
+TryAttachUnboxedSetPropStub(JSContext* cx, HandleScript script,
+                            ICSetProp_Fallback* stub, HandleId id,
+                            HandleObject obj, HandleValue rhs, bool* attached)
+{
+    MOZ_ASSERT(!*attached);
+
+    if (!cx->runtime()->jitSupportsFloatingPoint)
+        return true;
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (!property)
+        return true;
+
+    ICSetProp_Unboxed::Compiler compiler(cx, obj->group(),
+                                         property->offset + UnboxedPlainObject::offsetOfData(),
+                                         property->type);
+    ICUpdatedStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+    if (!newStub)
+        return false;
+    if (compiler.needsUpdateStubs() && !newStub->addUpdateStubForValue(cx, script, obj, id, rhs))
+        return false;
+
+    stub->addNewStub(newStub);
+
+    StripPreliminaryObjectStubs(cx, stub);
+
+    *attached = true;
+    return true;
+}
+
+static bool
 TryAttachTypedObjectSetPropStub(JSContext* cx, HandleScript script,
                                 ICSetProp_Fallback* stub, HandleId id,
                                 HandleObject obj, HandleValue rhs, bool* attached)
@@ -4291,6 +4418,12 @@ DoSetPropFallback(JSContext* cx, BaselineFrame* frame, ICSetProp_Fallback* stub_
         return false;
     RootedReceiverGuard oldGuard(cx, ReceiverGuard(obj));
 
+    if (obj->is<UnboxedPlainObject>()) {
+        MOZ_ASSERT(!oldShape);
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+            oldShape = expando->lastProperty();
+    }
+
     bool attached = false;
     // There are some reasons we can fail to attach a stub that are temporary.
     // We want to avoid calling noteUnoptimizableAccess() if the reason we
@@ -4363,6 +4496,15 @@ DoSetPropFallback(JSContext* cx, BaselineFrame* frame, ICSetProp_Fallback* stub_
 
     if (!attached &&
         lhs.isObject() &&
+        !TryAttachUnboxedSetPropStub(cx, script, stub, id, obj, rhs, &attached))
+    {
+        return false;
+    }
+    if (attached)
+        return true;
+
+    if (!attached &&
+        lhs.isObject() &&
         !TryAttachTypedObjectSetPropStub(cx, script, stub, id, obj, rhs, &attached))
     {
         return false;
@@ -4445,7 +4587,20 @@ GuardGroupAndShapeMaybeUnboxedExpando(MacroAssembler& masm, JSObject* obj,
 
     // Guard against shape or expando shape.
     masm.loadPtr(Address(ICStubReg, offsetOfShape), scratch);
-    masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+    if (obj->is<UnboxedPlainObject>()) {
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+        Label done;
+        masm.push(object);
+        masm.loadPtr(expandoAddress, object);
+        masm.branchTestObjShape(Assembler::Equal, object, scratch, &done);
+        masm.pop(object);
+        masm.jump(failure);
+        masm.bind(&done);
+        masm.pop(object);
+    } else {
+        masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+    }
 }
 
 bool
@@ -4484,7 +4639,13 @@ ICSetProp_Native::Compiler::generateStubCode(MacroAssembler& masm)
     regs.takeUnchecked(objReg);
 
     Register holderReg;
-    if (isFixedSlot_) {
+    if (obj_->is<UnboxedPlainObject>()) {
+        // We are loading off the expando object, so use that for the holder.
+        holderReg = regs.takeAny();
+        masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+        if (!isFixedSlot_)
+            masm.loadPtr(Address(holderReg, NativeObject::offsetOfSlots()), holderReg);
+    } else if (isFixedSlot_) {
         holderReg = objReg;
     } else {
         holderReg = regs.takeAny();
@@ -4621,17 +4782,31 @@ ICSetPropNativeAddCompiler::generateStubCode(MacroAssembler& masm)
     regs.add(R0);
     regs.takeUnchecked(objReg);
 
-    // Write the object's new shape.
-    Address shapeAddr(objReg, ShapedObject::offsetOfShape());
-    EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
-    masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
-    masm.storePtr(scratch, shapeAddr);
+    if (obj_->is<UnboxedPlainObject>()) {
+        holderReg = regs.takeAny();
+        masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
 
-    if (isFixedSlot_) {
-        holderReg = objReg;
+        // Write the expando object's new shape.
+        Address shapeAddr(holderReg, ShapedObject::offsetOfShape());
+        EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
+        masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
+        masm.storePtr(scratch, shapeAddr);
+
+        if (!isFixedSlot_)
+            masm.loadPtr(Address(holderReg, NativeObject::offsetOfSlots()), holderReg);
     } else {
-        holderReg = regs.takeAny();
-        masm.loadPtr(Address(objReg, NativeObject::offsetOfSlots()), holderReg);
+        // Write the object's new shape.
+        Address shapeAddr(objReg, ShapedObject::offsetOfShape());
+        EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
+        masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
+        masm.storePtr(scratch, shapeAddr);
+
+        if (isFixedSlot_) {
+            holderReg = objReg;
+        } else {
+            holderReg = regs.takeAny();
+            masm.loadPtr(Address(objReg, NativeObject::offsetOfSlots()), holderReg);
+        }
     }
 
     // Perform the store.  No write barrier required since this is a new
@@ -4663,6 +4838,70 @@ ICSetPropNativeAddCompiler::generateStubCode(MacroAssembler& masm)
 }
 
 bool
+ICSetProp_Unboxed::Compiler::generateStubCode(MacroAssembler& masm)
+{
+    MOZ_ASSERT(engine_ == Engine::Baseline);
+
+    Label failure;
+
+    // Guard input is an object.
+    masm.branchTestObject(Assembler::NotEqual, R0, &failure);
+
+    AllocatableGeneralRegisterSet regs(availableGeneralRegs(2));
+    Register scratch = regs.takeAny();
+
+    // Unbox and group guard.
+    Register object = masm.extractObject(R0, ExtractTemp0);
+    masm.loadPtr(Address(ICStubReg, ICSetProp_Unboxed::offsetOfGroup()), scratch);
+    masm.branchPtr(Assembler::NotEqual, Address(object, JSObject::offsetOfGroup()), scratch,
+                   &failure);
+
+    if (needsUpdateStubs()) {
+        // Stow both R0 and R1 (object and value).
+        EmitStowICValues(masm, 2);
+
+        // Move RHS into R0 for TypeUpdate check.
+        masm.moveValue(R1, R0);
+
+        // Call the type update stub.
+        if (!callTypeUpdateIC(masm, sizeof(Value)))
+            return false;
+
+        // Unstow R0 and R1 (object and key)
+        EmitUnstowICValues(masm, 2);
+
+        // The TypeUpdate IC may have smashed object. Rederive it.
+        masm.unboxObject(R0, object);
+
+        // Trigger post barriers here on the values being written. Fields which
+        // objects can be written to also need update stubs.
+        LiveGeneralRegisterSet saveRegs;
+        saveRegs.add(R0);
+        saveRegs.add(R1);
+        saveRegs.addUnchecked(object);
+        saveRegs.add(ICStubReg);
+        emitPostWriteBarrierSlot(masm, object, R1, scratch, saveRegs);
+    }
+
+    // Compute the address being written to.
+    masm.load32(Address(ICStubReg, ICSetProp_Unboxed::offsetOfFieldOffset()), scratch);
+    BaseIndex address(object, scratch, TimesOne);
+
+    EmitUnboxedPreBarrierForBaseline(masm, address, fieldType_);
+    masm.storeUnboxedProperty(address, fieldType_,
+                              ConstantOrRegister(TypedOrValueRegister(R1)), &failure);
+
+    // The RHS has to be in R0.
+    masm.moveValue(R1, R0);
+
+    EmitReturnFromIC(masm);
+
+    masm.bind(&failure);
+    EmitStubGuardFailure(masm);
+    return true;
+}
+
+bool
 ICSetProp_TypedObject::Compiler::generateStubCode(MacroAssembler& masm)
 {
     MOZ_ASSERT(engine_ == Engine::Baseline);
@@ -5421,7 +5660,7 @@ TryAttachCallStub(JSContext* cx, ICCall_Fallback* stub, HandleScript script, jsb
             if (!thisObject)
                 return false;
 
-            if (thisObject->is<PlainObject>())
+            if (thisObject->is<PlainObject>() || thisObject->is<UnboxedPlainObject>())
                 templateObject = thisObject;
         }
 
diff --git a/js/src/jit/BaselineIC.h b/js/src/jit/BaselineIC.h
index e1ad12559f..a1291a3bbf 100644
--- a/js/src/jit/BaselineIC.h
+++ b/js/src/jit/BaselineIC.h
@@ -22,6 +22,7 @@
 #include "jit/SharedICRegisters.h"
 #include "js/GCVector.h"
 #include "vm/ArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 namespace js {
 namespace jit {
@@ -1822,7 +1823,8 @@ class ICSetProp_Native : public ICUpdatedStub
         virtual int32_t getKey() const {
             return static_cast<int32_t>(engine_) |
                   (static_cast<int32_t>(kind) << 1) |
-                  (static_cast<int32_t>(isFixedSlot_) << 17);
+                  (static_cast<int32_t>(isFixedSlot_) << 17) |
+                  (static_cast<int32_t>(obj_->is<UnboxedPlainObject>()) << 18);
         }
 
         MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
@@ -1927,6 +1929,7 @@ class ICSetPropNativeAddCompiler : public ICStubCompiler
         return static_cast<int32_t>(engine_) |
               (static_cast<int32_t>(kind) << 1) |
               (static_cast<int32_t>(isFixedSlot_) << 17) |
+              (static_cast<int32_t>(obj_->is<UnboxedPlainObject>()) << 18) |
               (static_cast<int32_t>(protoChainDepth_) << 19);
     }
 
@@ -1951,7 +1954,10 @@ class ICSetPropNativeAddCompiler : public ICStubCompiler
             newGroup = nullptr;
 
         RootedShape newShape(cx);
-        newShape = obj_->as<NativeObject>().lastProperty();
+        if (obj_->isNative())
+            newShape = obj_->as<NativeObject>().lastProperty();
+        else
+            newShape = obj_->as<UnboxedPlainObject>().maybeExpando()->lastProperty();
 
         return newStub<ICSetProp_NativeAddImpl<ProtoChainDepth>>(
             space, getStubCode(), oldGroup_, shapes, newShape, newGroup, offset_);
diff --git a/js/src/jit/BaselineInspector.cpp b/js/src/jit/BaselineInspector.cpp
index 3b852debf3..bcb527516c 100644
--- a/js/src/jit/BaselineInspector.cpp
+++ b/js/src/jit/BaselineInspector.cpp
@@ -104,11 +104,19 @@ AddReceiver(const ReceiverGuard& receiver,
 static bool
 GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* receiver)
 {
-    // We match:
+    // We match either:
     //
     //   GuardIsObject 0
     //   GuardShape 0
     //   LoadFixedSlotResult 0 or LoadDynamicSlotResult 0
+    //
+    // or
+    //
+    //   GuardIsObject 0
+    //   GuardGroup 0
+    //   1: GuardAndLoadUnboxedExpando 0
+    //   GuardShape 1
+    //   LoadFixedSlotResult 1 or LoadDynamicSlotResult 1
 
     *receiver = ReceiverGuard();
     CacheIRReader reader(stub->stubInfo());
@@ -117,6 +125,14 @@ GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* re
     if (!reader.matchOp(CacheOp::GuardIsObject, objId))
         return false;
 
+    if (reader.matchOp(CacheOp::GuardGroup, objId)) {
+        receiver->group = stub->stubInfo()->getStubField<ObjectGroup*>(stub, reader.stubOffset());
+
+        if (!reader.matchOp(CacheOp::GuardAndLoadUnboxedExpando, objId))
+            return false;
+        objId = reader.objOperandId();
+    }
+
     if (reader.matchOp(CacheOp::GuardShape, objId)) {
         receiver->shape = stub->stubInfo()->getStubField<Shape*>(stub, reader.stubOffset());
         return reader.matchOpEither(CacheOp::LoadFixedSlotResult, CacheOp::LoadDynamicSlotResult);
@@ -125,6 +141,29 @@ GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* re
     return false;
 }
 
+static bool
+GetCacheIRReceiverForUnboxedProperty(ICCacheIR_Monitored* stub, ReceiverGuard* receiver)
+{
+    // We match:
+    //
+    //   GuardIsObject 0
+    //   GuardGroup 0
+    //   LoadUnboxedPropertyResult 0 ..
+
+    *receiver = ReceiverGuard();
+    CacheIRReader reader(stub->stubInfo());
+
+    ObjOperandId objId = ObjOperandId(0);
+    if (!reader.matchOp(CacheOp::GuardIsObject, objId))
+        return false;
+
+    if (!reader.matchOp(CacheOp::GuardGroup, objId))
+        return false;
+    receiver->group = stub->stubInfo()->getStubField<ObjectGroup*>(stub, reader.stubOffset());
+
+    return reader.matchOp(CacheOp::LoadUnboxedPropertyResult, objId);
+}
+
 bool
 BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers)
 {
@@ -143,7 +182,8 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
     while (stub->next()) {
         ReceiverGuard receiver;
         if (stub->isCacheIR_Monitored()) {
-            if (!GetCacheIRReceiverForNativeReadSlot(stub->toCacheIR_Monitored(), &receiver))
+            if (!GetCacheIRReceiverForNativeReadSlot(stub->toCacheIR_Monitored(), &receiver) &&
+                !GetCacheIRReceiverForUnboxedProperty(stub->toCacheIR_Monitored(), &receiver))
             {
                 receivers.clear();
                 return true;
@@ -151,6 +191,8 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
         } else if (stub->isSetProp_Native()) {
             receiver = ReceiverGuard(stub->toSetProp_Native()->group(),
                                      stub->toSetProp_Native()->shape());
+        } else if (stub->isSetProp_Unboxed()) {
+            receiver = ReceiverGuard(stub->toSetProp_Unboxed()->group(), nullptr);
         } else {
             receivers.clear();
             return true;
diff --git a/js/src/jit/CacheIR.cpp b/js/src/jit/CacheIR.cpp
index d184ea40c5..6822a70afe 100644
--- a/js/src/jit/CacheIR.cpp
+++ b/js/src/jit/CacheIR.cpp
@@ -10,7 +10,8 @@
 #include "jit/IonCaches.h"
 
 #include "jsobjinlines.h"
-#include "vm/NativeObject-inl.h"
+
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -59,6 +60,10 @@ GetPropIRGenerator::tryAttachStub(Maybe<CacheIRWriter>& writer)
             return false;
         if (!emitted_ && !tryAttachNative(*writer, obj, objId))
             return false;
+        if (!emitted_ && !tryAttachUnboxed(*writer, obj, objId))
+            return false;
+        if (!emitted_ && !tryAttachUnboxedExpando(*writer, obj, objId))
+            return false;
         if (!emitted_ && !tryAttachTypedObject(*writer, obj, objId))
             return false;
         if (!emitted_ && !tryAttachModuleNamespace(*writer, obj, objId))
@@ -158,9 +163,19 @@ GeneratePrototypeGuards(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
 }
 
 static void
-TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOperandId objId)
+TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOperandId objId,
+                     Maybe<ObjOperandId>* expandoId)
 {
-    if (obj->is<TypedObject>()) {
+    if (obj->is<UnboxedPlainObject>()) {
+        writer.guardGroup(objId, obj->group());
+
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+            expandoId->emplace(writer.guardAndLoadUnboxedExpando(objId));
+            writer.guardShape(expandoId->ref(), expando->lastProperty());
+        } else {
+            writer.guardNoUnboxedExpando(objId);
+        }
+    } else if (obj->is<TypedObject>()) {
         writer.guardGroup(objId, obj->group());
     } else {
         Shape* shape = obj->maybeShape();
@@ -173,7 +188,8 @@ static void
 EmitReadSlotResult(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
                    Shape* shape, ObjOperandId objId)
 {
-    TestMatchingReceiver(writer, obj, shape, objId);
+    Maybe<ObjOperandId> expandoId;
+    TestMatchingReceiver(writer, obj, shape, objId, &expandoId);
 
     ObjOperandId holderId;
     if (obj != holder) {
@@ -196,6 +212,9 @@ EmitReadSlotResult(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
                 lastObjId = protoId;
             }
         }
+    } else if (obj->is<UnboxedPlainObject>()) {
+        holder = obj->as<UnboxedPlainObject>().maybeExpando();
+        holderId = *expandoId;
     } else {
         holderId = objId;
     }
@@ -247,6 +266,51 @@ GetPropIRGenerator::tryAttachNative(CacheIRWriter& writer, HandleObject obj, Obj
 }
 
 bool
+GetPropIRGenerator::tryAttachUnboxed(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
+{
+    MOZ_ASSERT(!emitted_);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(name_);
+    if (!property)
+        return true;
+
+    if (!cx_->runtime()->jitSupportsFloatingPoint)
+        return true;
+
+    writer.guardGroup(objId, obj->group());
+    writer.loadUnboxedPropertyResult(objId, property->type,
+                                     UnboxedPlainObject::offsetOfData() + property->offset);
+    emitted_ = true;
+    preliminaryObjectAction_ = PreliminaryObjectAction::Unlink;
+    return true;
+}
+
+bool
+GetPropIRGenerator::tryAttachUnboxedExpando(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
+{
+    MOZ_ASSERT(!emitted_);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+    if (!expando)
+        return true;
+
+    Shape* shape = expando->lookup(cx_, NameToId(name_));
+    if (!shape || !shape->hasDefaultGetter() || !shape->hasSlot())
+        return true;
+
+    emitted_ = true;
+
+    EmitReadSlotResult(writer, obj, obj, shape, objId);
+    return true;
+}
+
+bool
 GetPropIRGenerator::tryAttachTypedObject(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
 {
     MOZ_ASSERT(!emitted_);
diff --git a/js/src/jit/CacheIR.h b/js/src/jit/CacheIR.h
index ae55cfebb4..4fd8575f01 100644
--- a/js/src/jit/CacheIR.h
+++ b/js/src/jit/CacheIR.h
@@ -87,10 +87,13 @@ class ObjOperandId : public OperandId
     _(GuardClass)                         \
     _(GuardSpecificObject)                \
     _(GuardNoDetachedTypedObjects)        \
+    _(GuardNoUnboxedExpando)              \
+    _(GuardAndLoadUnboxedExpando)         \
     _(LoadObject)                         \
     _(LoadProto)                          \
     _(LoadFixedSlotResult)                \
     _(LoadDynamicSlotResult)              \
+    _(LoadUnboxedPropertyResult)          \
     _(LoadTypedObjectResult)              \
     _(LoadInt32ArrayLengthResult)         \
     _(LoadArgumentsObjectLengthResult)    \
@@ -271,6 +274,15 @@ class MOZ_RAII CacheIRWriter
     void guardNoDetachedTypedObjects() {
         writeOp(CacheOp::GuardNoDetachedTypedObjects);
     }
+    void guardNoUnboxedExpando(ObjOperandId obj) {
+        writeOpWithOperandId(CacheOp::GuardNoUnboxedExpando, obj);
+    }
+    ObjOperandId guardAndLoadUnboxedExpando(ObjOperandId obj) {
+        ObjOperandId res(nextOperandId_++);
+        writeOpWithOperandId(CacheOp::GuardAndLoadUnboxedExpando, obj);
+        writeOperandId(res);
+        return res;
+    }
 
     ObjOperandId loadObject(JSObject* obj) {
         ObjOperandId res(nextOperandId_++);
@@ -296,6 +308,11 @@ class MOZ_RAII CacheIRWriter
         writeOpWithOperandId(CacheOp::LoadDynamicSlotResult, obj);
         addStubWord(offset, StubField::GCType::NoGCThing);
     }
+    void loadUnboxedPropertyResult(ObjOperandId obj, JSValueType type, size_t offset) {
+        writeOpWithOperandId(CacheOp::LoadUnboxedPropertyResult, obj);
+        buffer_.writeByte(uint32_t(type));
+        addStubWord(offset, StubField::GCType::NoGCThing);
+    }
     void loadTypedObjectResult(ObjOperandId obj, uint32_t offset, TypedThingLayout layout,
                                uint32_t typeDescr) {
         MOZ_ASSERT(uint32_t(layout) <= UINT8_MAX);
@@ -389,6 +406,9 @@ class MOZ_RAII GetPropIRGenerator
     PreliminaryObjectAction preliminaryObjectAction_;
 
     MOZ_MUST_USE bool tryAttachNative(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId);
+    MOZ_MUST_USE bool tryAttachUnboxed(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId);
+    MOZ_MUST_USE bool tryAttachUnboxedExpando(CacheIRWriter& writer, HandleObject obj,
+                                              ObjOperandId objId);
     MOZ_MUST_USE bool tryAttachTypedObject(CacheIRWriter& writer, HandleObject obj,
                                            ObjOperandId objId);
     MOZ_MUST_USE bool tryAttachObjectLength(CacheIRWriter& writer, HandleObject obj,
diff --git a/js/src/jit/CodeGenerator.cpp b/js/src/jit/CodeGenerator.cpp
index 2b1c671d1b..901e9ea939 100644
--- a/js/src/jit/CodeGenerator.cpp
+++ b/js/src/jit/CodeGenerator.cpp
@@ -25,7 +25,6 @@
 #include "builtin/Eval.h"
 #include "builtin/TypedObject.h"
 #include "gc/Nursery.h"
-#include "gc/StoreBuffer-inl.h"
 #include "irregexp/NativeRegExpMacroAssembler.h"
 #include "jit/AtomicOperations.h"
 #include "jit/BaselineCompiler.h"
@@ -3029,7 +3028,7 @@ CodeGenerator::visitStoreSlotV(LStoreSlotV* lir)
 
 static void
 GuardReceiver(MacroAssembler& masm, const ReceiverGuard& guard,
-              Register obj, Register scratch, Label* miss)
+              Register obj, Register scratch, Label* miss, bool checkNullExpando)
 {
     if (guard.group) {
         masm.branchTestObjGroup(Assembler::NotEqual, obj, guard.group, miss);
@@ -3051,11 +3050,13 @@ CodeGenerator::emitGetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
 
         Label next;
         masm.comment("GuardReceiver");
-        GuardReceiver(masm, receiver, obj, scratch, &next);
+        GuardReceiver(masm, receiver, obj, scratch, &next, /* checkNullExpando = */ false);
 
         if (receiver.shape) {
             masm.comment("loadTypedOrValue");
-            Register target = obj;
+            // If this is an unboxed expando access, GuardReceiver loaded the
+            // expando object into scratch.
+            Register target = receiver.group ? scratch : obj;
 
             Shape* shape = mir->shape(i);
             if (shape->slot() < shape->numFixedSlots()) {
@@ -3121,10 +3122,12 @@ CodeGenerator::emitSetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
         ReceiverGuard receiver = mir->receiver(i);
 
         Label next;
-        GuardReceiver(masm, receiver, obj, scratch, &next);
+        GuardReceiver(masm, receiver, obj, scratch, &next, /* checkNullExpando = */ false);
 
         if (receiver.shape) {
-            Register target = obj;
+            // If this is an unboxed expando access, GuardReceiver loaded the
+            // expando object into scratch.
+            Register target = receiver.group ? scratch : obj;
 
             Shape* shape = mir->shape(i);
             if (shape->slot() < shape->numFixedSlots()) {
@@ -3290,7 +3293,7 @@ CodeGenerator::visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir)
         const ReceiverGuard& receiver = mir->receiver(i);
 
         Label next;
-        GuardReceiver(masm, receiver, obj, temp, &next);
+        GuardReceiver(masm, receiver, obj, temp, &next, /* checkNullExpando = */ true);
 
         if (i == mir->numReceivers() - 1) {
             bailoutFrom(&next, lir->snapshot());
@@ -8379,6 +8382,11 @@ CodeGenerator::visitStoreUnboxedPointer(LStoreUnboxedPointer* lir)
     }
 }
 
+typedef bool (*ConvertUnboxedObjectToNativeFn)(JSContext*, JSObject*);
+static const VMFunction ConvertUnboxedPlainObjectToNativeInfo =
+    FunctionInfo<ConvertUnboxedObjectToNativeFn>(UnboxedPlainObject::convertToNative,
+                                                 "UnboxedPlainObject::convertToNative");
+
 typedef bool (*ArrayPopShiftFn)(JSContext*, HandleObject, MutableHandleValue);
 static const VMFunction ArrayPopDenseInfo =
     FunctionInfo<ArrayPopShiftFn>(jit::ArrayPopDense, "ArrayPopDense");
@@ -8671,11 +8679,11 @@ CodeGenerator::visitIteratorStartO(LIteratorStartO* lir)
     masm.loadPtr(Address(niTemp, offsetof(NativeIterator, guard_array)), temp2);
 
     // Compare object with the first receiver guard. The last iterator can only
-    // match for native objects.
+    // match for native objects and unboxed objects.
     {
         Address groupAddr(temp2, offsetof(ReceiverGuard, group));
         Address shapeAddr(temp2, offsetof(ReceiverGuard, shape));
-        Label guardDone, shapeMismatch;
+        Label guardDone, shapeMismatch, noExpando;
         masm.loadObjShape(obj, temp1);
         masm.branchPtr(Assembler::NotEqual, shapeAddr, temp1, &shapeMismatch);
 
@@ -8687,6 +8695,12 @@ CodeGenerator::visitIteratorStartO(LIteratorStartO* lir)
         masm.bind(&shapeMismatch);
         masm.loadObjGroup(obj, temp1);
         masm.branchPtr(Assembler::NotEqual, groupAddr, temp1, ool->entry());
+        masm.loadPtr(Address(obj, UnboxedPlainObject::offsetOfExpando()), temp1);
+        masm.branchTestPtr(Assembler::Zero, temp1, temp1, &noExpando);
+        branchIfNotEmptyObjectElements(temp1, ool->entry());
+        masm.loadObjShape(temp1, temp1);
+        masm.bind(&noExpando);
+        masm.branchPtr(Assembler::NotEqual, shapeAddr, temp1, ool->entry());
         masm.bind(&guardDone);
     }
 
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index 1e12f5dbe1..f00167d929 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -10940,6 +10940,63 @@ IonBuilder::getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_
     return slot;
 }
 
+uint32_t
+IonBuilder::getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name, JSValueType* punboxedType)
+{
+    if (!types || types->unknownObject() || !types->objectOrSentinel()) {
+        trackOptimizationOutcome(TrackedOutcome::NoTypeInfo);
+        return UINT32_MAX;
+    }
+
+    uint32_t offset = UINT32_MAX;
+
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (!key)
+            continue;
+
+        if (key->unknownProperties()) {
+            trackOptimizationOutcome(TrackedOutcome::UnknownProperties);
+            return UINT32_MAX;
+        }
+
+        if (key->isSingleton()) {
+            trackOptimizationOutcome(TrackedOutcome::Singleton);
+            return UINT32_MAX;
+        }
+
+        UnboxedLayout* layout = key->group()->maybeUnboxedLayout();
+        if (!layout) {
+            trackOptimizationOutcome(TrackedOutcome::NotUnboxed);
+            return UINT32_MAX;
+        }
+
+        const UnboxedLayout::Property* property = layout->lookup(name);
+        if (!property) {
+            trackOptimizationOutcome(TrackedOutcome::StructNoField);
+            return UINT32_MAX;
+        }
+
+        if (layout->nativeGroup()) {
+            trackOptimizationOutcome(TrackedOutcome::UnboxedConvertedToNative);
+            return UINT32_MAX;
+        }
+
+        if (offset == UINT32_MAX) {
+            offset = property->offset;
+            *punboxedType = property->type;
+        } else if (offset != property->offset) {
+            trackOptimizationOutcome(TrackedOutcome::InconsistentFieldOffset);
+            return UINT32_MAX;
+        } else if (*punboxedType != property->type) {
+            trackOptimizationOutcome(TrackedOutcome::InconsistentFieldType);
+            return UINT32_MAX;
+        }
+    }
+
+    return offset;
+}
+
 bool
 IonBuilder::jsop_runonce()
 {
@@ -11906,6 +11963,72 @@ IonBuilder::getPropTryModuleNamespace(bool* emitted, MDefinition* obj, PropertyN
     return true;
 }
 
+MInstruction*
+IonBuilder::loadUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                BarrierKind barrier, TemporaryTypeSet* types)
+{
+    // loadUnboxedValue is designed to load any value as if it were contained in
+    // an array. Thus a property offset is converted to an index, when the
+    // object is reinterpreted as an array of properties of the same size.
+    size_t index = offset / UnboxedTypeSize(unboxedType);
+    MInstruction* indexConstant = MConstant::New(alloc(), Int32Value(index));
+    current->add(indexConstant);
+
+    return loadUnboxedValue(obj, UnboxedPlainObject::offsetOfData(),
+                            indexConstant, unboxedType, barrier, types);
+}
+
+MInstruction*
+IonBuilder::loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
+                             MDefinition* index, JSValueType unboxedType,
+                             BarrierKind barrier, TemporaryTypeSet* types)
+{
+    MInstruction* load;
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Uint8,
+                                       DoesNotRequireMemoryBarrier, elementsOffset);
+        load->setResultType(MIRType::Boolean);
+        break;
+
+      case JSVAL_TYPE_INT32:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Int32,
+                                       DoesNotRequireMemoryBarrier, elementsOffset);
+        load->setResultType(MIRType::Int32);
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Float64,
+                                       DoesNotRequireMemoryBarrier, elementsOffset,
+                                       /* canonicalizeDoubles = */ false);
+        load->setResultType(MIRType::Double);
+        break;
+
+      case JSVAL_TYPE_STRING:
+        load = MLoadUnboxedString::New(alloc(), elements, index, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_OBJECT: {
+        MLoadUnboxedObjectOrNull::NullBehavior nullBehavior;
+        if (types->hasType(TypeSet::NullType()))
+            nullBehavior = MLoadUnboxedObjectOrNull::HandleNull;
+        else if (barrier != BarrierKind::NoBarrier)
+            nullBehavior = MLoadUnboxedObjectOrNull::BailOnNull;
+        else
+            nullBehavior = MLoadUnboxedObjectOrNull::NullNotPossible;
+        load = MLoadUnboxedObjectOrNull::New(alloc(), elements, index, nullBehavior,
+                                             elementsOffset);
+        break;
+      }
+
+      default:
+        MOZ_CRASH();
+    }
+
+    current->add(load);
+    return load;
+}
+
 MDefinition*
 IonBuilder::addShapeGuardsForGetterSetter(MDefinition* obj, JSObject* holder, Shape* holderShape,
                 const BaselineInspector::ReceiverVector& receivers,
@@ -12729,6 +12852,66 @@ IonBuilder::setPropTryDefiniteSlot(bool* emitted, MDefinition* obj,
     return true;
 }
 
+MInstruction*
+IonBuilder::storeUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                 MDefinition* value)
+{
+    size_t scaledOffsetConstant = offset / UnboxedTypeSize(unboxedType);
+    MInstruction* scaledOffset = MConstant::New(alloc(), Int32Value(scaledOffsetConstant));
+    current->add(scaledOffset);
+
+    return storeUnboxedValue(obj, obj, UnboxedPlainObject::offsetOfData(),
+                             scaledOffset, unboxedType, value);
+}
+
+MInstruction*
+IonBuilder::storeUnboxedValue(MDefinition* obj, MDefinition* elements, int32_t elementsOffset,
+                              MDefinition* scaledOffset, JSValueType unboxedType,
+                              MDefinition* value, bool preBarrier /* = true */)
+{
+    MInstruction* store;
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Uint8,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_INT32:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Int32,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Float64,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_STRING:
+        store = MStoreUnboxedString::New(alloc(), elements, scaledOffset, value,
+                                         elementsOffset, preBarrier);
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        MOZ_ASSERT(value->type() == MIRType::Object ||
+                   value->type() == MIRType::Null ||
+                   value->type() == MIRType::Value);
+        MOZ_ASSERT(!value->mightBeType(MIRType::Undefined),
+                   "MToObjectOrNull slow path is invalid for unboxed objects");
+        store = MStoreUnboxedObjectOrNull::New(alloc(), elements, scaledOffset, value, obj,
+                                               elementsOffset, preBarrier);
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+
+    current->add(store);
+    return store;
+}
+
 bool
 IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
                                    PropertyName* name, MDefinition* value,
diff --git a/js/src/jit/IonBuilder.h b/js/src/jit/IonBuilder.h
index 6a3b61232f..78af0e412e 100644
--- a/js/src/jit/IonBuilder.h
+++ b/js/src/jit/IonBuilder.h
@@ -1050,6 +1050,19 @@ class IonBuilder
     ResultWithOOM<bool> testNotDefinedProperty(MDefinition* obj, jsid id);
 
     uint32_t getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_t* pnfixed);
+    uint32_t getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name,
+                              JSValueType* punboxedType);
+    MInstruction* loadUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                      BarrierKind barrier, TemporaryTypeSet* types);
+    MInstruction* loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
+                                   MDefinition* scaledOffset, JSValueType unboxedType,
+                                   BarrierKind barrier, TemporaryTypeSet* types);
+    MInstruction* storeUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                       MDefinition* value);
+    MInstruction* storeUnboxedValue(MDefinition* obj,
+                                    MDefinition* elements, int32_t elementsOffset,
+                                    MDefinition* scaledOffset, JSValueType unboxedType,
+                                    MDefinition* value, bool preBarrier = true);
     MOZ_MUST_USE bool checkPreliminaryGroups(MDefinition *obj);
     MOZ_MUST_USE bool freezePropTypeSets(TemporaryTypeSet* types,
                                          JSObject* foundProto, PropertyName* name);
diff --git a/js/src/jit/IonCaches.cpp b/js/src/jit/IonCaches.cpp
index fb42911886..f5e4659c1a 100644
--- a/js/src/jit/IonCaches.cpp
+++ b/js/src/jit/IonCaches.cpp
@@ -31,6 +31,7 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/Interpreter-inl.h"
 #include "vm/Shape-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -619,7 +620,26 @@ TestMatchingReceiver(MacroAssembler& masm, IonCache::StubAttacher& attacher,
                      Register object, JSObject* obj, Label* failure,
                      bool alwaysCheckGroup = false)
 {
-    if (obj->is<TypedObject>()) {
+    if (obj->is<UnboxedPlainObject>()) {
+        MOZ_ASSERT(failure);
+
+        masm.branchTestObjGroup(Assembler::NotEqual, object, obj->group(), failure);
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+            masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+            Label success;
+            masm.push(object);
+            masm.loadPtr(expandoAddress, object);
+            masm.branchTestObjShape(Assembler::Equal, object, expando->lastProperty(),
+                                    &success);
+            masm.pop(object);
+            masm.jump(failure);
+            masm.bind(&success);
+            masm.pop(object);
+        } else {
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), failure);
+        }
+    } else if (obj->is<TypedObject>()) {
         attacher.branchNextStubOrLabel(masm, Assembler::NotEqual,
                                        Address(object, JSObject::offsetOfGroup()),
                                        ImmGCPtr(obj->group()), failure);
@@ -736,6 +756,7 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     // jump directly. Otherwise, jump to the end of the stub, so there's a
     // common point to patch.
     bool multipleFailureJumps = (obj != holder)
+                             || obj->is<UnboxedPlainObject>()
                              || (checkTDZ && output.hasValue())
                              || (failures != nullptr && failures->used());
 
@@ -754,6 +775,7 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     Register scratchReg = Register::FromCode(0); // Quell compiler warning.
 
     if (obj != holder ||
+        obj->is<UnboxedPlainObject>() ||
         !holder->as<NativeObject>().isFixedSlot(shape->slot()))
     {
         if (output.hasValue()) {
@@ -814,6 +836,10 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
 
             holderReg = InvalidReg;
         }
+    } else if (obj->is<UnboxedPlainObject>()) {
+        holder = obj->as<UnboxedPlainObject>().maybeExpando();
+        holderReg = scratchReg;
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), holderReg);
     } else {
         holderReg = object;
     }
@@ -841,6 +867,30 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     attacher.jumpNextStub(masm);
 }
 
+static void
+GenerateReadUnboxed(JSContext* cx, IonScript* ion, MacroAssembler& masm,
+                    IonCache::StubAttacher& attacher, JSObject* obj,
+                    const UnboxedLayout::Property* property,
+                    Register object, TypedOrValueRegister output,
+                    Label* failures = nullptr)
+{
+    // Guard on the group of the object.
+    attacher.branchNextStubOrLabel(masm, Assembler::NotEqual,
+                                   Address(object, JSObject::offsetOfGroup()),
+                                   ImmGCPtr(obj->group()), failures);
+
+    Address address(object, UnboxedPlainObject::offsetOfData() + property->offset);
+
+    masm.loadUnboxedProperty(address, property->type, output);
+
+    attacher.jumpRejoin(masm);
+
+    if (failures) {
+        masm.bind(failures);
+        attacher.jumpNextStub(masm);
+    }
+}
+
 static bool
 EmitGetterCall(JSContext* cx, MacroAssembler& masm,
                IonCache::StubAttacher& attacher, JSObject* obj,
@@ -1448,6 +1498,67 @@ GetPropertyIC::tryAttachNative(JSContext* cx, HandleScript outerScript, IonScrip
 }
 
 bool
+GetPropertyIC::tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                HandleObject obj, HandleId id, void* returnAddr, bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (!property)
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+    Label* maybeFailures = failures.used() ? &failures : nullptr;
+
+    StubAttacher attacher(*this);
+    GenerateReadUnboxed(cx, ion, masm, attacher, obj, property, object(), output(), maybeFailures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "read unboxed",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedRead);
+}
+
+bool
+GetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, void* returnAddr, bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando)
+        return true;
+
+    Shape* shape = expando->lookup(cx, id);
+    if (!shape || !shape->hasDefaultGetter() || !shape->hasSlot())
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+    Label* maybeFailures = failures.used() ? &failures : nullptr;
+
+    StubAttacher attacher(*this);
+    GenerateReadSlot(cx, ion, masm, attacher, DontCheckTDZ, obj, obj,
+                     shape, object(), output(), maybeFailures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "read unboxed expando",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedReadExpando);
+}
+
+bool
 GetPropertyIC::tryAttachTypedArrayLength(JSContext* cx, HandleScript outerScript, IonScript* ion,
                                          HandleObject obj, HandleId id, bool* emitted)
 {
@@ -2016,6 +2127,12 @@ GetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
         if (!*emitted && !tryAttachNative(cx, outerScript, ion, obj, id, returnAddr, emitted))
             return false;
 
+        if (!*emitted && !tryAttachUnboxed(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedExpando(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
         if (!*emitted && !tryAttachTypedArrayLength(cx, outerScript, ion, obj, id, emitted))
             return false;
     }
@@ -2194,6 +2311,12 @@ GenerateSetSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
 
     NativeObject::slotsSizeMustNotOverflow();
 
+    if (obj->is<UnboxedPlainObject>()) {
+        obj = obj->as<UnboxedPlainObject>().maybeExpando();
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), tempReg);
+        object = tempReg;
+    }
+
     if (obj->as<NativeObject>().isFixedSlot(shape->slot())) {
         Address addr(object, NativeObject::getFixedSlotOffset(shape->slot()));
 
@@ -2831,13 +2954,23 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     masm.branchTestObjGroup(Assembler::NotEqual, object, oldGroup, failures);
     if (obj->maybeShape()) {
         masm.branchTestObjShape(Assembler::NotEqual, object, oldShape, failures);
+    } else {
+        MOZ_ASSERT(obj->is<UnboxedPlainObject>());
+
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failures);
+
+        masm.loadPtr(expandoAddress, tempReg);
+        masm.branchTestObjShape(Assembler::NotEqual, tempReg, oldShape, failures);
     }
 
     Shape* newShape = obj->maybeShape();
+    if (!newShape)
+        newShape = obj->as<UnboxedPlainObject>().maybeExpando()->lastProperty();
 
     // Guard that the incoming value is in the type set for the property
     // if a type barrier is required.
-    if (newShape && checkTypeset)
+    if (checkTypeset)
         CheckTypeSetForWrite(masm, obj, newShape->propid(), tempReg, value, failures);
 
     // Guard shapes along prototype chain.
@@ -2858,7 +2991,9 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     }
 
     // Call a stub to (re)allocate dynamic slots, if necessary.
-    uint32_t newNumDynamicSlots = obj->as<NativeObject>().numDynamicSlots();
+    uint32_t newNumDynamicSlots = obj->is<UnboxedPlainObject>()
+                                  ? obj->as<UnboxedPlainObject>().maybeExpando()->numDynamicSlots()
+                                  : obj->as<NativeObject>().numDynamicSlots();
     if (NativeObject::dynamicSlotsCount(oldShape) != newNumDynamicSlots) {
         AllocatableRegisterSet regs(RegisterSet::Volatile());
         LiveRegisterSet save(regs.asLiveSet());
@@ -2869,6 +3004,12 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
         Register temp1 = regs.takeAnyGeneral();
         Register temp2 = regs.takeAnyGeneral();
 
+        if (obj->is<UnboxedPlainObject>()) {
+            // Pass the expando object to the stub.
+            masm.Push(object);
+            masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), object);
+        }
+
         masm.setupUnalignedABICall(temp1);
         masm.loadJSContext(temp1);
         masm.passABIArg(temp1);
@@ -2885,16 +3026,27 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
         masm.jump(&allocDone);
 
         masm.bind(&allocFailed);
+        if (obj->is<UnboxedPlainObject>())
+            masm.Pop(object);
         masm.PopRegsInMask(save);
         masm.jump(failures);
 
         masm.bind(&allocDone);
         masm.setFramePushed(framePushedAfterCall);
+        if (obj->is<UnboxedPlainObject>())
+            masm.Pop(object);
         masm.PopRegsInMask(save);
     }
 
     bool popObject = false;
 
+    if (obj->is<UnboxedPlainObject>()) {
+        masm.push(object);
+        popObject = true;
+        obj = obj->as<UnboxedPlainObject>().maybeExpando();
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), object);
+    }
+
     // Write the object or expando object's new shape.
     Address shapeAddr(object, ShapedObject::offsetOfShape());
     if (cx->zone()->needsIncrementalBarrier())
@@ -2902,6 +3054,8 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     masm.storePtr(ImmGCPtr(newShape), shapeAddr);
 
     if (oldGroup != obj->group()) {
+        MOZ_ASSERT(!obj->is<UnboxedPlainObject>());
+
         // Changing object's group from a partially to fully initialized group,
         // per the acquired properties analysis. Only change the group if the
         // old group still has a newScript.
@@ -3144,6 +3298,141 @@ CanAttachNativeSetProp(JSContext* cx, HandleObject obj, HandleId id, const Const
     return SetPropertyIC::CanAttachNone;
 }
 
+static void
+GenerateSetUnboxed(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& attacher,
+                   JSObject* obj, jsid id, uint32_t unboxedOffset, JSValueType unboxedType,
+                   Register object, Register tempReg, const ConstantOrRegister& value,
+                   bool checkTypeset, Label* failures)
+{
+    // Guard on the type of the object.
+    masm.branchPtr(Assembler::NotEqual,
+                   Address(object, JSObject::offsetOfGroup()),
+                   ImmGCPtr(obj->group()), failures);
+
+    if (checkTypeset)
+        CheckTypeSetForWrite(masm, obj, id, tempReg, value, failures);
+
+    Address address(object, UnboxedPlainObject::offsetOfData() + unboxedOffset);
+
+    if (cx->zone()->needsIncrementalBarrier()) {
+        if (unboxedType == JSVAL_TYPE_OBJECT)
+            masm.callPreBarrier(address, MIRType::Object);
+        else if (unboxedType == JSVAL_TYPE_STRING)
+            masm.callPreBarrier(address, MIRType::String);
+        else
+            MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(unboxedType));
+    }
+
+    masm.storeUnboxedProperty(address, unboxedType, value, failures);
+
+    attacher.jumpRejoin(masm);
+
+    masm.bind(failures);
+    attacher.jumpNextStub(masm);
+}
+
+static bool
+CanAttachSetUnboxed(JSContext* cx, HandleObject obj, HandleId id, const ConstantOrRegister& val,
+                    bool needsTypeBarrier, bool* checkTypeset,
+                    uint32_t* unboxedOffset, JSValueType* unboxedType)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (property) {
+        *checkTypeset = false;
+        if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+            return false;
+        *unboxedOffset = property->offset;
+        *unboxedType = property->type;
+        return true;
+    }
+
+    return false;
+}
+
+static bool
+CanAttachSetUnboxedExpando(JSContext* cx, HandleObject obj, HandleId id,
+                           const ConstantOrRegister& val,
+                           bool needsTypeBarrier, bool* checkTypeset, Shape** pshape)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando)
+        return false;
+
+    Shape* shape = expando->lookupPure(id);
+    if (!shape || !shape->hasDefaultSetter() || !shape->hasSlot() || !shape->writable())
+        return false;
+
+    *checkTypeset = false;
+    if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+        return false;
+
+    *pshape = shape;
+    return true;
+}
+
+static bool
+CanAttachAddUnboxedExpando(JSContext* cx, HandleObject obj, HandleShape oldShape,
+                           HandleId id, const ConstantOrRegister& val,
+                           bool needsTypeBarrier, bool* checkTypeset)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando || expando->inDictionaryMode())
+        return false;
+
+    Shape* newShape = expando->lastProperty();
+    if (newShape->isEmptyShape() || newShape->propid() != id || newShape->previous() != oldShape)
+        return false;
+
+    MOZ_ASSERT(newShape->hasDefaultSetter() && newShape->hasSlot() && newShape->writable());
+
+    if (PrototypeChainShadowsPropertyAdd(cx, obj, id))
+        return false;
+
+    *checkTypeset = false;
+    if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+        return false;
+
+    return true;
+}
+
+bool
+SetPropertyIC::tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                HandleObject obj, HandleId id, bool* emitted)
+{
+    MOZ_ASSERT(!*emitted);
+
+    bool checkTypeset = false;
+    uint32_t unboxedOffset;
+    JSValueType unboxedType;
+    if (!CanAttachSetUnboxed(cx, obj, id, value(), needsTypeBarrier(), &checkTypeset,
+                             &unboxedOffset, &unboxedType))
+    {
+        return true;
+    }
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+    StubAttacher attacher(*this);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+
+    GenerateSetUnboxed(cx, masm, attacher, obj, id, unboxedOffset, unboxedType,
+                       object(), temp(), value(), checkTypeset, &failures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "set_unboxed",
+                             JS::TrackedOutcome::ICSetPropStub_SetUnboxed);
+}
+
 bool
 SetPropertyIC::tryAttachProxy(JSContext* cx, HandleScript outerScript, IonScript* ion,
                               HandleObject obj, HandleId id, bool* emitted)
@@ -3225,6 +3514,26 @@ SetPropertyIC::tryAttachNative(JSContext* cx, HandleScript outerScript, IonScrip
 }
 
 bool
+SetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, bool* emitted)
+{
+    MOZ_ASSERT(!*emitted);
+
+    RootedShape shape(cx);
+    bool checkTypeset = false;
+    if (!CanAttachSetUnboxedExpando(cx, obj, id, value(), needsTypeBarrier(),
+                                    &checkTypeset, shape.address()))
+    {
+        return true;
+    }
+
+    if (!attachSetSlot(cx, outerScript, ion, obj, shape, checkTypeset))
+        return false;
+    *emitted = true;
+    return true;
+}
+
+bool
 SetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript* ion,
                              HandleObject obj, HandleValue idval, HandleValue value,
                              MutableHandleId id, bool* emitted, bool* tryNativeAddSlot)
@@ -3249,6 +3558,12 @@ SetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
 
         if (!*emitted && !tryAttachNative(cx, outerScript, ion, obj, id, emitted, tryNativeAddSlot))
             return false;
+
+        if (!*emitted && !tryAttachUnboxed(cx, outerScript, ion, obj, id, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedExpando(cx, outerScript, ion, obj, id, emitted))
+            return false;
     }
 
     if (idval.isInt32()) {
@@ -3300,6 +3615,16 @@ SetPropertyIC::tryAttachAddSlot(JSContext* cx, HandleScript outerScript, IonScri
         return true;
     }
 
+    checkTypeset = false;
+    if (CanAttachAddUnboxedExpando(cx, obj, oldShape, id, value(), needsTypeBarrier(),
+                                   &checkTypeset))
+    {
+        if (!attachAddSlot(cx, outerScript, ion, obj, id, oldShape, oldGroup, checkTypeset))
+            return false;
+        *emitted = true;
+        return true;
+    }
+
     return true;
 }
 
@@ -3321,6 +3646,11 @@ SetPropertyIC::update(JSContext* cx, HandleScript outerScript, size_t cacheIndex
             return false;
 
         oldShape = obj->maybeShape();
+        if (obj->is<UnboxedPlainObject>()) {
+            MOZ_ASSERT(!oldShape);
+            if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+                oldShape = expando->lastProperty();
+        }
     }
 
     RootedId id(cx);
diff --git a/js/src/jit/IonCaches.h b/js/src/jit/IonCaches.h
index b006465383..173e06c6ba 100644
--- a/js/src/jit/IonCaches.h
+++ b/js/src/jit/IonCaches.h
@@ -529,6 +529,18 @@ class GetPropertyIC : public IonCache
                                       HandleObject obj, HandleId id, void* returnAddr,
                                       bool* emitted);
 
+    MOZ_MUST_USE bool tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, void* returnAddr,
+                                       bool* emitted);
+
+    MOZ_MUST_USE bool tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript,
+                                              IonScript* ion, HandleObject obj, HandleId id,
+                                              void* returnAddr, bool* emitted);
+
+    MOZ_MUST_USE bool tryAttachUnboxedArrayLength(JSContext* cx, HandleScript outerScript,
+                                                  IonScript* ion, HandleObject obj, HandleId id,
+                                                  void* returnAddr, bool* emitted);
+
     MOZ_MUST_USE bool tryAttachTypedArrayLength(JSContext* cx, HandleScript outerScript,
                                                 IonScript* ion, HandleObject obj, HandleId id,
                                                 bool* emitted);
diff --git a/js/src/jit/JitOptions.cpp b/js/src/jit/JitOptions.cpp
index 3f9d9db888..b9a7c7b276 100644
--- a/js/src/jit/JitOptions.cpp
+++ b/js/src/jit/JitOptions.cpp
@@ -221,6 +221,9 @@ DefaultJitOptions::DefaultJitOptions()
             Warn(forcedRegisterAllocatorEnv, env);
     }
 
+    // Toggles whether unboxed plain objects can be created by the VM.
+    SET_DEFAULT(disableUnboxedObjects, true);
+
     // Test whether Atomics are allowed in asm.js code.
     SET_DEFAULT(asmJSAtomicsEnable, false);
 
diff --git a/js/src/jit/JitOptions.h b/js/src/jit/JitOptions.h
index 719ee14d95..076980b4e5 100644
--- a/js/src/jit/JitOptions.h
+++ b/js/src/jit/JitOptions.h
@@ -91,6 +91,9 @@ struct DefaultJitOptions
     mozilla::Maybe<uint32_t> forcedDefaultIonSmallFunctionWarmUpThreshold;
     mozilla::Maybe<IonRegisterAllocator> forcedRegisterAllocator;
 
+    // The options below affect the rest of the VM, and not just the JIT.
+    bool disableUnboxedObjects;
+
     DefaultJitOptions();
     bool isSmallFunction(JSScript* script) const;
     void setEagerCompilation();
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index 2363545300..a1c336391b 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -30,6 +30,7 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using mozilla::ArrayLength;
 using mozilla::AssertedCast;
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index 0cf31adb33..1e4ee170f1 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -4783,14 +4783,15 @@ MCreateThisWithTemplate::canRecoverOnBailout() const
 
 MObjectState::MObjectState(MObjectState* state)
   : numSlots_(state->numSlots_),
-    numFixedSlots_(state->numFixedSlots_)
+    numFixedSlots_(state->numFixedSlots_),
+    operandIndex_(state->operandIndex_)
 {
     // This instruction is only used as a summary for bailout paths.
     setResultType(MIRType::Object);
     setRecoveredOnBailout();
 }
 
-MObjectState::MObjectState(JSObject* templateObject)
+MObjectState::MObjectState(JSObject *templateObject, OperandIndexMap* operandIndex)
 {
     // This instruction is only used as a summary for bailout paths.
     setResultType(MIRType::Object);
@@ -4801,6 +4802,8 @@ MObjectState::MObjectState(JSObject* templateObject)
     NativeObject* nativeObject = &templateObject->as<NativeObject>();
     numSlots_ = nativeObject->slotSpan();
     numFixedSlots_ = nativeObject->numFixedSlots();
+
+    operandIndex_ = operandIndex;
 }
 
 JSObject*
@@ -4860,7 +4863,7 @@ MObjectState::New(TempAllocator& alloc, MDefinition* obj)
     JSObject* templateObject = templateObjectOf(obj);
     MOZ_ASSERT(templateObject, "Unexpected object creation.");
 
-    MObjectState* res = new(alloc) MObjectState(templateObject);
+    MObjectState* res = new(alloc) MObjectState(templateObject, nullptr);
     if (!res || !res->init(alloc, obj))
         return nullptr;
     return res;
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index 0c1e77f802..cafdbab714 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -375,7 +375,7 @@ class AliasSet {
         Element           = 1 << 1, // A Value member of obj->elements or
                                     // a typed object.
         UnboxedElement    = 1 << 2, // An unboxed scalar or reference member of
-                                    // typed object.
+                                    // typed object or unboxed object.
         DynamicSlot       = 1 << 3, // A Value member of obj->slots.
         FixedSlot         = 1 << 4, // A Value member of obj->fixedSlots().
         DOMProperty       = 1 << 5, // A DOM property
@@ -3758,9 +3758,14 @@ class MObjectState
 {
   private:
     uint32_t numSlots_;
-    uint32_t numFixedSlots_;
+    uint32_t numFixedSlots_;        // valid if isUnboxed() == false.
+    OperandIndexMap* operandIndex_; // valid if isUnboxed() == true.
 
-    MObjectState(JSObject *templateObject);
+    bool isUnboxed() const {
+        return operandIndex_ != nullptr;
+    }
+
+    MObjectState(JSObject *templateObject, OperandIndexMap* operandIndex);
     explicit MObjectState(MObjectState* state);
 
     MOZ_MUST_USE bool init(TempAllocator& alloc, MDefinition* obj);
@@ -3820,6 +3825,18 @@ class MObjectState
         setSlot(slot + numFixedSlots(), def);
     }
 
+    // Interface reserved for unboxed objects.
+    bool hasOffset(uint32_t offset) const {
+        MOZ_ASSERT(isUnboxed());
+        return offset < operandIndex_->map.length() && operandIndex_->map[offset] != 0;
+    }
+    MDefinition* getOffset(uint32_t offset) const {
+        return getOperand(operandIndex_->map[offset]);
+    }
+    void setOffset(uint32_t offset, MDefinition* def) {
+        replaceOperand(operandIndex_->map[offset], def);
+    }
+
     MOZ_MUST_USE bool writeRecoverData(CompactBufferWriter& writer) const override;
     bool canRecoverOnBailout() const override {
         return true;
diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index a739b93256..e50f687227 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -126,14 +126,20 @@ MacroAssembler::guardTypeSetMightBeIncomplete(TypeSet* types, Register obj, Regi
 {
     // Type set guards might miss when an object's group changes. In this case
     // either its old group's properties will become unknown, or it will change
-    // to a native object. Jump to label if this might have happened for the
-    // input object.
+    // to a native object with an original unboxed group. Jump to label if this
+    // might have happened for the input object.
 
     if (types->unknownObject()) {
         jump(label);
         return;
     }
 
+    loadPtr(Address(obj, JSObject::offsetOfGroup()), scratch);
+    load32(Address(scratch, ObjectGroup::offsetOfFlags()), scratch);
+    and32(Imm32(OBJECT_FLAG_ADDENDUM_MASK), scratch);
+    branch32(Assembler::Equal,
+             scratch, Imm32(ObjectGroup::addendumOriginalUnboxedGroupValue()), label);
+
     for (size_t i = 0; i < types->getObjectCount(); i++) {
         if (JSObject* singleton = types->getSingletonNoBarrier(i)) {
             movePtr(ImmGCPtr(singleton), scratch);
@@ -462,6 +468,243 @@ template void MacroAssembler::loadFromTypedArray(Scalar::Type arrayType, const A
 template void MacroAssembler::loadFromTypedArray(Scalar::Type arrayType, const BaseIndex& src, const ValueOperand& dest,
                                                  bool allowDouble, Register temp, Label* fail);
 
+template <typename T>
+void
+MacroAssembler::loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output)
+{
+    switch (type) {
+      case JSVAL_TYPE_INT32: {
+          // Handle loading an int32 into a double reg.
+          if (output.type() == MIRType::Double) {
+              convertInt32ToDouble(address, output.typedReg().fpu());
+              break;
+          }
+          MOZ_FALLTHROUGH;
+      }
+
+      case JSVAL_TYPE_BOOLEAN:
+      case JSVAL_TYPE_STRING: {
+        Register outReg;
+        if (output.hasValue()) {
+            outReg = output.valueReg().scratchReg();
+        } else {
+            MOZ_ASSERT(output.type() == MIRTypeFromValueType(type));
+            outReg = output.typedReg().gpr();
+        }
+
+        switch (type) {
+          case JSVAL_TYPE_BOOLEAN:
+            load8ZeroExtend(address, outReg);
+            break;
+          case JSVAL_TYPE_INT32:
+            load32(address, outReg);
+            break;
+          case JSVAL_TYPE_STRING:
+            loadPtr(address, outReg);
+            break;
+          default:
+            MOZ_CRASH();
+        }
+
+        if (output.hasValue())
+            tagValue(type, outReg, output.valueReg());
+        break;
+      }
+
+      case JSVAL_TYPE_OBJECT:
+        if (output.hasValue()) {
+            Register scratch = output.valueReg().scratchReg();
+            loadPtr(address, scratch);
+
+            Label notNull, done;
+            branchPtr(Assembler::NotEqual, scratch, ImmWord(0), &notNull);
+
+            moveValue(NullValue(), output.valueReg());
+            jump(&done);
+
+            bind(&notNull);
+            tagValue(JSVAL_TYPE_OBJECT, scratch, output.valueReg());
+
+            bind(&done);
+        } else {
+            // Reading null can't be possible here, as otherwise the result
+            // would be a value (either because null has been read before or
+            // because there is a barrier).
+            Register reg = output.typedReg().gpr();
+            loadPtr(address, reg);
+#ifdef DEBUG
+            Label ok;
+            branchTestPtr(Assembler::NonZero, reg, reg, &ok);
+            assumeUnreachable("Null not possible");
+            bind(&ok);
+#endif
+        }
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        // Note: doubles in unboxed objects are not accessed through other
+        // views and do not need canonicalization.
+        if (output.hasValue())
+            loadValue(address, output.valueReg());
+        else
+            loadDouble(address, output.typedReg().fpu());
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+template void
+MacroAssembler::loadUnboxedProperty(Address address, JSValueType type,
+                                    TypedOrValueRegister output);
+
+template void
+MacroAssembler::loadUnboxedProperty(BaseIndex address, JSValueType type,
+                                    TypedOrValueRegister output);
+
+static void
+StoreUnboxedFailure(MacroAssembler& masm, Label* failure)
+{
+    // Storing a value to an unboxed property is a fallible operation and
+    // the caller must provide a failure label if a particular unboxed store
+    // might fail. Sometimes, however, a store that cannot succeed (such as
+    // storing a string to an int32 property) will be marked as infallible.
+    // This can only happen if the code involved is unreachable.
+    if (failure)
+        masm.jump(failure);
+    else
+        masm.assumeUnreachable("Incompatible write to unboxed property");
+}
+
+template <typename T>
+void
+MacroAssembler::storeUnboxedProperty(T address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        if (value.constant()) {
+            if (value.value().isBoolean())
+                store8(Imm32(value.value().toBoolean()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Boolean)
+                store8(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestBoolean(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ 1);
+        }
+        break;
+
+      case JSVAL_TYPE_INT32:
+        if (value.constant()) {
+            if (value.value().isInt32())
+                store32(Imm32(value.value().toInt32()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Int32)
+                store32(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestInt32(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ 4);
+        }
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        if (value.constant()) {
+            if (value.value().isNumber()) {
+                loadConstantDouble(value.value().toNumber(), ScratchDoubleReg);
+                storeDouble(ScratchDoubleReg, address);
+            } else {
+                StoreUnboxedFailure(*this, failure);
+            }
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Int32) {
+                convertInt32ToDouble(value.reg().typedReg().gpr(), ScratchDoubleReg);
+                storeDouble(ScratchDoubleReg, address);
+            } else if (value.reg().type() == MIRType::Double) {
+                storeDouble(value.reg().typedReg().fpu(), address);
+            } else {
+                StoreUnboxedFailure(*this, failure);
+            }
+        } else {
+            ValueOperand reg = value.reg().valueReg();
+            Label notInt32, end;
+            branchTestInt32(Assembler::NotEqual, reg, &notInt32);
+            int32ValueToDouble(reg, ScratchDoubleReg);
+            storeDouble(ScratchDoubleReg, address);
+            jump(&end);
+            bind(&notInt32);
+            if (failure)
+                branchTestDouble(Assembler::NotEqual, reg, failure);
+            storeValue(reg, address);
+            bind(&end);
+        }
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        if (value.constant()) {
+            if (value.value().isObjectOrNull())
+                storePtr(ImmGCPtr(value.value().toObjectOrNull()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            MOZ_ASSERT(value.reg().type() != MIRType::Null);
+            if (value.reg().type() == MIRType::Object)
+                storePtr(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure) {
+                Label ok;
+                branchTestNull(Assembler::Equal, value.reg().valueReg(), &ok);
+                branchTestObject(Assembler::NotEqual, value.reg().valueReg(), failure);
+                bind(&ok);
+            }
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ sizeof(uintptr_t));
+        }
+        break;
+
+      case JSVAL_TYPE_STRING:
+        if (value.constant()) {
+            if (value.value().isString())
+                storePtr(ImmGCPtr(value.value().toString()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::String)
+                storePtr(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestString(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ sizeof(uintptr_t));
+        }
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+template void
+MacroAssembler::storeUnboxedProperty(Address address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure);
+
+template void
+MacroAssembler::storeUnboxedProperty(BaseIndex address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure);
+
 // Inlined version of gc::CheckAllocatorState that checks the bare essentials
 // and bails for anything that cannot be handled with our jit allocators.
 void
@@ -1009,6 +1252,10 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
             nbytes = (nbytes < sizeof(uintptr_t)) ? 0 : nbytes - sizeof(uintptr_t);
             offset += sizeof(uintptr_t);
         }
+    } else if (templateObj->is<UnboxedPlainObject>()) {
+        storePtr(ImmWord(0), Address(obj, UnboxedPlainObject::offsetOfExpando()));
+        if (initContents)
+            initUnboxedObjectContents(obj, &templateObj->as<UnboxedPlainObject>());
     } else {
         MOZ_CRASH("Unknown object");
     }
@@ -1030,6 +1277,29 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
 }
 
 void
+MacroAssembler::initUnboxedObjectContents(Register object, UnboxedPlainObject* templateObject)
+{
+    const UnboxedLayout& layout = templateObject->layoutDontCheckGeneration();
+
+    // Initialize reference fields of the object, per UnboxedPlainObject::create.
+    if (const int32_t* list = layout.traceList()) {
+        while (*list != -1) {
+            storePtr(ImmGCPtr(GetJitContext()->runtime->names().empty),
+                     Address(object, UnboxedPlainObject::offsetOfData() + *list));
+            list++;
+        }
+        list++;
+        while (*list != -1) {
+            storePtr(ImmWord(0),
+                     Address(object, UnboxedPlainObject::offsetOfData() + *list));
+            list++;
+        }
+        // Unboxed objects don't have Values to initialize.
+        MOZ_ASSERT(*(list + 1) == -1);
+    }
+}
+
+void
 MacroAssembler::compareStrings(JSOp op, Register left, Register right, Register result,
                                Label* fail)
 {
diff --git a/js/src/jit/MacroAssembler.h b/js/src/jit/MacroAssembler.h
index d5cc95839d..6ee989463d 100644
--- a/js/src/jit/MacroAssembler.h
+++ b/js/src/jit/MacroAssembler.h
@@ -36,6 +36,7 @@
 #include "vm/ProxyObject.h"
 #include "vm/Shape.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 using mozilla::FloatingPoint;
 
@@ -1625,6 +1626,17 @@ class MacroAssembler : public MacroAssemblerSpecific
     void storeToTypedFloatArray(Scalar::Type arrayType, FloatRegister value, const Address& dest,
                                 unsigned numElems = 0);
 
+    // Load a property from an UnboxedPlainObject.
+    template <typename T>
+    void loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output);
+
+    // Store a property to an UnboxedPlainObject, without triggering barriers.
+    // If failure is null, the value definitely has a type suitable for storing
+    // in the property.
+    template <typename T>
+    void storeUnboxedProperty(T address, JSValueType type,
+                              const ConstantOrRegister& value, Label* failure);
+
     Register extractString(const Address& address, Register scratch) {
         return extractObject(address, scratch);
     }
@@ -1701,6 +1713,8 @@ class MacroAssembler : public MacroAssemblerSpecific
                              LiveRegisterSet liveRegs, Label* fail,
                              TypedArrayObject* templateObj, TypedArrayLength lengthKind);
 
+    void initUnboxedObjectContents(Register object, UnboxedPlainObject* templateObject);
+
     void newGCString(Register result, Register temp, Label* fail);
     void newGCFatInlineString(Register result, Register temp, Label* fail);
 
diff --git a/js/src/jit/OptimizationTracking.cpp b/js/src/jit/OptimizationTracking.cpp
index 7d72795a01..b42634d436 100644
--- a/js/src/jit/OptimizationTracking.cpp
+++ b/js/src/jit/OptimizationTracking.cpp
@@ -15,9 +15,11 @@
 #include "jit/JitcodeMap.h"
 #include "jit/JitSpewer.h"
 #include "js/TrackedOptimizationInfo.h"
+#include "vm/UnboxedObject.h"
 
 #include "vm/ObjectGroup-inl.h"
 #include "vm/TypeInference-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -844,6 +846,8 @@ MaybeConstructorFromType(TypeSet::Type ty)
         return nullptr;
     ObjectGroup* obj = ty.group();
     TypeNewScript* newScript = obj->newScript();
+    if (!newScript && obj->maybeUnboxedLayout())
+        newScript = obj->unboxedLayout().newScript();
     return newScript ? newScript->function() : nullptr;
 }
 
diff --git a/js/src/jit/Recover.cpp b/js/src/jit/Recover.cpp
index 793b631df0..8fe6ee3fb5 100644
--- a/js/src/jit/Recover.cpp
+++ b/js/src/jit/Recover.cpp
@@ -30,6 +30,7 @@
 
 #include "vm/Interpreter-inl.h"
 #include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -1539,12 +1540,37 @@ RObjectState::recover(JSContext* cx, SnapshotIterator& iter) const
     RootedObject object(cx, &iter.read().toObject());
     RootedValue val(cx);
 
-    RootedNativeObject nativeObject(cx, &object->as<NativeObject>());
-    MOZ_ASSERT(nativeObject->slotSpan() == numSlots());
+    if (object->is<UnboxedPlainObject>()) {
+        const UnboxedLayout& layout = object->as<UnboxedPlainObject>().layout();
 
-    for (size_t i = 0; i < numSlots(); i++) {
-        val = iter.read();
-        nativeObject->setSlot(i, val);
+        RootedId id(cx);
+        RootedValue receiver(cx, ObjectValue(*object));
+        const UnboxedLayout::PropertyVector& properties = layout.properties();
+        for (size_t i = 0; i < properties.length(); i++) {
+            val = iter.read();
+
+            // This is the default placeholder value of MObjectState, when no
+            // properties are defined yet.
+            if (val.isUndefined())
+                continue;
+
+            id = NameToId(properties[i].name);
+            ObjectOpResult result;
+
+            // SetProperty can only fail due to OOM.
+            if (!SetProperty(cx, object, id, val, receiver, result))
+                return false;
+            if (!result)
+                return result.reportError(cx, object, id);
+        }
+    } else {
+        RootedNativeObject nativeObject(cx, &object->as<NativeObject>());
+        MOZ_ASSERT(nativeObject->slotSpan() == numSlots());
+
+        for (size_t i = 0; i < numSlots(); i++) {
+            val = iter.read();
+            nativeObject->setSlot(i, val);
+        }
     }
 
     val.setObject(*object);
diff --git a/js/src/jit/ScalarReplacement.cpp b/js/src/jit/ScalarReplacement.cpp
index 97ba52349f..be9ceee2ee 100644
--- a/js/src/jit/ScalarReplacement.cpp
+++ b/js/src/jit/ScalarReplacement.cpp
@@ -285,6 +285,10 @@ class ObjectMemoryView : public MDefinitionVisitorDefaultNoop
     void visitGuardShape(MGuardShape* ins);
     void visitFunctionEnvironment(MFunctionEnvironment* ins);
     void visitLambda(MLambda* ins);
+
+  private:
+    void storeOffset(MInstruction* ins, size_t offset, MDefinition* value);
+    void loadOffset(MInstruction* ins, size_t offset);
 };
 
 const char* ObjectMemoryView::phaseName = "Scalar Replacement of Object";
@@ -626,6 +630,35 @@ ObjectMemoryView::visitLambda(MLambda* ins)
     ins->setIncompleteObject();
 }
 
+void
+ObjectMemoryView::storeOffset(MInstruction* ins, size_t offset, MDefinition* value)
+{
+    // Clone the state and update the slot value.
+    MOZ_ASSERT(state_->hasOffset(offset));
+    state_ = BlockState::Copy(alloc_, state_);
+    if (!state_) {
+        oom_ = true;
+        return;
+    }
+
+    state_->setOffset(offset, value);
+    ins->block()->insertBefore(ins, state_);
+
+    // Remove original instruction.
+    ins->block()->discard(ins);
+}
+
+void
+ObjectMemoryView::loadOffset(MInstruction* ins, size_t offset)
+{
+    // Replace load by the slot value.
+    MOZ_ASSERT(state_->hasOffset(offset));
+    ins->replaceAllUsesWith(state_->getOffset(offset));
+
+    // Remove original instruction.
+    ins->block()->discard(ins);
+}
+
 static bool
 IndexOf(MDefinition* ins, int32_t* res)
 {
diff --git a/js/src/jit/SharedIC.cpp b/js/src/jit/SharedIC.cpp
index 05a95824fb..3139574629 100644
--- a/js/src/jit/SharedIC.cpp
+++ b/js/src/jit/SharedIC.cpp
@@ -2244,7 +2244,8 @@ IsCacheableProtoChain(JSObject* obj, JSObject* holder, bool isDOMProxy)
     if (!isDOMProxy && !obj->isNative()) {
         if (obj == holder)
             return false;
-        if (!obj->is<TypedObject>())
+        if (!obj->is<UnboxedPlainObject>() &&
+            !obj->is<TypedObject>())
         {
             return false;
         }
@@ -2572,6 +2573,9 @@ CheckHasNoSuchProperty(JSContext* cx, JSObject* obj, PropertyName* name,
         } else if (curObj != obj) {
             // Non-native objects are only handled as the original receiver.
             return false;
+        } else if (curObj->is<UnboxedPlainObject>()) {
+            if (curObj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, NameToId(name)))
+                return false;
         } else if (curObj->is<TypedObject>()) {
             if (curObj->as<TypedObject>().typeDescr().hasProperty(cx->names(), NameToId(name)))
                 return false;
@@ -2836,15 +2840,34 @@ GuardReceiverObject(MacroAssembler& masm, ReceiverGuard guard,
 {
     Address groupAddress(ICStubReg, receiverGuardOffset + HeapReceiverGuard::offsetOfGroup());
     Address shapeAddress(ICStubReg, receiverGuardOffset + HeapReceiverGuard::offsetOfShape());
+    Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
 
     if (guard.group) {
         masm.loadPtr(groupAddress, scratch);
         masm.branchTestObjGroup(Assembler::NotEqual, object, scratch, failure);
+
+        if (guard.group->clasp() == &UnboxedPlainObject::class_ && !guard.shape) {
+            // Guard the unboxed object has no expando object.
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), failure);
+        }
     }
 
     if (guard.shape) {
         masm.loadPtr(shapeAddress, scratch);
-        masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+        if (guard.group && guard.group->clasp() == &UnboxedPlainObject::class_) {
+            // Guard the unboxed object has a matching expando object.
+            masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+            Label done;
+            masm.push(object);
+            masm.loadPtr(expandoAddress, object);
+            masm.branchTestObjShape(Assembler::Equal, object, scratch, &done);
+            masm.pop(object);
+            masm.jump(failure);
+            masm.bind(&done);
+            masm.pop(object);
+        } else {
+            masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+        }
     }
 }
 
@@ -4228,7 +4251,8 @@ DoNewObject(JSContext* cx, void* payload, ICNewObject_Fallback* stub, MutableHan
                 return false;
 
             if (!stub->invalid() &&
-                !templateObject->as<PlainObject>().hasDynamicSlots())
+                (templateObject->is<UnboxedPlainObject>() ||
+                 !templateObject->as<PlainObject>().hasDynamicSlots()))
             {
                 JitCode* code = GenerateNewObjectWithTemplateCode(cx, templateObject);
                 if (!code)
diff --git a/js/src/jit/VMFunctions.cpp b/js/src/jit/VMFunctions.cpp
index 652c23bf15..1cb731de85 100644
--- a/js/src/jit/VMFunctions.cpp
+++ b/js/src/jit/VMFunctions.cpp
@@ -28,7 +28,7 @@
 #include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
 #include "vm/TypeInference-inl.h"
-#include "gc/StoreBuffer-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
diff --git a/js/src/jsarray.cpp b/js/src/jsarray.cpp
index 73243d9181..87d1a5accc 100644
--- a/js/src/jsarray.cpp
+++ b/js/src/jsarray.cpp
@@ -45,6 +45,7 @@
 #include "vm/Caches-inl.h"
 #include "vm/Interpreter-inl.h"
 #include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::gc;
diff --git a/js/src/jscompartment.h b/js/src/jscompartment.h
index 05ff40b439..d7c6765ff8 100644
--- a/js/src/jscompartment.h
+++ b/js/src/jscompartment.h
@@ -528,6 +528,9 @@ struct JSCompartment
     // table manages references from such typed objects to their buffers.
     js::ObjectWeakMap* lazyArrayBuffers;
 
+    // All unboxed layouts in the compartment.
+    mozilla::LinkedList<js::UnboxedLayout> unboxedLayouts;
+
     // WebAssembly state for the compartment.
     js::wasm::Compartment wasm;
 
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index bdb3c0a4da..f818bb2902 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -268,7 +268,7 @@ js::GetBuiltinClass(JSContext* cx, HandleObject obj, ESClass* cls)
     if (MOZ_UNLIKELY(obj->is<ProxyObject>()))
         return Proxy::getBuiltinClass(cx, obj, cls);
 
-    if (obj->is<PlainObject>())
+    if (obj->is<PlainObject>() || obj->is<UnboxedPlainObject>())
         *cls = ESClass::Object;
     else if (obj->is<ArrayObject>())
         *cls = ESClass::Array;
diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index ef12910797..3d7f294fe7 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -42,7 +42,6 @@
 #include "frontend/BytecodeCompiler.h"
 #include "gc/Marking.h"
 #include "gc/Policy.h"
-#include "gc/StoreBuffer-inl.h"
 #include "jit/BaselineJIT.h"
 #include "js/MemoryMetrics.h"
 #include "js/Proxy.h"
@@ -54,6 +53,7 @@
 #include "vm/RegExpStaticsObject.h"
 #include "vm/Shape.h"
 #include "vm/TypedArrayCommon.h"
+#include "vm/UnboxedObject-inl.h"
 
 #include "jsatominlines.h"
 #include "jsboolinlines.h"
diff --git a/js/src/jsobjinlines.h b/js/src/jsobjinlines.h
index 98e7401427..889033143e 100644
--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@@ -32,6 +32,19 @@
 #include "vm/ShapedObject-inl.h"
 #include "vm/TypeInference-inl.h"
 
+namespace js {
+
+// This is needed here for ensureShape() below.
+inline bool
+MaybeConvertUnboxedObjectToNative(ExclusiveContext* cx, JSObject* obj)
+{
+    if (obj->is<UnboxedPlainObject>())
+        return UnboxedPlainObject::convertToNative(cx->asJSContext(), obj);
+    return true;
+}
+
+} // namespace js
+
 inline js::Shape*
 JSObject::maybeShape() const
 {
@@ -44,6 +57,8 @@ JSObject::maybeShape() const
 inline js::Shape*
 JSObject::ensureShape(js::ExclusiveContext* cx)
 {
+    if (!js::MaybeConvertUnboxedObjectToNative(cx, this))
+        return nullptr;
     js::Shape* shape = maybeShape();
     MOZ_ASSERT(shape);
     return shape;
diff --git a/js/src/moz.build b/js/src/moz.build
index 5287aef00a..c0cef9929b 100644
--- a/js/src/moz.build
+++ b/js/src/moz.build
@@ -355,6 +355,7 @@ UNIFIED_SOURCES += [
     'vm/UbiNode.cpp',
     'vm/UbiNodeCensus.cpp',
     'vm/UbiNodeShortestPaths.cpp',
+    'vm/UnboxedObject.cpp',
     'vm/Unicode.cpp',
     'vm/Value.cpp',
     'vm/WeakMapPtr.cpp',
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 4b0c858a40..e077bbbf88 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -7539,6 +7539,7 @@ SetWorkerContextOptions(JSContext* cx)
                              .setWasm(enableWasm)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
+                             .setUnboxedArrays(enableUnboxedArrays)
                              .setArrayProtoValues(enableArrayProtoValues);
     cx->setOffthreadIonCompilationEnabled(offthreadCompilation);
     cx->profilingScripts = enableCodeCoverage || enableDisassemblyDumps;
@@ -7708,6 +7709,7 @@ main(int argc, char** argv, char** envp)
         || !op.addBoolOption('\0', "no-asmjs", "Disable asm.js compilation")
         || !op.addBoolOption('\0', "no-wasm", "Disable WebAssembly compilation")
         || !op.addBoolOption('\0', "no-native-regexp", "Disable native regexp compilation")
+        || !op.addBoolOption('\0', "no-unboxed-objects", "Disable creating unboxed plain objects")
         || !op.addBoolOption('\0', "wasm-always-baseline", "Enable wasm baseline compiler when possible")
         || !op.addBoolOption('\0', "wasm-check-bce", "Always generate wasm bounds check, even redundant ones.")
         || !op.addBoolOption('\0', "no-array-proto-values", "Remove Array.prototype.values")
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 3cf9b57f60..b87d12924d 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -5016,6 +5016,11 @@ js::NewObjectOperationWithTemplate(JSContext* cx, HandleObject templateObject)
 
     NewObjectKind newKind = templateObject->group()->shouldPreTenure() ? TenuredObject : GenericObject;
 
+    if (templateObject->group()->maybeUnboxedLayout()) {
+        RootedObjectGroup group(cx, templateObject->group());
+        return UnboxedPlainObject::create(cx, group, newKind);
+    }
+
     JSObject* obj = CopyInitializerObject(cx, templateObject.as<PlainObject>(), newKind);
     if (!obj)
         return nullptr;
diff --git a/js/src/vm/NativeObject.cpp b/js/src/vm/NativeObject.cpp
index e7de36f13e..8b7543d128 100644
--- a/js/src/vm/NativeObject.cpp
+++ b/js/src/vm/NativeObject.cpp
@@ -388,6 +388,33 @@ NativeObject::setLastPropertyMakeNonNative(Shape* shape)
     shape_ = shape;
 }
 
+void
+NativeObject::setLastPropertyMakeNative(ExclusiveContext* cx, Shape* shape)
+{
+    MOZ_ASSERT(getClass()->isNative());
+    MOZ_ASSERT(shape->getObjectClass()->isNative());
+    MOZ_ASSERT(!shape->inDictionary());
+
+    // This method is used to convert unboxed objects into native objects. In
+    // this case, the shape_ field was previously used to store other data and
+    // this should be treated as an initialization.
+    shape_.init(shape);
+
+    slots_ = nullptr;
+    elements_ = emptyObjectElements;
+
+    size_t oldSpan = shape->numFixedSlots();
+    size_t newSpan = shape->slotSpan();
+
+    initializeSlotRange(0, oldSpan);
+
+    // A failure at this point will leave the object as a mutant, and we
+    // can't recover.
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (oldSpan != newSpan && !updateSlotsForSpan(cx, oldSpan, newSpan))
+        oomUnsafe.crash("NativeObject::setLastPropertyMakeNative");
+}
+
 bool
 NativeObject::setSlotSpan(ExclusiveContext* cx, uint32_t span)
 {
diff --git a/js/src/vm/NativeObject.h b/js/src/vm/NativeObject.h
index b17920ef9e..c774308aff 100644
--- a/js/src/vm/NativeObject.h
+++ b/js/src/vm/NativeObject.h
@@ -470,6 +470,11 @@ class NativeObject : public ShapedObject
     // that are (temporarily) inconsistent.
     void setLastPropertyMakeNonNative(Shape* shape);
 
+    // As for setLastProperty(), but changes the class associated with the
+    // object to a native one. The object's type has already been changed, and
+    // this brings the shape into sync with it.
+    void setLastPropertyMakeNative(ExclusiveContext* cx, Shape* shape);
+
     // Newly-created TypedArrays that map a SharedArrayBuffer are
     // marked as shared by giving them an ObjectElements that has the
     // ObjectElements::SHARED_MEMORY flag set.
diff --git a/js/src/vm/ObjectGroup-inl.h b/js/src/vm/ObjectGroup-inl.h
index d41343be64..9074f4d974 100644
--- a/js/src/vm/ObjectGroup-inl.h
+++ b/js/src/vm/ObjectGroup-inl.h
@@ -108,6 +108,20 @@ ObjectGroup::maybePreliminaryObjects()
     return maybePreliminaryObjectsDontCheckGeneration();
 }
 
+inline UnboxedLayout*
+ObjectGroup::maybeUnboxedLayout()
+{
+    maybeSweep(nullptr);
+    return maybeUnboxedLayoutDontCheckGeneration();
+}
+
+inline UnboxedLayout&
+ObjectGroup::unboxedLayout()
+{
+    maybeSweep(nullptr);
+    return unboxedLayoutDontCheckGeneration();
+}
+
 } // namespace js
 
 #endif /* vm_ObjectGroup_inl_h */
diff --git a/js/src/vm/ObjectGroup.cpp b/js/src/vm/ObjectGroup.cpp
index 91070b3f6b..95fcada946 100644
--- a/js/src/vm/ObjectGroup.cpp
+++ b/js/src/vm/ObjectGroup.cpp
@@ -18,10 +18,11 @@
 #include "vm/ArrayObject.h"
 #include "vm/Shape.h"
 #include "vm/TaggedProto.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsobjinlines.h"
 
-#include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 
@@ -55,6 +56,7 @@ ObjectGroup::finalize(FreeOp* fop)
     if (newScriptDontCheckGeneration())
         newScriptDontCheckGeneration()->clear();
     fop->delete_(newScriptDontCheckGeneration());
+    fop->delete_(maybeUnboxedLayoutDontCheckGeneration());
     if (maybePreliminaryObjectsDontCheckGeneration())
         maybePreliminaryObjectsDontCheckGeneration()->clear();
     fop->delete_(maybePreliminaryObjectsDontCheckGeneration());
@@ -81,6 +83,8 @@ ObjectGroup::sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const
     size_t n = 0;
     if (TypeNewScript* newScript = newScriptDontCheckGeneration())
         n += newScript->sizeOfIncludingThis(mallocSizeOf);
+    if (UnboxedLayout* layout = maybeUnboxedLayoutDontCheckGeneration())
+        n += layout->sizeOfIncludingThis(mallocSizeOf);
     return n;
 }
 
@@ -529,7 +533,8 @@ ObjectGroup::defaultNewGroup(ExclusiveContext* cx, const Class* clasp,
     if (p) {
         ObjectGroup* group = p->group;
         MOZ_ASSERT_IF(clasp, group->clasp() == clasp);
-        MOZ_ASSERT_IF(!clasp, group->clasp() == &PlainObject::class_);
+        MOZ_ASSERT_IF(!clasp, group->clasp() == &PlainObject::class_ ||
+                              group->clasp() == &UnboxedPlainObject::class_);
         MOZ_ASSERT(group->proto() == proto);
         return group;
     }
@@ -969,6 +974,46 @@ js::CombinePlainObjectPropertyTypes(ExclusiveContext* cx, JSObject* newObj,
                 }
             }
         }
+    } else if (newObj->is<UnboxedPlainObject>()) {
+        const UnboxedLayout& layout = newObj->as<UnboxedPlainObject>().layout();
+        const int32_t* traceList = layout.traceList();
+        if (!traceList)
+            return true;
+
+        uint8_t* newData = newObj->as<UnboxedPlainObject>().data();
+        uint8_t* oldData = oldObj->as<UnboxedPlainObject>().data();
+
+        for (; *traceList != -1; traceList++) {}
+        traceList++;
+        for (; *traceList != -1; traceList++) {
+            JSObject* newInnerObj = *reinterpret_cast<JSObject**>(newData + *traceList);
+            JSObject* oldInnerObj = *reinterpret_cast<JSObject**>(oldData + *traceList);
+
+            if (!newInnerObj || !oldInnerObj || SameGroup(oldInnerObj, newInnerObj))
+                continue;
+
+            if (!GiveObjectGroup(cx, newInnerObj, oldInnerObj))
+                return false;
+
+            if (SameGroup(oldInnerObj, newInnerObj))
+                continue;
+
+            if (!GiveObjectGroup(cx, oldInnerObj, newInnerObj))
+                return false;
+
+            if (SameGroup(oldInnerObj, newInnerObj)) {
+                for (size_t i = 1; i < ncompare; i++) {
+                    if (compare[i].isObject() && SameGroup(&compare[i].toObject(), newObj)) {
+                        uint8_t* otherData = compare[i].toObject().as<UnboxedPlainObject>().data();
+                        JSObject* otherInnerObj = *reinterpret_cast<JSObject**>(otherData + *traceList);
+                        if (otherInnerObj && !SameGroup(otherInnerObj, newInnerObj)) {
+                            if (!GiveObjectGroup(cx, otherInnerObj, newInnerObj))
+                                return false;
+                        }
+                    }
+                }
+            }
+        }
     }
 
     return true;
@@ -1192,6 +1237,12 @@ ObjectGroup::newPlainObject(ExclusiveContext* cx, IdValuePair* properties, size_
 
     RootedObjectGroup group(cx, p->value().group);
 
+    // Watch for existing groups which now use an unboxed layout.
+    if (group->maybeUnboxedLayout()) {
+        MOZ_ASSERT(group->unboxedLayout().properties().length() == nproperties);
+        return UnboxedPlainObject::createWithProperties(cx, group, newKind, properties);
+    }
+
     // Update property types according to the properties we are about to add.
     // Do this before we do anything which can GC, which might move or remove
     // this table entry.
diff --git a/js/src/vm/ObjectGroup.h b/js/src/vm/ObjectGroup.h
index 0b6eaee51b..553cb83667 100644
--- a/js/src/vm/ObjectGroup.h
+++ b/js/src/vm/ObjectGroup.h
@@ -20,6 +20,7 @@
 namespace js {
 
 class TypeDescr;
+class UnboxedLayout;
 
 class PreliminaryObjectArrayWithTemplate;
 class TypeNewScript;
@@ -153,6 +154,16 @@ class ObjectGroup : public gc::TenuredCell
         // For some plain objects, the addendum stores a PreliminaryObjectArrayWithTemplate.
         Addendum_PreliminaryObjects,
 
+        // When objects in this group have an unboxed representation, the
+        // addendum stores an UnboxedLayout (which might have a TypeNewScript
+        // as well, if the group is also constructed using 'new').
+        Addendum_UnboxedLayout,
+
+        // If this group is used by objects that have been converted from an
+        // unboxed representation and/or have the same allocation kind as such
+        // objects, the addendum points to that unboxed group.
+        Addendum_OriginalUnboxedGroup,
+
         // When used by typed objects, the addendum stores a TypeDescr.
         Addendum_TypeDescr
     };
@@ -174,6 +185,7 @@ class ObjectGroup : public gc::TenuredCell
         return nullptr;
     }
 
+    TypeNewScript* anyNewScript();
     void detachNewScript(bool writeBarrier, ObjectGroup* replacement);
 
     ObjectGroupFlags flagsDontCheckGeneration() const {
@@ -213,6 +225,34 @@ class ObjectGroup : public gc::TenuredCell
                maybePreliminaryObjectsDontCheckGeneration();
     }
 
+    inline UnboxedLayout* maybeUnboxedLayout();
+    inline UnboxedLayout& unboxedLayout();
+
+    UnboxedLayout* maybeUnboxedLayoutDontCheckGeneration() const {
+        if (addendumKind() == Addendum_UnboxedLayout)
+            return reinterpret_cast<UnboxedLayout*>(addendum_);
+        return nullptr;
+    }
+
+    UnboxedLayout& unboxedLayoutDontCheckGeneration() const {
+        MOZ_ASSERT(addendumKind() == Addendum_UnboxedLayout);
+        return *maybeUnboxedLayoutDontCheckGeneration();
+    }
+
+    void setUnboxedLayout(UnboxedLayout* layout) {
+        setAddendum(Addendum_UnboxedLayout, layout);
+    }
+
+    ObjectGroup* maybeOriginalUnboxedGroup() const {
+        if (addendumKind() == Addendum_OriginalUnboxedGroup)
+            return reinterpret_cast<ObjectGroup*>(addendum_);
+        return nullptr;
+    }
+
+    void setOriginalUnboxedGroup(ObjectGroup* group) {
+        setAddendum(Addendum_OriginalUnboxedGroup, group);
+    }
+
     TypeDescr* maybeTypeDescr() {
         // Note: there is no need to sweep when accessing the type descriptor
         // of an object, as it is strongly held and immutable.
@@ -273,8 +313,9 @@ class ObjectGroup : public gc::TenuredCell
      * that can be read out of that property in actual JS objects. In native
      * objects, property types account for plain data properties (those with a
      * slot and no getter or setter hook) and dense elements. In typed objects
-     * property types account for object and value properties and elements in
-     * the object.
+     * and unboxed objects, property types account for object and value
+     * properties and elements in the object, and expando properties in unboxed
+     * objects.
      *
      * For accesses on these properties, the correspondence is as follows:
      *
@@ -297,9 +338,10 @@ class ObjectGroup : public gc::TenuredCell
      * 2. Array lengths are special cased by the compiler and VM and are not
      *    reflected in property types.
      *
-     * 3. In typed objects, the initial values of properties (null pointers and
-     *    undefined values) are not reflected in the property types. These
-     *    values are always possible when reading the property.
+     * 3. In typed objects (but not unboxed objects), the initial values of
+     *    properties (null pointers and undefined values) are not reflected in
+     *    the property types. These values are always possible when reading the
+     *    property.
      *
      * We establish these by using write barriers on calls to setProperty and
      * defineProperty which are on native properties, and on any jitcode which
@@ -413,6 +455,12 @@ class ObjectGroup : public gc::TenuredCell
         return &flags_;
     }
 
+    // Get the bit pattern stored in an object's addendum when it has an
+    // original unboxed group.
+    static inline int32_t addendumOriginalUnboxedGroupValue() {
+        return Addendum_OriginalUnboxedGroup << OBJECT_FLAG_ADDENDUM_SHIFT;
+    }
+
     inline uint32_t basePropertyCount();
 
   private:
@@ -463,8 +511,8 @@ class ObjectGroup : public gc::TenuredCell
                                        NewObjectKind newKind,
                                        NewArrayKind arrayKind = NewArrayKind::Normal);
 
-    // Create a PlainObject with the specified properties and a group specialized
-    // for those properties.
+    // Create a PlainObject or UnboxedPlainObject with the specified properties
+    // and a group specialized for those properties.
     static JSObject* newPlainObject(ExclusiveContext* cx,
                                     IdValuePair* properties, size_t nproperties,
                                     NewObjectKind newKind);
diff --git a/js/src/vm/ReceiverGuard.cpp b/js/src/vm/ReceiverGuard.cpp
index e95e8a208a..e37bf8ee52 100644
--- a/js/src/vm/ReceiverGuard.cpp
+++ b/js/src/vm/ReceiverGuard.cpp
@@ -15,7 +15,11 @@ ReceiverGuard::ReceiverGuard(JSObject* obj)
   : group(nullptr), shape(nullptr)
 {
     if (obj) {
-        if (obj->is<TypedObject>()) {
+        if (obj->is<UnboxedPlainObject>()) {
+            group = obj->group();
+            if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+                shape = expando->lastProperty();
+        } else if (obj->is<TypedObject>()) {
             group = obj->group();
         } else {
             shape = obj->maybeShape();
@@ -28,7 +32,9 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
 {
     if (group) {
         const Class* clasp = group->clasp();
-        if (IsTypedObjectClass(clasp)) {
+        if (clasp == &UnboxedPlainObject::class_) {
+            // Keep both group and shape.
+        } else if (IsTypedObjectClass(clasp)) {
             this->shape = nullptr;
         } else {
             this->group = nullptr;
@@ -39,6 +45,10 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
 /* static */ int32_t
 HeapReceiverGuard::keyBits(JSObject* obj)
 {
+    if (obj->is<UnboxedPlainObject>()) {
+        // Both the group and shape need to be guarded for unboxed plain objects.
+        return obj->as<UnboxedPlainObject>().maybeExpando() ? 0 : 1;
+    }
     if (obj->is<TypedObject>()) {
         // Only the group needs to be guarded for typed objects.
         return 2;
diff --git a/js/src/vm/TypeInference-inl.h b/js/src/vm/TypeInference-inl.h
index 2af252cea1..da47fa898b 100644
--- a/js/src/vm/TypeInference-inl.h
+++ b/js/src/vm/TypeInference-inl.h
@@ -23,6 +23,7 @@
 #include "vm/SharedArrayObject.h"
 #include "vm/StringObject.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 #include "jscntxtinlines.h"
 
@@ -284,6 +285,10 @@ TypeIdString(jsid id)
  */
 struct AutoEnterAnalysis
 {
+    // For use when initializing an UnboxedLayout.  The UniquePtr's destructor
+    // must run when GC is not suppressed.
+    UniquePtr<UnboxedLayout> unboxedLayoutToCleanUp;
+
     // Prevent GC activity in the middle of analysis.
     gc::AutoSuppressGC suppressGC;
 
diff --git a/js/src/vm/TypeInference.cpp b/js/src/vm/TypeInference.cpp
index 2b1fa0e3bb..39206539be 100644
--- a/js/src/vm/TypeInference.cpp
+++ b/js/src/vm/TypeInference.cpp
@@ -35,6 +35,7 @@
 #include "vm/Opcodes.h"
 #include "vm/Shape.h"
 #include "vm/Time.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsatominlines.h"
 #include "jsscriptinlines.h"
@@ -296,6 +297,9 @@ js::ObjectGroupHasProperty(JSContext* cx, ObjectGroup* group, jsid id, const Val
                         return true;
                 }
             }
+            JSObject* obj = &value.toObject();
+            if (!obj->hasLazyGroup() && obj->group()->maybeOriginalUnboxedGroup())
+                return true;
         }
 
         if (!types->hasType(type)) {
@@ -1944,6 +1948,33 @@ class ConstraintDataFreezeObjectForTypedArrayData
     }
 };
 
+// Constraint which triggers recompilation if an unboxed object in some group
+// is converted to a native object.
+class ConstraintDataFreezeObjectForUnboxedConvertedToNative
+{
+  public:
+    ConstraintDataFreezeObjectForUnboxedConvertedToNative()
+    {}
+
+    const char* kind() { return "freezeObjectForUnboxedConvertedToNative"; }
+
+    bool invalidateOnNewType(TypeSet::Type type) { return false; }
+    bool invalidateOnNewPropertyState(TypeSet* property) { return false; }
+    bool invalidateOnNewObjectState(ObjectGroup* group) {
+        return group->unboxedLayout().nativeGroup() != nullptr;
+    }
+
+    bool constraintHolds(JSContext* cx,
+                         const HeapTypeSetKey& property, TemporaryTypeSet* expected)
+    {
+        return !invalidateOnNewObjectState(property.object()->maybeGroup());
+    }
+
+    bool shouldSweep() { return false; }
+
+    JSCompartment* maybeCompartment() { return nullptr; }
+};
+
 } /* anonymous namespace */
 
 void
@@ -2478,6 +2509,8 @@ TemporaryTypeSet::propertyNeedsBarrier(CompilerConstraintList* constraints, jsid
 bool
 js::ClassCanHaveExtraProperties(const Class* clasp)
 {
+    if (clasp == &UnboxedPlainObject::class_)
+        return false;
     return clasp->getResolve()
         || clasp->getOpsLookupProperty()
         || clasp->getOpsGetProperty()
@@ -2768,6 +2801,15 @@ js::AddTypePropertyId(ExclusiveContext* cx, ObjectGroup* group, JSObject* obj, j
     // from acquiring the fully initialized group.
     if (group->newScript() && group->newScript()->initializedGroup())
         AddTypePropertyId(cx, group->newScript()->initializedGroup(), nullptr, id, type);
+
+    // Maintain equivalent type information for unboxed object groups and their
+    // corresponding native group. Since type sets might contain the unboxed
+    // group but not the native group, this ensures optimizations based on the
+    // unboxed group are valid for the native group.
+    if (group->maybeUnboxedLayout() && group->maybeUnboxedLayout()->nativeGroup())
+        AddTypePropertyId(cx, group->maybeUnboxedLayout()->nativeGroup(), nullptr, id, type);
+    if (ObjectGroup* unboxedGroup = group->maybeOriginalUnboxedGroup())
+        AddTypePropertyId(cx, unboxedGroup, nullptr, id, type);
 }
 
 void
@@ -2839,6 +2881,12 @@ ObjectGroup::setFlags(ExclusiveContext* cx, ObjectGroupFlags flags)
     // acquired properties analysis.
     if (newScript() && newScript()->initializedGroup())
         newScript()->initializedGroup()->setFlags(cx, flags);
+
+    // Propagate flag changes between unboxed and corresponding native groups.
+    if (maybeUnboxedLayout() && maybeUnboxedLayout()->nativeGroup())
+        maybeUnboxedLayout()->nativeGroup()->setFlags(cx, flags);
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        unboxedGroup->setFlags(cx, flags);
 }
 
 void
@@ -2871,6 +2919,23 @@ ObjectGroup::markUnknown(ExclusiveContext* cx)
             prop->types.setNonDataProperty(cx);
         }
     }
+
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        MarkObjectGroupUnknownProperties(cx, unboxedGroup);
+    if (maybeUnboxedLayout() && maybeUnboxedLayout()->nativeGroup())
+        MarkObjectGroupUnknownProperties(cx, maybeUnboxedLayout()->nativeGroup());
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        MarkObjectGroupUnknownProperties(cx, unboxedGroup);
+}
+
+TypeNewScript*
+ObjectGroup::anyNewScript()
+{
+    if (newScript())
+        return newScript();
+    if (maybeUnboxedLayout())
+        return unboxedLayout().newScript();
+    return nullptr;
 }
 
 void
@@ -2880,7 +2945,7 @@ ObjectGroup::detachNewScript(bool writeBarrier, ObjectGroup* replacement)
     // analyzed, remove it from the newObjectGroups table so that it will not be
     // produced by calling 'new' on the associated function anymore.
     // The TypeNewScript is not actually destroyed.
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     MOZ_ASSERT(newScript);
 
     if (newScript->analyzed()) {
@@ -2899,7 +2964,10 @@ ObjectGroup::detachNewScript(bool writeBarrier, ObjectGroup* replacement)
         MOZ_ASSERT(!replacement);
     }
 
-    setAddendum(Addendum_None, nullptr, writeBarrier);
+    if (this->newScript())
+        setAddendum(Addendum_None, nullptr, writeBarrier);
+    else
+        unboxedLayout().setNewScript(nullptr, writeBarrier);
 }
 
 void
@@ -2910,7 +2978,7 @@ ObjectGroup::maybeClearNewScriptOnOOM()
     if (!isMarked())
         return;
 
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     if (!newScript)
         return;
 
@@ -2925,7 +2993,7 @@ ObjectGroup::maybeClearNewScriptOnOOM()
 void
 ObjectGroup::clearNewScript(ExclusiveContext* cx, ObjectGroup* replacement /* = nullptr*/)
 {
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     if (!newScript)
         return;
 
@@ -3390,6 +3458,22 @@ PreliminaryObjectArray::sweep()
     for (size_t i = 0; i < COUNT; i++) {
         JSObject** ptr = &objects[i];
         if (*ptr && IsAboutToBeFinalizedUnbarriered(ptr)) {
+            // Before we clear this reference, change the object's group to the
+            // Object.prototype group. This is done to ensure JSObject::finalize
+            // sees a NativeObject Class even if we change the current group's
+            // Class to one of the unboxed object classes in the meantime. If
+            // the compartment's global is dead, we don't do anything as the
+            // group's Class is not going to change in that case.
+            JSObject* obj = *ptr;
+            GlobalObject* global = obj->compartment()->unsafeUnbarrieredMaybeGlobal();
+            if (global && !obj->isSingleton()) {
+                JSObject* objectProto = GetBuiltinPrototypePure(global, JSProto_Object);
+                obj->setGroup(objectProto->groupRaw());
+                MOZ_ASSERT(obj->is<NativeObject>());
+                MOZ_ASSERT(obj->getClass() == objectProto->getClass());
+                MOZ_ASSERT(!obj->getClass()->hasFinalize());
+            }
+
             *ptr = nullptr;
         }
     }
@@ -3489,11 +3573,16 @@ PreliminaryObjectArrayWithTemplate::maybeAnalyze(ExclusiveContext* cx, ObjectGro
         }
     }
 
-    // Since the preliminary objects still reflect the template object's
-    // properties, and all objects in the future will be created with those
-    // properties, the properties can be marked as definitive for objects in
-    // the group.
-    group->addDefiniteProperties(cx, shape());
+    if (group->maybeUnboxedLayout())
+        return;
+
+    if (shape()) {
+        // We weren't able to use an unboxed layout, but since the preliminary
+        // objects still reflect the template object's properties, and all
+        // objects in the future will be created with those properties, the
+        // properties can be marked as definite for objects in the group.
+        group->addDefiniteProperties(cx, shape());
+    }
 }
 
 /////////////////////////////////////////////////////////////////////
@@ -3507,6 +3596,7 @@ TypeNewScript::make(JSContext* cx, ObjectGroup* group, JSFunction* fun)
 {
     MOZ_ASSERT(cx->zone()->types.activeAnalysis);
     MOZ_ASSERT(!group->newScript());
+    MOZ_ASSERT(!group->maybeUnboxedLayout());
 
     // rollbackPartiallyInitializedObjects expects function_ to be
     // canonicalized.
@@ -3814,6 +3904,27 @@ TypeNewScript::maybeAnalyze(JSContext* cx, ObjectGroup* group, bool* regenerate,
     js_delete(preliminaryObjects);
     preliminaryObjects = nullptr;
 
+    if (group->maybeUnboxedLayout()) {
+        // An unboxed layout was constructed for the group, and this has already
+        // been hooked into it.
+        MOZ_ASSERT(group->unboxedLayout().newScript() == this);
+        destroyNewScript.group = nullptr;
+
+        // Clear out the template object, which is not used for TypeNewScripts
+        // with an unboxed layout. Currently it is a mutant object with a
+        // non-native group and native shape, so make it safe for GC by changing
+        // its group to the default for its prototype.
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        ObjectGroup* plainGroup = ObjectGroup::defaultNewGroup(cx, &PlainObject::class_,
+                                                               group->proto());
+        if (!plainGroup)
+            oomUnsafe.crash("TypeNewScript::maybeAnalyze");
+        templateObject_->setGroup(plainGroup);
+        templateObject_ = nullptr;
+
+        return true;
+    }
+
     if (prefixShape->slotSpan() == templateObject()->slotSpan()) {
         // The definite properties analysis found exactly the properties that
         // are held in common by the preliminary objects. No further analysis
@@ -3927,6 +4038,12 @@ TypeNewScript::rollbackPartiallyInitializedObjects(JSContext* cx, ObjectGroup* g
             continue;
         }
 
+        if (thisv.toObject().is<UnboxedPlainObject>()) {
+            AutoEnterOOMUnsafeRegion oomUnsafe;
+            if (!UnboxedPlainObject::convertToNative(cx, &thisv.toObject()))
+                oomUnsafe.crash("rollbackPartiallyInitializedObjects");
+        }
+
         // Found a matching frame.
         RootedPlainObject obj(cx, &thisv.toObject().as<PlainObject>());
 
@@ -4120,6 +4237,12 @@ ConstraintTypeSet::sweep(Zone* zone, AutoClearTypeInferenceStateOnOOM& oom)
                 // Object sets containing objects with unknown properties might
                 // not be complete. Mark the type set as unknown, which it will
                 // be treated as during Ion compilation.
+                //
+                // Note that we don't have to do this when the type set might
+                // be missing the native group corresponding to an unboxed
+                // object group. In this case, the native group points to the
+                // unboxed object group via its addendum, so as long as objects
+                // with either group exist, neither group will be finalized.
                 flags |= TYPE_FLAG_ANYOBJECT;
                 clearObjects();
                 objectCount = 0;
@@ -4203,6 +4326,21 @@ ObjectGroup::sweep(AutoClearTypeInferenceStateOnOOM* oom)
     Maybe<AutoClearTypeInferenceStateOnOOM> fallbackOOM;
     EnsureHasAutoClearTypeInferenceStateOnOOM(oom, zone(), fallbackOOM);
 
+    if (maybeUnboxedLayout()) {
+        // Remove unboxed layouts that are about to be finalized from the
+        // compartment wide list while we are still on the main thread.
+        ObjectGroup* group = this;
+        if (IsAboutToBeFinalizedUnbarriered(&group))
+            unboxedLayout().detachFromCompartment();
+
+        if (unboxedLayout().newScript())
+            unboxedLayout().newScript()->sweep();
+
+        // Discard constructor code to avoid holding onto ExecutablePools.
+        if (zone()->isGCCompacting())
+            unboxedLayout().setConstructorCode(nullptr);
+    }
+
     if (maybePreliminaryObjects())
         maybePreliminaryObjects()->sweep();
 
diff --git a/js/src/vm/UnboxedObject-inl.h b/js/src/vm/UnboxedObject-inl.h
new file mode 100644
index 0000000000..c1468a5b18
--- /dev/null
+++ b/js/src/vm/UnboxedObject-inl.h
@@ -0,0 +1,177 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef vm_UnboxedObject_inl_h
+#define vm_UnboxedObject_inl_h
+
+#include "vm/UnboxedObject.h"
+
+#include "gc/StoreBuffer-inl.h"
+#include "vm/ArrayObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+namespace js {
+
+static inline Value
+GetUnboxedValue(uint8_t* p, JSValueType type, bool maybeUninitialized)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        return BooleanValue(*p != 0);
+
+      case JSVAL_TYPE_INT32:
+        return Int32Value(*reinterpret_cast<int32_t*>(p));
+
+      case JSVAL_TYPE_DOUBLE: {
+        // During unboxed plain object creation, non-GC thing properties are
+        // left uninitialized. This is normally fine, since the properties will
+        // be filled in shortly, but if they are read before that happens we
+        // need to make sure that doubles are canonical.
+        double d = *reinterpret_cast<double*>(p);
+        if (maybeUninitialized)
+            return DoubleValue(JS::CanonicalizeNaN(d));
+        return DoubleValue(d);
+      }
+
+      case JSVAL_TYPE_STRING:
+        return StringValue(*reinterpret_cast<JSString**>(p));
+
+      case JSVAL_TYPE_OBJECT:
+        return ObjectOrNullValue(*reinterpret_cast<JSObject**>(p));
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+static inline void
+SetUnboxedValueNoTypeChange(JSObject* unboxedObject,
+                            uint8_t* p, JSValueType type, const Value& v,
+                            bool preBarrier)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        *p = v.toBoolean();
+        return;
+
+      case JSVAL_TYPE_INT32:
+        *reinterpret_cast<int32_t*>(p) = v.toInt32();
+        return;
+
+      case JSVAL_TYPE_DOUBLE:
+        *reinterpret_cast<double*>(p) = v.toNumber();
+        return;
+
+      case JSVAL_TYPE_STRING: {
+        MOZ_ASSERT(!IsInsideNursery(v.toString()));
+        JSString** np = reinterpret_cast<JSString**>(p);
+        if (preBarrier)
+            JSString::writeBarrierPre(*np);
+        *np = v.toString();
+        return;
+      }
+
+      case JSVAL_TYPE_OBJECT: {
+        JSObject** np = reinterpret_cast<JSObject**>(p);
+
+        // Manually trigger post barriers on the whole object. If we treat
+        // the pointer as a HeapPtrObject we will get confused later if the
+        // object is converted to its native representation.
+        JSObject* obj = v.toObjectOrNull();
+        if (IsInsideNursery(obj) && !IsInsideNursery(unboxedObject)) {
+            JSRuntime* rt = unboxedObject->runtimeFromMainThread();
+            rt->gc.storeBuffer.putWholeCell(unboxedObject);
+        }
+
+        if (preBarrier)
+            JSObject::writeBarrierPre(*np);
+        *np = obj;
+        return;
+      }
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+static inline bool
+SetUnboxedValue(ExclusiveContext* cx, JSObject* unboxedObject, jsid id,
+                uint8_t* p, JSValueType type, const Value& v, bool preBarrier)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        if (v.isBoolean()) {
+            *p = v.toBoolean();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_INT32:
+        if (v.isInt32()) {
+            *reinterpret_cast<int32_t*>(p) = v.toInt32();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_DOUBLE:
+        if (v.isNumber()) {
+            *reinterpret_cast<double*>(p) = v.toNumber();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_STRING:
+        if (v.isString()) {
+            MOZ_ASSERT(!IsInsideNursery(v.toString()));
+            JSString** np = reinterpret_cast<JSString**>(p);
+            if (preBarrier)
+                JSString::writeBarrierPre(*np);
+            *np = v.toString();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_OBJECT:
+        if (v.isObjectOrNull()) {
+            JSObject** np = reinterpret_cast<JSObject**>(p);
+
+            // Update property types when writing object properties. Types for
+            // other properties were captured when the unboxed layout was
+            // created.
+            AddTypePropertyId(cx, unboxedObject, id, v);
+
+            // As above, trigger post barriers on the whole object.
+            JSObject* obj = v.toObjectOrNull();
+            if (IsInsideNursery(v.toObjectOrNull()) && !IsInsideNursery(unboxedObject)) {
+                JSRuntime* rt = unboxedObject->runtimeFromMainThread();
+                rt->gc.storeBuffer.putWholeCell(unboxedObject);
+            }
+
+            if (preBarrier)
+                JSObject::writeBarrierPre(*np);
+            *np = obj;
+            return true;
+        }
+        return false;
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedPlainObject
+/////////////////////////////////////////////////////////////////////
+
+inline const UnboxedLayout&
+UnboxedPlainObject::layout() const
+{
+    return group()->unboxedLayout();
+}
+
+} // namespace js
+
+#endif // vm_UnboxedObject_inl_h
diff --git a/js/src/vm/UnboxedObject.cpp b/js/src/vm/UnboxedObject.cpp
new file mode 100644
index 0000000000..2e017ca3bd
--- /dev/null
+++ b/js/src/vm/UnboxedObject.cpp
@@ -0,0 +1,946 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "vm/UnboxedObject-inl.h"
+
+#include "jit/BaselineIC.h"
+#include "jit/ExecutableAllocator.h"
+#include "jit/JitCommon.h"
+#include "jit/Linker.h"
+
+#include "jsobjinlines.h"
+
+#include "gc/Nursery-inl.h"
+#include "jit/MacroAssembler-inl.h"
+#include "vm/Shape-inl.h"
+
+using mozilla::ArrayLength;
+using mozilla::DebugOnly;
+using mozilla::PodCopy;
+
+using namespace js;
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedLayout
+/////////////////////////////////////////////////////////////////////
+
+void
+UnboxedLayout::trace(JSTracer* trc)
+{
+    for (size_t i = 0; i < properties_.length(); i++)
+        TraceManuallyBarrieredEdge(trc, &properties_[i].name, "unboxed_layout_name");
+
+    if (newScript())
+        newScript()->trace(trc);
+
+    TraceNullableEdge(trc, &nativeGroup_, "unboxed_layout_nativeGroup");
+    TraceNullableEdge(trc, &nativeShape_, "unboxed_layout_nativeShape");
+    TraceNullableEdge(trc, &allocationScript_, "unboxed_layout_allocationScript");
+    TraceNullableEdge(trc, &replacementGroup_, "unboxed_layout_replacementGroup");
+    TraceNullableEdge(trc, &constructorCode_, "unboxed_layout_constructorCode");
+}
+
+size_t
+UnboxedLayout::sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf)
+{
+    return mallocSizeOf(this)
+         + properties_.sizeOfExcludingThis(mallocSizeOf)
+         + (newScript() ? newScript()->sizeOfIncludingThis(mallocSizeOf) : 0)
+         + mallocSizeOf(traceList());
+}
+
+void
+UnboxedLayout::setNewScript(TypeNewScript* newScript, bool writeBarrier /* = true */)
+{
+    if (newScript_ && writeBarrier)
+        TypeNewScript::writeBarrierPre(newScript_);
+    newScript_ = newScript;
+}
+
+// Constructor code returns a 0x1 value to indicate the constructor code should
+// be cleared.
+static const uintptr_t CLEAR_CONSTRUCTOR_CODE_TOKEN = 0x1;
+
+/* static */ bool
+UnboxedLayout::makeConstructorCode(JSContext* cx, HandleObjectGroup group)
+{
+    gc::AutoSuppressGC suppress(cx);
+
+    using namespace jit;
+
+    if (!cx->compartment()->ensureJitCompartmentExists(cx))
+        return false;
+
+    UnboxedLayout& layout = group->unboxedLayout();
+    MOZ_ASSERT(!layout.constructorCode());
+
+    UnboxedPlainObject* templateObject = UnboxedPlainObject::create(cx, group, TenuredObject);
+    if (!templateObject)
+        return false;
+
+    JitContext jitContext(cx, nullptr);
+
+    MacroAssembler masm;
+
+    Register propertiesReg, newKindReg;
+#ifdef JS_CODEGEN_X86
+    propertiesReg = eax;
+    newKindReg = ecx;
+    masm.loadPtr(Address(masm.getStackPointer(), sizeof(void*)), propertiesReg);
+    masm.loadPtr(Address(masm.getStackPointer(), 2 * sizeof(void*)), newKindReg);
+#else
+    propertiesReg = IntArgReg0;
+    newKindReg = IntArgReg1;
+#endif
+
+#ifdef JS_CODEGEN_ARM64
+    // ARM64 communicates stack address via sp, but uses a pseudo-sp for addressing.
+    masm.initStackPtr();
+#endif
+
+    MOZ_ASSERT(propertiesReg.volatile_());
+    MOZ_ASSERT(newKindReg.volatile_());
+
+    AllocatableGeneralRegisterSet regs(GeneralRegisterSet::All());
+    regs.take(propertiesReg);
+    regs.take(newKindReg);
+    Register object = regs.takeAny(), scratch1 = regs.takeAny(), scratch2 = regs.takeAny();
+
+    LiveGeneralRegisterSet savedNonVolatileRegisters = SavedNonVolatileRegisters(regs);
+    masm.PushRegsInMask(savedNonVolatileRegisters);
+
+    // The scratch double register might be used by MacroAssembler methods.
+    if (ScratchDoubleReg.volatile_())
+        masm.push(ScratchDoubleReg);
+
+    Label failure, tenuredObject, allocated;
+    masm.branch32(Assembler::NotEqual, newKindReg, Imm32(GenericObject), &tenuredObject);
+    masm.branchTest32(Assembler::NonZero, AbsoluteAddress(group->addressOfFlags()),
+                      Imm32(OBJECT_FLAG_PRE_TENURE), &tenuredObject);
+
+    // Allocate an object in the nursery
+    masm.createGCObject(object, scratch1, templateObject, gc::DefaultHeap, &failure,
+                        /* initFixedSlots = */ false);
+
+    masm.jump(&allocated);
+    masm.bind(&tenuredObject);
+
+    // Allocate an object in the tenured heap.
+    masm.createGCObject(object, scratch1, templateObject, gc::TenuredHeap, &failure,
+                        /* initFixedSlots = */ false);
+
+    // If any of the properties being stored are in the nursery, add a store
+    // buffer entry for the new object.
+    Label postBarrier;
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            Address valueAddress(propertiesReg, i * sizeof(IdValuePair) + offsetof(IdValuePair, value));
+            Label notObject;
+            masm.branchTestObject(Assembler::NotEqual, valueAddress, &notObject);
+            Register valueObject = masm.extractObject(valueAddress, scratch1);
+            masm.branchPtrInNurseryChunk(Assembler::Equal, valueObject, scratch2, &postBarrier);
+            masm.bind(&notObject);
+        }
+    }
+
+    masm.jump(&allocated);
+    masm.bind(&postBarrier);
+
+    LiveGeneralRegisterSet liveVolatileRegisters;
+    liveVolatileRegisters.add(propertiesReg);
+    if (object.volatile_())
+        liveVolatileRegisters.add(object);
+    masm.PushRegsInMask(liveVolatileRegisters);
+
+    masm.mov(ImmPtr(cx->runtime()), scratch1);
+    masm.setupUnalignedABICall(scratch2);
+    masm.passABIArg(scratch1);
+    masm.passABIArg(object);
+    masm.callWithABI(JS_FUNC_TO_DATA_PTR(void*, PostWriteBarrier));
+
+    masm.PopRegsInMask(liveVolatileRegisters);
+
+    masm.bind(&allocated);
+
+    ValueOperand valueOperand;
+#ifdef JS_NUNBOX32
+    valueOperand = ValueOperand(scratch1, scratch2);
+#else
+    valueOperand = ValueOperand(scratch1);
+#endif
+
+    Label failureStoreOther, failureStoreObject;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        Address valueAddress(propertiesReg, i * sizeof(IdValuePair) + offsetof(IdValuePair, value));
+        Address targetAddress(object, UnboxedPlainObject::offsetOfData() + property.offset);
+
+        masm.loadValue(valueAddress, valueOperand);
+
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            HeapTypeSet* types = group->maybeGetProperty(IdToTypeId(NameToId(property.name)));
+
+            Label notObject;
+            masm.branchTestObject(Assembler::NotEqual, valueOperand,
+                                  types->mightBeMIRType(MIRType::Null) ? &notObject : &failureStoreObject);
+
+            Register payloadReg = masm.extractObject(valueOperand, scratch1);
+
+            if (!types->hasType(TypeSet::AnyObjectType())) {
+                Register scratch = (payloadReg == scratch1) ? scratch2 : scratch1;
+                masm.guardObjectType(payloadReg, types, scratch, &failureStoreObject);
+            }
+
+            masm.storeUnboxedProperty(targetAddress, JSVAL_TYPE_OBJECT,
+                                      TypedOrValueRegister(MIRType::Object,
+                                                           AnyRegister(payloadReg)), nullptr);
+
+            if (notObject.used()) {
+                Label done;
+                masm.jump(&done);
+                masm.bind(&notObject);
+                masm.branchTestNull(Assembler::NotEqual, valueOperand, &failureStoreOther);
+                masm.storeUnboxedProperty(targetAddress, JSVAL_TYPE_OBJECT, NullValue(), nullptr);
+                masm.bind(&done);
+            }
+        } else {
+            masm.storeUnboxedProperty(targetAddress, property.type,
+                                      ConstantOrRegister(valueOperand), &failureStoreOther);
+        }
+    }
+
+    Label done;
+    masm.bind(&done);
+
+    if (object != ReturnReg)
+        masm.movePtr(object, ReturnReg);
+
+    // Restore non-volatile registers which were saved on entry.
+    if (ScratchDoubleReg.volatile_())
+        masm.pop(ScratchDoubleReg);
+    masm.PopRegsInMask(savedNonVolatileRegisters);
+
+    masm.abiret();
+
+    masm.bind(&failureStoreOther);
+
+    // There was a failure while storing a value which cannot be stored at all
+    // in the unboxed object. Initialize the object so it is safe for GC and
+    // return null.
+    masm.initUnboxedObjectContents(object, templateObject);
+
+    masm.bind(&failure);
+
+    masm.movePtr(ImmWord(0), object);
+    masm.jump(&done);
+
+    masm.bind(&failureStoreObject);
+
+    // There was a failure while storing a value to an object slot of the
+    // unboxed object. If the value is storable, the failure occurred due to
+    // incomplete type information in the object, so return a token to trigger
+    // regeneration of the jitcode after a new object is created in the VM.
+    {
+        Label isObject;
+        masm.branchTestObject(Assembler::Equal, valueOperand, &isObject);
+        masm.branchTestNull(Assembler::NotEqual, valueOperand, &failureStoreOther);
+        masm.bind(&isObject);
+    }
+
+    // Initialize the object so it is safe for GC.
+    masm.initUnboxedObjectContents(object, templateObject);
+
+    masm.movePtr(ImmWord(CLEAR_CONSTRUCTOR_CODE_TOKEN), object);
+    masm.jump(&done);
+
+    Linker linker(masm);
+    AutoFlushICache afc("UnboxedObject");
+    JitCode* code = linker.newCode<NoGC>(cx, OTHER_CODE);
+    if (!code)
+        return false;
+
+    layout.setConstructorCode(code);
+    return true;
+}
+
+void
+UnboxedLayout::detachFromCompartment()
+{
+    if (isInList())
+        remove();
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedPlainObject
+/////////////////////////////////////////////////////////////////////
+
+bool
+UnboxedPlainObject::setValue(ExclusiveContext* cx, const UnboxedLayout::Property& property,
+                             const Value& v)
+{
+    uint8_t* p = &data_[property.offset];
+    return SetUnboxedValue(cx, this, NameToId(property.name), p, property.type, v,
+                           /* preBarrier = */ true);
+}
+
+Value
+UnboxedPlainObject::getValue(const UnboxedLayout::Property& property,
+                             bool maybeUninitialized /* = false */)
+{
+    uint8_t* p = &data_[property.offset];
+    return GetUnboxedValue(p, property.type, maybeUninitialized);
+}
+
+void
+UnboxedPlainObject::trace(JSTracer* trc, JSObject* obj)
+{
+    if (obj->as<UnboxedPlainObject>().expando_) {
+        TraceManuallyBarrieredEdge(trc,
+            reinterpret_cast<NativeObject**>(&obj->as<UnboxedPlainObject>().expando_),
+            "unboxed_expando");
+    }
+
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+    const int32_t* list = layout.traceList();
+    if (!list)
+        return;
+
+    uint8_t* data = obj->as<UnboxedPlainObject>().data();
+    while (*list != -1) {
+        GCPtrString* heap = reinterpret_cast<GCPtrString*>(data + *list);
+        TraceEdge(trc, heap, "unboxed_string");
+        list++;
+    }
+    list++;
+    while (*list != -1) {
+        GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(data + *list);
+        TraceNullableEdge(trc, heap, "unboxed_object");
+        list++;
+    }
+
+    // Unboxed objects don't have Values to trace.
+    MOZ_ASSERT(*(list + 1) == -1);
+}
+
+/* static */ UnboxedExpandoObject*
+UnboxedPlainObject::ensureExpando(JSContext* cx, Handle<UnboxedPlainObject*> obj)
+{
+    if (obj->expando_)
+        return obj->expando_;
+
+    UnboxedExpandoObject* expando =
+        NewObjectWithGivenProto<UnboxedExpandoObject>(cx, nullptr, gc::AllocKind::OBJECT4);
+    if (!expando)
+        return nullptr;
+
+    // Don't track property types for expando objects. This allows Baseline
+    // and Ion AddSlot ICs to guard on the unboxed group without guarding on
+    // the expando group.
+    MarkObjectGroupUnknownProperties(cx, expando->group());
+
+    // If the expando is tenured then the original object must also be tenured.
+    // Otherwise barriers triggered on the original object for writes to the
+    // expando (as can happen in the JIT) won't see the tenured->nursery edge.
+    // See WholeCellEdges::mark.
+    MOZ_ASSERT_IF(!IsInsideNursery(expando), !IsInsideNursery(obj));
+
+    // As with setValue(), we need to manually trigger post barriers on the
+    // whole object. If we treat the field as a GCPtrObject and later
+    // convert the object to its native representation, we will end up with a
+    // corrupted store buffer entry.
+    if (IsInsideNursery(expando) && !IsInsideNursery(obj))
+        cx->runtime()->gc.storeBuffer.putWholeCell(obj);
+
+    obj->expando_ = expando;
+    return expando;
+}
+
+bool
+UnboxedPlainObject::containsUnboxedOrExpandoProperty(ExclusiveContext* cx, jsid id) const
+{
+    if (layout().lookup(id))
+        return true;
+
+    if (maybeExpando() && maybeExpando()->containsShapeOrElement(cx, id))
+        return true;
+
+    return false;
+}
+
+static bool
+PropagatePropertyTypes(JSContext* cx, jsid id, ObjectGroup* oldGroup, ObjectGroup* newGroup)
+{
+    HeapTypeSet* typeProperty = oldGroup->maybeGetProperty(id);
+    TypeSet::TypeList types;
+    if (!typeProperty->enumerateTypes(&types)) {
+        ReportOutOfMemory(cx);
+        return false;
+    }
+    for (size_t j = 0; j < types.length(); j++)
+        AddTypePropertyId(cx, newGroup, nullptr, id, types[j]);
+    return true;
+}
+
+static PlainObject*
+MakeReplacementTemplateObject(JSContext* cx, HandleObjectGroup group, const UnboxedLayout &layout)
+{
+    PlainObject* obj = NewObjectWithGroup<PlainObject>(cx, group, layout.getAllocKind(),
+                                                       TenuredObject);
+    if (!obj)
+        return nullptr;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        if (!obj->addDataProperty(cx, NameToId(property.name), i, JSPROP_ENUMERATE))
+            return nullptr;
+        MOZ_ASSERT(obj->slotSpan() == i + 1);
+        MOZ_ASSERT(!obj->inDictionaryMode());
+    }
+
+    return obj;
+}
+
+/* static */ bool
+UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
+{
+    AutoEnterAnalysis enter(cx);
+
+    UnboxedLayout& layout = group->unboxedLayout();
+    Rooted<TaggedProto> proto(cx, group->proto());
+
+    MOZ_ASSERT(!layout.nativeGroup());
+
+    RootedObjectGroup replacementGroup(cx);
+
+    // Immediately clear any new script on the group. This is done by replacing
+    // the existing new script with one for a replacement default new group.
+    // This is done so that the size of the replacment group's objects is the
+    // same as that for the unboxed group, so that we do not see polymorphic
+    // slot accesses later on for sites that see converted objects from this
+    // group and objects that were allocated using the replacement new group.
+    if (layout.newScript()) {
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto);
+        if (!replacementGroup)
+            return false;
+
+        PlainObject* templateObject = MakeReplacementTemplateObject(cx, replacementGroup, layout);
+        if (!templateObject)
+            return false;
+
+        TypeNewScript* replacementNewScript =
+            TypeNewScript::makeNativeVersion(cx, layout.newScript(), templateObject);
+        if (!replacementNewScript)
+            return false;
+
+        replacementGroup->setNewScript(replacementNewScript);
+        gc::TraceTypeNewScript(replacementGroup);
+
+        group->clearNewScript(cx, replacementGroup);
+    }
+
+    // Similarly, if this group is keyed to an allocation site, replace its
+    // entry with a new group that has no unboxed layout.
+    if (layout.allocationScript()) {
+        RootedScript script(cx, layout.allocationScript());
+        jsbytecode* pc = layout.allocationPc();
+
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto);
+        if (!replacementGroup)
+            return false;
+
+        PlainObject* templateObject = &script->getObject(pc)->as<PlainObject>();
+        replacementGroup->addDefiniteProperties(cx, templateObject->lastProperty());
+
+        cx->compartment()->objectGroups.replaceAllocationSiteGroup(script, pc, JSProto_Object,
+                                                                   replacementGroup);
+
+        // Clear any baseline information at this opcode which might use the old group.
+        if (script->hasBaselineScript()) {
+            jit::ICEntry& entry = script->baselineScript()->icEntryFromPCOffset(script->pcToOffset(pc));
+            jit::ICFallbackStub* fallback = entry.fallbackStub();
+            for (jit::ICStubIterator iter = fallback->beginChain(); !iter.atEnd(); iter++)
+                iter.unlink(cx);
+            if (fallback->isNewObject_Fallback())
+                fallback->toNewObject_Fallback()->setTemplateObject(nullptr);
+            else if (fallback->isNewArray_Fallback())
+                fallback->toNewArray_Fallback()->setTemplateGroup(replacementGroup);
+        }
+    }
+
+    size_t nfixed = gc::GetGCKindSlots(layout.getAllocKind());
+
+    RootedShape shape(cx, EmptyShape::getInitialShape(cx, &PlainObject::class_, proto, nfixed, 0));
+    if (!shape)
+        return false;
+
+    // Add shapes for each property, if this is for a plain object.
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+
+        Rooted<StackShape> child(cx, StackShape(shape->base()->unowned(), NameToId(property.name),
+                                                i, JSPROP_ENUMERATE, 0));
+        shape = cx->zone()->propertyTree.getChild(cx, shape, child);
+        if (!shape)
+            return false;
+    }
+
+    ObjectGroup* nativeGroup =
+        ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto,
+                                          group->flags() & OBJECT_FLAG_DYNAMIC_MASK);
+    if (!nativeGroup)
+        return false;
+
+    // No sense propagating if we don't know what we started with.
+    if (!group->unknownProperties()) {
+        // Propagate all property types from the old group to the new group.
+        for (size_t i = 0; i < layout.properties().length(); i++) {
+            const UnboxedLayout::Property& property = layout.properties()[i];
+            jsid id = NameToId(property.name);
+            if (!PropagatePropertyTypes(cx, id, group, nativeGroup))
+                return false;
+
+            // If we are OOM we may not be able to propagate properties.
+            if (nativeGroup->unknownProperties())
+                break;
+
+            HeapTypeSet* nativeProperty = nativeGroup->maybeGetProperty(id);
+            if (nativeProperty && nativeProperty->canSetDefinite(i))
+                nativeProperty->setDefinite(i);
+        }
+    } else {
+        // If we skip, though, the new group had better agree.
+        MOZ_ASSERT(nativeGroup->unknownProperties());
+    }
+
+    layout.nativeGroup_ = nativeGroup;
+    layout.nativeShape_ = shape;
+    layout.replacementGroup_ = replacementGroup;
+
+    nativeGroup->setOriginalUnboxedGroup(group);
+
+    group->markStateChange(cx);
+
+    return true;
+}
+
+/* static */ bool
+UnboxedPlainObject::convertToNative(JSContext* cx, JSObject* obj)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+    UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+
+    if (!layout.nativeGroup()) {
+        if (!UnboxedLayout::makeNativeGroup(cx, obj->group()))
+            return false;
+
+        // makeNativeGroup can reentrantly invoke this method.
+        if (obj->is<PlainObject>())
+            return true;
+    }
+
+    AutoValueVector values(cx);
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        // We might be reading properties off the object which have not been
+        // initialized yet. Make sure any double values we read here are
+        // canonicalized.
+        if (!values.append(obj->as<UnboxedPlainObject>().getValue(layout.properties()[i], true)))
+            return false;
+    }
+
+    // We are eliminating the expando edge with the conversion, so trigger a
+    // pre barrier.
+    JSObject::writeBarrierPre(expando);
+
+    // Additionally trigger a post barrier on the expando itself. Whole cell
+    // store buffer entries can be added on the original unboxed object for
+    // writes to the expando (see WholeCellEdges::trace), so after conversion
+    // we need to make sure the expando itself will still be traced.
+    if (expando && !IsInsideNursery(expando))
+        cx->runtime()->gc.storeBuffer.putWholeCell(expando);
+
+    obj->setGroup(layout.nativeGroup());
+    obj->as<PlainObject>().setLastPropertyMakeNative(cx, layout.nativeShape());
+
+    for (size_t i = 0; i < values.length(); i++)
+        obj->as<PlainObject>().initSlotUnchecked(i, values[i]);
+
+    if (expando) {
+        // Add properties from the expando object to the object, in order.
+        // Suppress GC here, so that callers don't need to worry about this
+        // method collecting. The stuff below can only fail due to OOM, in
+        // which case the object will not have been completely filled back in.
+        gc::AutoSuppressGC suppress(cx);
+
+        Vector<jsid> ids(cx);
+        for (Shape::Range<NoGC> r(expando->lastProperty()); !r.empty(); r.popFront()) {
+            if (!ids.append(r.front().propid()))
+                return false;
+        }
+        for (size_t i = 0; i < expando->getDenseInitializedLength(); i++) {
+            if (!expando->getDenseElement(i).isMagic(JS_ELEMENTS_HOLE)) {
+                if (!ids.append(INT_TO_JSID(i)))
+                    return false;
+            }
+        }
+        ::Reverse(ids.begin(), ids.end());
+
+        RootedPlainObject nobj(cx, &obj->as<PlainObject>());
+        Rooted<UnboxedExpandoObject*> nexpando(cx, expando);
+        RootedId id(cx);
+        Rooted<PropertyDescriptor> desc(cx);
+        for (size_t i = 0; i < ids.length(); i++) {
+            id = ids[i];
+            if (!GetOwnPropertyDescriptor(cx, nexpando, id, &desc))
+                return false;
+            ObjectOpResult result;
+            if (!DefineProperty(cx, nobj, id, desc, result))
+                return false;
+            MOZ_ASSERT(result.ok());
+        }
+    }
+
+    return true;
+}
+
+/* static */
+UnboxedPlainObject*
+UnboxedPlainObject::create(ExclusiveContext* cx, HandleObjectGroup group, NewObjectKind newKind)
+{
+    AutoSetNewObjectMetadata metadata(cx);
+
+    MOZ_ASSERT(group->clasp() == &class_);
+    gc::AllocKind allocKind = group->unboxedLayout().getAllocKind();
+
+    UnboxedPlainObject* res =
+        NewObjectWithGroup<UnboxedPlainObject>(cx, group, allocKind, newKind);
+    if (!res)
+        return nullptr;
+
+    // Overwrite the dummy shape which was written to the object's expando field.
+    res->initExpando();
+
+    // Initialize reference fields of the object. All fields in the object will
+    // be overwritten shortly, but references need to be safe for the GC.
+    const int32_t* list = res->layout().traceList();
+    if (list) {
+        uint8_t* data = res->data();
+        while (*list != -1) {
+            GCPtrString* heap = reinterpret_cast<GCPtrString*>(data + *list);
+            heap->init(cx->names().empty);
+            list++;
+        }
+        list++;
+        while (*list != -1) {
+            GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(data + *list);
+            heap->init(nullptr);
+            list++;
+        }
+        // Unboxed objects don't have Values to initialize.
+        MOZ_ASSERT(*(list + 1) == -1);
+    }
+
+    return res;
+}
+
+/* static */ JSObject*
+UnboxedPlainObject::createWithProperties(ExclusiveContext* cx, HandleObjectGroup group,
+                                         NewObjectKind newKind, IdValuePair* properties)
+{
+    MOZ_ASSERT(newKind == GenericObject || newKind == TenuredObject);
+
+    UnboxedLayout& layout = group->unboxedLayout();
+
+    if (layout.constructorCode()) {
+        MOZ_ASSERT(cx->isJSContext());
+
+        typedef JSObject* (*ConstructorCodeSignature)(IdValuePair*, NewObjectKind);
+        ConstructorCodeSignature function =
+            reinterpret_cast<ConstructorCodeSignature>(layout.constructorCode()->raw());
+
+        JSObject* obj;
+        {
+            JS::AutoSuppressGCAnalysis nogc;
+            obj = reinterpret_cast<JSObject*>(CALL_GENERATED_2(function, properties, newKind));
+        }
+        if (obj > reinterpret_cast<JSObject*>(CLEAR_CONSTRUCTOR_CODE_TOKEN))
+            return obj;
+
+        if (obj == reinterpret_cast<JSObject*>(CLEAR_CONSTRUCTOR_CODE_TOKEN))
+            layout.setConstructorCode(nullptr);
+    }
+
+    UnboxedPlainObject* obj = UnboxedPlainObject::create(cx, group, newKind);
+    if (!obj)
+        return nullptr;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        if (!obj->setValue(cx, layout.properties()[i], properties[i].value))
+            return NewPlainObjectWithProperties(cx, properties, layout.properties().length(), newKind);
+    }
+
+#ifndef JS_CODEGEN_NONE
+    if (cx->isJSContext() &&
+        !group->unknownProperties() &&
+        !layout.constructorCode() &&
+        cx->asJSContext()->runtime()->jitSupportsFloatingPoint &&
+        jit::CanLikelyAllocateMoreExecutableMemory())
+    {
+        if (!UnboxedLayout::makeConstructorCode(cx->asJSContext(), group))
+            return nullptr;
+    }
+#endif
+
+    return obj;
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                       HandleId id, MutableHandleObject objp,
+                                       MutableHandleShape propp)
+{
+    if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+        MarkNonNativePropertyFound<CanGC>(propp);
+        objp.set(obj);
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        objp.set(nullptr);
+        propp.set(nullptr);
+        return true;
+    }
+
+    return LookupProperty(cx, proto, id, objp, propp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       Handle<PropertyDescriptor> desc,
+                                       ObjectOpResult& result)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        if (!desc.getter() && !desc.setter() && desc.attributes() == JSPROP_ENUMERATE) {
+            // This define is equivalent to setting an existing property.
+            if (obj->as<UnboxedPlainObject>().setValue(cx, *property, desc.value()))
+                return result.succeed();
+        }
+
+        // Trying to incompatibly redefine an existing property requires the
+        // object to be converted to a native object.
+        if (!convertToNative(cx, obj))
+            return false;
+
+        return DefineProperty(cx, obj, id, desc, result);
+    }
+
+    // Define the property on the expando object.
+    Rooted<UnboxedExpandoObject*> expando(cx, ensureExpando(cx, obj.as<UnboxedPlainObject>()));
+    if (!expando)
+        return false;
+
+    // Update property types on the unboxed object as well.
+    AddTypePropertyId(cx, obj, id, desc.value());
+
+    return DefineProperty(cx, expando, id, desc, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp)
+{
+    if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+        *foundp = true;
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        *foundp = false;
+        return true;
+    }
+
+    return HasProperty(cx, proto, id, foundp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                    HandleId id, MutableHandleValue vp)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        vp.set(obj->as<UnboxedPlainObject>().getValue(*property));
+        return true;
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            RootedObject nexpando(cx, expando);
+            return GetProperty(cx, nexpando, receiver, id, vp);
+        }
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        vp.setUndefined();
+        return true;
+    }
+
+    return GetProperty(cx, proto, receiver, id, vp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                    HandleValue receiver, ObjectOpResult& result)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        if (receiver.isObject() && obj == &receiver.toObject()) {
+            if (obj->as<UnboxedPlainObject>().setValue(cx, *property, v))
+                return result.succeed();
+
+            if (!convertToNative(cx, obj))
+                return false;
+            return SetProperty(cx, obj, id, v, receiver, result);
+        }
+
+        return SetPropertyByDefining(cx, id, v, receiver, result);
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            // Update property types on the unboxed object as well.
+            AddTypePropertyId(cx, obj, id, v);
+
+            RootedObject nexpando(cx, expando);
+            return SetProperty(cx, nexpando, id, v, receiver, result);
+        }
+    }
+
+    return SetPropertyOnProto(cx, obj, id, v, receiver, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                                 MutableHandle<PropertyDescriptor> desc)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        desc.value().set(obj->as<UnboxedPlainObject>().getValue(*property));
+        desc.setAttributes(JSPROP_ENUMERATE);
+        desc.object().set(obj);
+        return true;
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            RootedObject nexpando(cx, expando);
+            if (!GetOwnPropertyDescriptor(cx, nexpando, id, desc))
+                return false;
+            if (desc.object() == nexpando)
+                desc.object().set(obj);
+            return true;
+        }
+    }
+
+    desc.object().set(nullptr);
+    return true;
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       ObjectOpResult& result)
+{
+    if (!convertToNative(cx, obj))
+        return false;
+    return DeleteProperty(cx, obj, id, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                                  bool enumerableOnly)
+{
+    // Ignore expando properties here, they are special-cased by the property
+    // enumeration code.
+
+    const UnboxedLayout::PropertyVector& unboxed = obj->as<UnboxedPlainObject>().layout().properties();
+    for (size_t i = 0; i < unboxed.length(); i++) {
+        if (!properties.append(NameToId(unboxed[i].name)))
+            return false;
+    }
+
+    return true;
+}
+
+const Class UnboxedExpandoObject::class_ = {
+    "UnboxedExpandoObject",
+    0
+};
+
+static const ClassOps UnboxedPlainObjectClassOps = {
+    nullptr,        /* addProperty */
+    nullptr,        /* delProperty */
+    nullptr,        /* getProperty */
+    nullptr,        /* setProperty */
+    nullptr,        /* enumerate   */
+    nullptr,        /* resolve     */
+    nullptr,        /* mayResolve  */
+    nullptr,        /* finalize    */
+    nullptr,        /* call        */
+    nullptr,        /* hasInstance */
+    nullptr,        /* construct   */
+    UnboxedPlainObject::trace,
+};
+
+static const ObjectOps UnboxedPlainObjectObjectOps = {
+    UnboxedPlainObject::obj_lookupProperty,
+    UnboxedPlainObject::obj_defineProperty,
+    UnboxedPlainObject::obj_hasProperty,
+    UnboxedPlainObject::obj_getProperty,
+    UnboxedPlainObject::obj_setProperty,
+    UnboxedPlainObject::obj_getOwnPropertyDescriptor,
+    UnboxedPlainObject::obj_deleteProperty,
+    nullptr,   /* getElements */
+    UnboxedPlainObject::obj_enumerate,
+    nullptr    /* funToString */
+};
+
+const Class UnboxedPlainObject::class_ = {
+    js_Object_str,
+    Class::NON_NATIVE |
+    JSCLASS_HAS_CACHED_PROTO(JSProto_Object) |
+    JSCLASS_DELAY_METADATA_BUILDER,
+    &UnboxedPlainObjectClassOps,
+    JS_NULL_CLASS_SPEC,
+    JS_NULL_CLASS_EXT,
+    &UnboxedPlainObjectObjectOps
+};
+
+/////////////////////////////////////////////////////////////////////
+// API
+/////////////////////////////////////////////////////////////////////
+
+static inline Value
+NextValue(Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    return values[(*valueCursor)++];
+}
+
+void
+UnboxedPlainObject::fillAfterConvert(ExclusiveContext* cx,
+                                     Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    initExpando();
+    memset(data(), 0, layout().size());
+    for (size_t i = 0; i < layout().properties().length(); i++)
+        JS_ALWAYS_TRUE(setValue(cx, layout().properties()[i], NextValue(values, valueCursor)));
+}
diff --git a/js/src/vm/UnboxedObject.h b/js/src/vm/UnboxedObject.h
new file mode 100644
index 0000000000..ba66434bc2
--- /dev/null
+++ b/js/src/vm/UnboxedObject.h
@@ -0,0 +1,319 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef vm_UnboxedObject_h
+#define vm_UnboxedObject_h
+
+#include "jsgc.h"
+#include "jsobj.h"
+
+#include "vm/Runtime.h"
+#include "vm/TypeInference.h"
+
+namespace js {
+
+// Memory required for an unboxed value of a given type. Returns zero for types
+// which can't be used for unboxed objects.
+static inline size_t
+UnboxedTypeSize(JSValueType type)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN: return 1;
+      case JSVAL_TYPE_INT32:   return 4;
+      case JSVAL_TYPE_DOUBLE:  return 8;
+      case JSVAL_TYPE_STRING:  return sizeof(void*);
+      case JSVAL_TYPE_OBJECT:  return sizeof(void*);
+      default:                 return 0;
+    }
+}
+
+static inline bool
+UnboxedTypeNeedsPreBarrier(JSValueType type)
+{
+    return type == JSVAL_TYPE_STRING || type == JSVAL_TYPE_OBJECT;
+}
+
+static inline bool
+UnboxedTypeNeedsPostBarrier(JSValueType type)
+{
+    return type == JSVAL_TYPE_OBJECT;
+}
+
+// Class tracking information specific to unboxed objects.
+class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
+{
+  public:
+    struct Property {
+        PropertyName* name;
+        uint32_t offset;
+        JSValueType type;
+
+        Property()
+          : name(nullptr), offset(UINT32_MAX), type(JSVAL_TYPE_MAGIC)
+        {}
+    };
+
+    typedef Vector<Property, 0, SystemAllocPolicy> PropertyVector;
+
+  private:
+    // If objects in this group have ever been converted to native objects,
+    // these store the corresponding native group and initial shape for such
+    // objects. Type information for this object is reflected in nativeGroup.
+    GCPtrObjectGroup nativeGroup_;
+    GCPtrShape nativeShape_;
+
+    // Any script/pc which the associated group is created for.
+    GCPtrScript allocationScript_;
+    jsbytecode* allocationPc_;
+
+    // If nativeGroup is set and this object originally had a TypeNewScript or
+    // was keyed to an allocation site, this points to the group which replaced
+    // this one. This link is only needed to keep the replacement group from
+    // being GC'ed. If it were GC'ed and a new one regenerated later, that new
+    // group might have a different allocation kind from this group.
+    GCPtrObjectGroup replacementGroup_;
+
+    // The following members are only used for unboxed plain objects.
+
+    // All properties on objects with this layout, in enumeration order.
+    PropertyVector properties_;
+
+    // Byte size of the data for objects with this layout.
+    size_t size_;
+
+    // Any 'new' script information associated with this layout.
+    TypeNewScript* newScript_;
+
+    // List for use in tracing objects with this layout. This has the same
+    // structure as the trace list on a TypeDescr.
+    int32_t* traceList_;
+
+    // If this layout has been used to construct script or JSON constant
+    // objects, this code might be filled in to more quickly fill in objects
+    // from an array of values.
+    GCPtrJitCode constructorCode_;
+
+  public:
+    UnboxedLayout()
+      : nativeGroup_(nullptr), nativeShape_(nullptr),
+        allocationScript_(nullptr), allocationPc_(nullptr), replacementGroup_(nullptr),
+        size_(0), newScript_(nullptr), traceList_(nullptr), constructorCode_(nullptr)
+    {}
+
+    bool initProperties(const PropertyVector& properties, size_t size) {
+        size_ = size;
+        return properties_.appendAll(properties);
+    }
+
+    ~UnboxedLayout() {
+        if (newScript_)
+            newScript_->clear();
+        js_delete(newScript_);
+        js_free(traceList_);
+
+        nativeGroup_.init(nullptr);
+        nativeShape_.init(nullptr);
+        replacementGroup_.init(nullptr);
+        constructorCode_.init(nullptr);
+    }
+
+    void detachFromCompartment();
+
+    const PropertyVector& properties() const {
+        return properties_;
+    }
+
+    TypeNewScript* newScript() const {
+        return newScript_;
+    }
+
+    void setNewScript(TypeNewScript* newScript, bool writeBarrier = true);
+
+    JSScript* allocationScript() const {
+        return allocationScript_;
+    }
+
+    jsbytecode* allocationPc() const {
+        return allocationPc_;
+    }
+
+    void setAllocationSite(JSScript* script, jsbytecode* pc) {
+        allocationScript_ = script;
+        allocationPc_ = pc;
+    }
+
+    const int32_t* traceList() const {
+        return traceList_;
+    }
+
+    void setTraceList(int32_t* traceList) {
+        traceList_ = traceList;
+    }
+
+    const Property* lookup(JSAtom* atom) const {
+        for (size_t i = 0; i < properties_.length(); i++) {
+            if (properties_[i].name == atom)
+                return &properties_[i];
+        }
+        return nullptr;
+    }
+
+    const Property* lookup(jsid id) const {
+        if (JSID_IS_STRING(id))
+            return lookup(JSID_TO_ATOM(id));
+        return nullptr;
+    }
+
+    size_t size() const {
+        return size_;
+    }
+
+    ObjectGroup* nativeGroup() const {
+        return nativeGroup_;
+    }
+
+    Shape* nativeShape() const {
+        return nativeShape_;
+    }
+
+    jit::JitCode* constructorCode() const {
+        return constructorCode_;
+    }
+
+    void setConstructorCode(jit::JitCode* code) {
+        constructorCode_ = code;
+    }
+
+    inline gc::AllocKind getAllocKind() const;
+
+    void trace(JSTracer* trc);
+
+    size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf);
+
+    static bool makeNativeGroup(JSContext* cx, ObjectGroup* group);
+    static bool makeConstructorCode(JSContext* cx, HandleObjectGroup group);
+};
+
+// Class for expando objects holding extra properties given to an unboxed plain
+// object. These objects behave identically to normal native plain objects, and
+// have a separate Class to distinguish them for memory usage reporting.
+class UnboxedExpandoObject : public NativeObject
+{
+  public:
+    static const Class class_;
+};
+
+// Class for a plain object using an unboxed representation. The physical
+// layout of these objects is identical to that of an InlineTypedObject, though
+// these objects use an UnboxedLayout instead of a TypeDescr to keep track of
+// how their properties are stored.
+class UnboxedPlainObject : public JSObject
+{
+    // Optional object which stores extra properties on this object. This is
+    // not automatically barriered to avoid problems if the object is converted
+    // to a native. See ensureExpando().
+    UnboxedExpandoObject* expando_;
+
+    // Start of the inline data, which immediately follows the group and extra properties.
+    uint8_t data_[1];
+
+  public:
+    static const Class class_;
+
+    static bool obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                   HandleId id, MutableHandleObject objp,
+                                   MutableHandleShape propp);
+
+    static bool obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   Handle<PropertyDescriptor> desc,
+                                   ObjectOpResult& result);
+
+    static bool obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp);
+
+    static bool obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                HandleId id, MutableHandleValue vp);
+
+    static bool obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                HandleValue receiver, ObjectOpResult& result);
+
+    static bool obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                             MutableHandle<PropertyDescriptor> desc);
+
+    static bool obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   ObjectOpResult& result);
+
+    static bool obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                              bool enumerableOnly);
+    static bool obj_watch(JSContext* cx, HandleObject obj, HandleId id, HandleObject callable);
+
+    inline const UnboxedLayout& layout() const;
+
+    const UnboxedLayout& layoutDontCheckGeneration() const {
+        return group()->unboxedLayoutDontCheckGeneration();
+    }
+
+    uint8_t* data() {
+        return &data_[0];
+    }
+
+    UnboxedExpandoObject* maybeExpando() const {
+        return expando_;
+    }
+
+    void initExpando() {
+        expando_ = nullptr;
+    }
+
+    // For use during GC.
+    JSObject** addressOfExpando() {
+        return reinterpret_cast<JSObject**>(&expando_);
+    }
+
+    bool containsUnboxedOrExpandoProperty(ExclusiveContext* cx, jsid id) const;
+
+    static UnboxedExpandoObject* ensureExpando(JSContext* cx, Handle<UnboxedPlainObject*> obj);
+
+    bool setValue(ExclusiveContext* cx, const UnboxedLayout::Property& property, const Value& v);
+    Value getValue(const UnboxedLayout::Property& property, bool maybeUninitialized = false);
+
+    static bool convertToNative(JSContext* cx, JSObject* obj);
+    static UnboxedPlainObject* create(ExclusiveContext* cx, HandleObjectGroup group,
+                                      NewObjectKind newKind);
+    static JSObject* createWithProperties(ExclusiveContext* cx, HandleObjectGroup group,
+                                          NewObjectKind newKind, IdValuePair* properties);
+
+    void fillAfterConvert(ExclusiveContext* cx,
+                          Handle<GCVector<Value>> values, size_t* valueCursor);
+
+    static void trace(JSTracer* trc, JSObject* object);
+
+    static size_t offsetOfExpando() {
+        return offsetof(UnboxedPlainObject, expando_);
+    }
+
+    static size_t offsetOfData() {
+        return offsetof(UnboxedPlainObject, data_[0]);
+    }
+};
+
+inline gc::AllocKind
+UnboxedLayout::getAllocKind() const
+{
+    MOZ_ASSERT(size());
+    return gc::GetGCObjectKindForBytes(UnboxedPlainObject::offsetOfData() + size());
+}
+
+} // namespace js
+
+namespace JS {
+
+template <>
+struct DeletePolicy<js::UnboxedLayout> : public js::GCManagedDeletePolicy<js::UnboxedLayout>
+{};
+
+} /* namespace JS */
+
+#endif /* vm_UnboxedObject_h */