1 files changed, 28 insertions, 8 deletions

diff --git a/mfbt/SegmentedVector.h b/mfbt/SegmentedVector.h
index 6d255102fa..6e4dc2a452 100644
--- a/mfbt/SegmentedVector.h
+++ b/mfbt/SegmentedVector.h
@@ -120,9 +120,9 @@ class SegmentedVector : private AllocPolicy
     ? (IdealSegmentSize - kSingleElementSegmentSize) / sizeof(T) + 1
     : 1;
 
+public:
   typedef SegmentImpl<kSegmentCapacity> Segment;
 
-public:
   // The |aIdealSegmentSize| is only for sanity checking. If it's specified, we
   // check that the actual segment size is as close as possible to it. This
   // serves as a sanity check for SegmentedVectorCapacity's capacity
@@ -259,7 +259,6 @@ public:
     // than we want to pop.
     MOZ_ASSERT(last);
     MOZ_ASSERT(last == mSegments.getLast());
-    MOZ_ASSERT(aNumElements != 0);
     MOZ_ASSERT(aNumElements < last->Length());
     for (uint32_t i = 0; i < aNumElements; ++i) {
       last->PopLast();
@@ -274,6 +273,10 @@ public:
   //    f(elem);
   //  }
   //
+  // Note, adding new entries to the SegmentedVector while using iterators
+  // is supported, but removing is not!
+  // If an iterator has entered Done() state, adding more entries to the
+  // vector doesn't affect it.
   class IterImpl
   {
     friend class SegmentedVector;
@@ -281,10 +284,14 @@ public:
     Segment* mSegment;
     size_t mIndex;
 
-    explicit IterImpl(SegmentedVector* aVector)
-      : mSegment(aVector->mSegments.getFirst())
-      , mIndex(0)
-    {}
+    explicit IterImpl(SegmentedVector* aVector, bool aFromFirst)
+      : mSegment(aFromFirst ? aVector->mSegments.getFirst() :
+                              aVector->mSegments.getLast())
+      , mIndex(aFromFirst ? 0 :
+                            (mSegment ? mSegment->Length() - 1 : 0))
+    {
+      MOZ_ASSERT_IF(mSegment, mSegment->Length() > 0);
+    }
 
   public:
     bool Done() const { return !mSegment; }
@@ -310,10 +317,23 @@ public:
         mIndex = 0;
       }
     }
-  };
 
-  IterImpl Iter() { return IterImpl(this); }
+    void Prev()
+    {
+      MOZ_ASSERT(!Done());
+      if (mIndex == 0) {
+        mSegment = mSegment->getPrevious();
+        if (mSegment) {
+          mIndex = mSegment->Length() - 1;
+        }
+      } else {
+        --mIndex;
+      }
+    }
+  };
 
+  IterImpl Iter() { return IterImpl(this, true); }
+  IterImpl IterFromLast() { return IterImpl(this, false); }
   // Measure the memory consumption of the vector excluding |this|. Note that
   // it only measures the vector itself. If the vector elements contain
   // pointers to other memory blocks, those blocks must be measured separately