Reduce number of allocations in AutoStopwatch

This patch fixes two related issues.
1. The AutoStopwatch uses a stack-allocated `mozilla::Vector` to
communicate with its callback during each compartment switch.
This vector was designed to allow its contents to be stack-allocated
but they turned out to be accidentally heap-allocated.

2. During each tick, the stopwatch fills a vector `recentGroups_`.
This vector always started with minimal capacity and had to grow
repeatedly as groups were added, causing repeated reallocations.
This patch preallocates `recentGroups_` to have the same capacity as the
previous tick. We expect that this should eventually reach a stable size
that closely matches the actual needs of the process.

2 files changed, 18 insertions, 8 deletions

diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 0983f034f8..799396a0af 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -6566,7 +6566,7 @@ struct JS_PUBLIC_API(PerformanceGroup) {
     uint64_t refCount_;
 };
 
-using PerformanceGroupVector = mozilla::Vector<RefPtr<js::PerformanceGroup>, 0, SystemAllocPolicy>;
+using PerformanceGroupVector = mozilla::Vector<RefPtr<js::PerformanceGroup>, 8, SystemAllocPolicy>;
 
 /**
  * Commit any Performance Monitoring data.
diff --git a/js/src/vm/Stopwatch.cpp b/js/src/vm/Stopwatch.cpp
index 7a6acb9704..49b70c4785 100644
--- a/js/src/vm/Stopwatch.cpp
+++ b/js/src/vm/Stopwatch.cpp
@@ -136,6 +136,9 @@ PerformanceMonitoring::start()
 bool
 PerformanceMonitoring::commit()
 {
+    // Maximum initialization size, in elements for the vector of groups.
+    static const size_t MAX_GROUPS_INIT_CAPACITY = 1024;
+
 #if !defined(MOZ_HAVE_RDTSC)
     // The AutoStopwatch is only executed if `MOZ_HAVE_RDTSC`.
     return false;
@@ -152,12 +155,19 @@ PerformanceMonitoring::commit()
         return true;
     }
 
-    PerformanceGroupVector recentGroups;
-    recentGroups_.swap(recentGroups);
+    // The move operation is generally constant time, unless `recentGroups_.length()` is very small, in which case it's
+    // fast anyway because it's small.
+    PerformanceGroupVector recentGroups(Move(recentGroups_));
+    recentGroups_ = PerformanceGroupVector(); // Reconstruct after `Move`.
 
     bool success = true;
-    if (stopwatchCommitCallback)
-        success = stopwatchCommitCallback(iteration_, recentGroups, stopwatchCommitClosure);
+    if (stopwatchCommitCallback) {
+      success = stopwatchCommitCallback(iteration_, recentGroups, stopwatchCommitClosure);
+    }
+
+    // Heuristic use: we expect to have roughly the same number of groups as in the previous iteration.
+    const size_t capacity = std::min(recentGroups.capacity(), MAX_GROUPS_INIT_CAPACITY);
+    success = recentGroups_.reserve(capacity) && success;
 
     // Reset immediately, to make sure that we're not hit by the end
     // of a nested event loop (which would cause `commit` to be called
@@ -227,7 +237,7 @@ AutoStopwatch::AutoStopwatch(JSContext* cx MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IM
     MOZ_GUARD_OBJECT_NOTIFIER_INIT;
 
     JSCompartment* compartment = cx_->compartment();
-    if (compartment->scheduledForDestruction)
+    if (MOZ_UNLIKELY(compartment->scheduledForDestruction))
         return;
 
     JSRuntime* runtime = cx_->runtime();
@@ -266,11 +276,11 @@ AutoStopwatch::~AutoStopwatch()
     }
 
     JSCompartment* compartment = cx_->compartment();
-    if (compartment->scheduledForDestruction)
+    if (MOZ_UNLIKELY(compartment->scheduledForDestruction))
         return;
 
     JSRuntime* runtime = cx_->runtime();
-    if (iteration_ != runtime->performanceMonitoring.iteration()) {
+    if (MOZ_UNLIKELY(iteration_ != runtime->performanceMonitoring.iteration())) {
         // We have entered a nested event loop at some point.
         // Any information we may have is obsolete.
         return;