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, 7 insertions, 3 deletions

diff --git a/toolkit/components/perfmonitoring/nsPerformanceStats.cpp b/toolkit/components/perfmonitoring/nsPerformanceStats.cpp
index 6c470356a..03e63a461 100644
--- a/toolkit/components/perfmonitoring/nsPerformanceStats.cpp
+++ b/toolkit/components/perfmonitoring/nsPerformanceStats.cpp
@@ -1310,8 +1310,12 @@ nsPerformanceStatsService::GetResources(uint64_t* userTime,
 
 void
 nsPerformanceStatsService::NotifyJankObservers(const mozilla::Vector<uint64_t>& aPreviousJankLevels) {
-  GroupVector alerts;
-  mPendingAlerts.swap(alerts);
+
+  // The move operation is generally constant time, unless `mPendingAlerts.length()` is very small, in which case it's
+  // fast anyway.
+  GroupVector alerts(Move(mPendingAlerts));
+  mPendingAlerts = GroupVector(); // Reconstruct after `Move`.
+
   if (!mPendingAlertsCollector) {
     // We are shutting down.
     return;
diff --git a/toolkit/components/perfmonitoring/nsPerformanceStats.h b/toolkit/components/perfmonitoring/nsPerformanceStats.h
index 6902c840d..661a78a1a 100644
--- a/toolkit/components/perfmonitoring/nsPerformanceStats.h
+++ b/toolkit/components/perfmonitoring/nsPerformanceStats.h
@@ -19,7 +19,7 @@
 class nsPerformanceGroup;
 class nsPerformanceGroupDetails;
 
-typedef mozilla::Vector<RefPtr<nsPerformanceGroup>> GroupVector;
+typedef mozilla::Vector<RefPtr<nsPerformanceGroup>, 8> GroupVector;
 
 /**
  * A data structure for registering observers interested in