4 files changed, 11 insertions, 25 deletions

diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 799396a0a..0983f034f 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>, 8, SystemAllocPolicy>;
+using PerformanceGroupVector = mozilla::Vector<RefPtr<js::PerformanceGroup>, 0, SystemAllocPolicy>;
 
 /**
  * Commit any Performance Monitoring data.
diff --git a/js/src/vm/Stopwatch.cpp b/js/src/vm/Stopwatch.cpp
index 49b70c478..7a6acb970 100644
--- a/js/src/vm/Stopwatch.cpp
+++ b/js/src/vm/Stopwatch.cpp
@@ -136,9 +136,6 @@ 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;
@@ -155,19 +152,12 @@ PerformanceMonitoring::commit()
         return true;
     }
 
-    // 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`.
+    PerformanceGroupVector recentGroups;
+    recentGroups_.swap(recentGroups);
 
     bool success = true;
-    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;
+    if (stopwatchCommitCallback)
+        success = stopwatchCommitCallback(iteration_, recentGroups, stopwatchCommitClosure);
 
     // 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
@@ -237,7 +227,7 @@ AutoStopwatch::AutoStopwatch(JSContext* cx MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IM
     MOZ_GUARD_OBJECT_NOTIFIER_INIT;
 
     JSCompartment* compartment = cx_->compartment();
-    if (MOZ_UNLIKELY(compartment->scheduledForDestruction))
+    if (compartment->scheduledForDestruction)
         return;
 
     JSRuntime* runtime = cx_->runtime();
@@ -276,11 +266,11 @@ AutoStopwatch::~AutoStopwatch()
     }
 
     JSCompartment* compartment = cx_->compartment();
-    if (MOZ_UNLIKELY(compartment->scheduledForDestruction))
+    if (compartment->scheduledForDestruction)
         return;
 
     JSRuntime* runtime = cx_->runtime();
-    if (MOZ_UNLIKELY(iteration_ != runtime->performanceMonitoring.iteration())) {
+    if (iteration_ != runtime->performanceMonitoring.iteration()) {
         // We have entered a nested event loop at some point.
         // Any information we may have is obsolete.
         return;
diff --git a/toolkit/components/perfmonitoring/nsPerformanceStats.cpp b/toolkit/components/perfmonitoring/nsPerformanceStats.cpp
index 03e63a461..6c470356a 100644
--- a/toolkit/components/perfmonitoring/nsPerformanceStats.cpp
+++ b/toolkit/components/perfmonitoring/nsPerformanceStats.cpp
@@ -1310,12 +1310,8 @@ nsPerformanceStatsService::GetResources(uint64_t* userTime,
 
 void
 nsPerformanceStatsService::NotifyJankObservers(const mozilla::Vector<uint64_t>& aPreviousJankLevels) {
-
-  // 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`.
-
+  GroupVector alerts;
+  mPendingAlerts.swap(alerts);
   if (!mPendingAlertsCollector) {
     // We are shutting down.
     return;
diff --git a/toolkit/components/perfmonitoring/nsPerformanceStats.h b/toolkit/components/perfmonitoring/nsPerformanceStats.h
index 661a78a1a..6902c840d 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>, 8> GroupVector;
+typedef mozilla::Vector<RefPtr<nsPerformanceGroup>> GroupVector;
 
 /**
  * A data structure for registering observers interested in