Bug 1751543 - Skip the "can I GC yet?" check entirely in the parent process r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D137353

dom/base/CCGCScheduler.cpp

@@ -113,7 +113,7 @@ void CCGCScheduler::NoteGCBegin() {
   // Treat all GC as incremental here; non-incremental GC will just appear to
   // be one slice.
   mInIncrementalGC = true;
-  mReadyForMajorGC = false;
+  mReadyForMajorGC = !mAskParentBeforeMajorGC;
 
   // Tell the parent process that we've started a GC (it might not know if
   // we hit a threshold in the JS engine).
@@ -129,7 +129,7 @@ void CCGCScheduler::NoteGCEnd() {
 
   mInIncrementalGC = false;
   mCCBlockStart = TimeStamp();
-  mReadyForMajorGC = false;
+  mReadyForMajorGC = !mAskParentBeforeMajorGC;
   mWantAtLeastRegularGC = false;
   mNeedsFullCC = CCReason::GC_FINISHED;
   mHasRunGC = true;
@@ -203,7 +203,7 @@ void CCGCScheduler::NoteCCEnd(TimeStamp aWhen) {
 }
 
 void CCGCScheduler::NoteWontGC() {
-  mReadyForMajorGC = false;
+  mReadyForMajorGC = !mAskParentBeforeMajorGC;
   mMajorGCReason = JS::GCReason::NO_REASON;
   mWantAtLeastRegularGC = false;
   // Don't clear the WantFullGC state, we will do a full GC the next time a
@@ -365,7 +365,13 @@ RefPtr<CCGCScheduler::MayGCPromise> CCGCScheduler::MayGCNow(
       break;
   }
 
-  return MayGCPromise::CreateAndResolve(true, __func__);
+  // We use synchronous task dispatch here to avoid a trip through the event
+  // loop if we're on the parent process or it's a GC reason that does not
+  // require permission to GC.
+  RefPtr<MayGCPromise::Private> p = MakeRefPtr<MayGCPromise::Private>(__func__);
+  p->UseSynchronousTaskDispatch(__func__);
+  p->Resolve(true, __func__);
+  return p;
 }
 
 void CCGCScheduler::RunNextCollectorTimer(JS::GCReason aReason,

dom/base/CCGCScheduler.h

@@ -109,7 +109,10 @@ struct CCRunnerStep {
 
 class CCGCScheduler {
  public:
-  CCGCScheduler() : mInterruptRequested(false) {}
+  CCGCScheduler()
+      : mAskParentBeforeMajorGC(XRE_IsContentProcess()),
+        mReadyForMajorGC(!mAskParentBeforeMajorGC),
+        mInterruptRequested(false) {}
 
   static bool CCRunnerFired(TimeStamp aDeadline);
 
@@ -434,12 +437,16 @@ class CCGCScheduler {
   // duration (or until it goes too long and is finished synchronously.)
   bool mInIncrementalGC = false;
 
+  // Whether to ask the parent process if now is a good time to GC (false for
+  // the parent process.)
+  const bool mAskParentBeforeMajorGC;
+
   // We've asked the parent process if now is a good time to GC (do not ask
   // again).
   bool mHaveAskedParent = false;
 
   // The parent process is ready for us to do a major GC.
-  bool mReadyForMajorGC = false;
+  bool mReadyForMajorGC;
 
   // Set when the IdleTaskRunner requests the current task be interrupted.
   // Cleared when the GC slice budget has detected the interrupt request.