Bug 1714561 - Allow single-zone JS holders to contain pointers into the atoms zone r=mccr8,sfink

Differential Revision: https://phabricator.services.mozilla.com/D116848

js/public/GCAPI.h

@@ -1172,6 +1172,8 @@ extern JS_PUBLIC_API void SetHostCleanupFinalizationRegistryCallback(
 extern JS_PUBLIC_API void ClearKeptObjects(JSContext* cx);
 
 extern JS_PUBLIC_API bool ZoneIsCollecting(Zone* zone);
+extern JS_PUBLIC_API bool AtomsZoneIsCollecting(JSRuntime* runtime);
+extern JS_PUBLIC_API bool IsAtomsZone(Zone* zone);
 
 }  // namespace JS
 

js/src/jsapi.cpp

@@ -1368,6 +1368,14 @@ JS_PUBLIC_API bool JS::ZoneIsCollecting(JS::Zone* zone) {
   return zone->wasGCStarted();
 }
 
+JS_PUBLIC_API bool JS::AtomsZoneIsCollecting(JSRuntime* runtime) {
+  return runtime->activeGCInAtomsZone();
+}
+
+JS_PUBLIC_API bool JS::IsAtomsZone(JS::Zone* zone) {
+  return zone->isAtomsZone();
+}
+
 JS_PUBLIC_API bool JS_AddWeakPointerZonesCallback(JSContext* cx,
                                                   JSWeakPointerZonesCallback cb,
                                                   void* data) {

xpcom/base/CycleCollectedJSRuntime.cpp

@@ -971,7 +971,14 @@ void CycleCollectedJSRuntime::TraceGrayJS(JSTracer* aTracer, void* aData) {
   CycleCollectedJSRuntime* self = static_cast<CycleCollectedJSRuntime*>(aData);
 
   // Mark these roots as gray so the CC can walk them later.
-  self->TraceNativeGrayRoots(aTracer, JSHolderMap::HoldersInCollectingZones);
+
+  JSHolderMap::WhichHolders which = JSHolderMap::HoldersInCollectingZones;
+  if (JS::AtomsZoneIsCollecting(self->Runtime())) {
+    // Any holder may point into the atoms zone.
+    which = JSHolderMap::AllHolders;
+  }
+
+  self->TraceNativeGrayRoots(aTracer, which);
 }
 
 /* static */
@@ -1268,6 +1275,11 @@ struct CheckZoneTracer : public TraceCallbacks {
       : mClassName(aClassName), mZone(aZone) {}
 
   void checkZone(JS::Zone* aZone, const char* aName) const {
+    if (JS::IsAtomsZone(aZone)) {
+      // Any holder may contain pointers into the atoms zone.
+      return;
+    }
+
     if (!mZone) {
       mZone = aZone;
       return;
@@ -1277,9 +1289,12 @@ struct CheckZoneTracer : public TraceCallbacks {
       return;
     }
 
-    // Most JS holders only contain pointers to GC things in a single zone. In
+    // Most JS holders only contain pointers to GC things in a single zone. We
-    // the future this will allow us to improve GC performance by only tracing
+    // group holders by referent zone where possible, allowing us to improve GC
-    // holders in zones that are being collected.
+    // performance by only tracing holders for zones that are being collected.
+    //
+    // Additionally, pointers from any holder into the atoms zone are allowed
+    // since all holders are traced when we collect the atoms zone.
     //
     // If you added a holder that has pointers into multiple zones please try to
     // remedy this. Some options are:
@@ -1306,7 +1321,7 @@ struct CheckZoneTracer : public TraceCallbacks {
                      void* aClosure) const override {
     jsid id = aPtr->unbarrieredGet();
     if (id.isGCThing()) {
-      checkZone(JS::GetTenuredGCThingZone(id.toGCCellPtr()), aName);
+      MOZ_ASSERT(JS::IsAtomsZone(JS::GetTenuredGCThingZone(id.toGCCellPtr())));
     }
   }
   virtual void Trace(JS::Heap<JSObject*>* aPtr, const char* aName,

xpcom/base/nsCycleCollectionParticipant.h

@@ -872,8 +872,8 @@ T* DowncastCCParticipant(void* aPtr) {
   _class::NS_CYCLE_COLLECTION_INNERCLASS _class::NS_CYCLE_COLLECTION_INNERNAME;
 
 // Most JS holder classes should only contain pointers to JS GC things in a
-// single JS zone, but there are some exceptions. Such classes should use this
+// single JS zone (excluding pointers into the atoms zone), but there are some
-// macro to tell the system about this.
+// exceptions. Such classes should use this macro to tell the system about this.
 #define NS_IMPL_CYCLE_COLLECTION_MULTI_ZONE_JSHOLDER_CLASS(_class) \
   _class::NS_CYCLE_COLLECTION_INNERCLASS                           \
       _class::NS_CYCLE_COLLECTION_INNERNAME(                       \