heap: Rework forced GCs

Forced GCs can either be invoked internally or communicate the fact that
they are forced externally via API. Before this CL, all uses were
passing kGCCallbackFlagForced to indicate that the GC was forced.

This flag is used by embedders though to trigger followup actions. E.g.,
it can be used to trigger a follow up call to
GarbageCollectionForTesting() call which requires --expose-gc.

This patch changes the semantics as follows:
- Internal forced GCs use a Heap GC flag (kForcedGC)
- External forced GCs and GC extension use kGCCallbackFlagForced

Bug: chromium:1074061
Change-Id: Ide7ea0ccdf88b8c8cac002289aef5b7eb0f9748c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2172747
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67498}

src/compiler/compilation-dependencies.cc

@@ -534,8 +534,7 @@ bool CompilationDependencies::Commit(Handle<Code> code) {
   // that triggers its deoptimization.
   if (FLAG_stress_gc_during_compilation) {
     broker_->isolate()->heap()->PreciseCollectAllGarbage(
-        Heap::kNoGCFlags, GarbageCollectionReason::kTesting,
-        kGCCallbackFlagForced);
+        Heap::kForcedGC, GarbageCollectionReason::kTesting, kNoGCCallbackFlags);
   }
 #ifdef DEBUG
   for (auto dep : dependencies_) {

src/heap/heap.cc

@@ -1380,16 +1380,15 @@ void Heap::CollectAllAvailableGarbage(GarbageCollectionReason gc_reason) {
   // The optimizing compiler may be unnecessarily holding on to memory.
   isolate()->AbortConcurrentOptimization(BlockingBehavior::kDontBlock);
   isolate()->ClearSerializerData();
-  set_current_gc_flags(kReduceMemoryFootprintMask);
+  set_current_gc_flags(
+      kReduceMemoryFootprintMask |
+      (gc_reason == GarbageCollectionReason::kLowMemoryNotification ? kForcedGC
+                                                                    : 0));
   isolate_->compilation_cache()->Clear();
   const int kMaxNumberOfAttempts = 7;
   const int kMinNumberOfAttempts = 2;
-  const v8::GCCallbackFlags callback_flags =
-      gc_reason == GarbageCollectionReason::kLowMemoryNotification
-          ? v8::kGCCallbackFlagForced
-          : v8::kGCCallbackFlagCollectAllAvailableGarbage;
   for (int attempt = 0; attempt < kMaxNumberOfAttempts; attempt++) {
-    if (!CollectGarbage(OLD_SPACE, gc_reason, callback_flags) &&
+    if (!CollectGarbage(OLD_SPACE, gc_reason, kNoGCCallbackFlags) &&
         attempt + 1 >= kMinNumberOfAttempts) {
       break;
     }
@@ -1512,7 +1511,8 @@ bool Heap::CollectGarbage(AllocationSpace space,
                           const v8::GCCallbackFlags gc_callback_flags) {
   const char* collector_reason = nullptr;
   GarbageCollector collector = SelectGarbageCollector(space, &collector_reason);
-  is_current_gc_forced_ = gc_callback_flags & v8::kGCCallbackFlagForced;
+  is_current_gc_forced_ = gc_callback_flags & v8::kGCCallbackFlagForced ||
+                          current_gc_flags_ & kForcedGC;
 
   DevToolsTraceEventScope devtools_trace_event_scope(
       this, IsYoungGenerationCollector(collector) ? "MinorGC" : "MajorGC",

src/heap/heap.h

@@ -300,6 +300,9 @@ class Heap {
 
   static const int kNoGCFlags = 0;
   static const int kReduceMemoryFootprintMask = 1;
+  // GCs that are forced, either through testing configurations (requring
+  // --expose-gc) or through DevTools (using LowMemoryNotificaton).
+  static const int kForcedGC = 2;
 
   // The minimum size of a HeapObject on the heap.
   static const int kMinObjectSizeInTaggedWords = 2;

src/wasm/c-api.cc

@@ -265,8 +265,8 @@ auto Engine::make(own<Config>&& config) -> own<Engine> {
 StoreImpl::~StoreImpl() {
 #ifdef DEBUG
   reinterpret_cast<i::Isolate*>(isolate_)->heap()->PreciseCollectAllGarbage(
-      i::Heap::kNoGCFlags, i::GarbageCollectionReason::kTesting,
-      v8::kGCCallbackFlagForced);
+      i::Heap::kForcedGC, i::GarbageCollectionReason::kTesting,
+      v8::kNoGCCallbackFlags);
 #endif
   context()->Exit();
   isolate_->Dispose();

test/cctest/test-api.cc

@@ -26850,8 +26850,8 @@ static void CallIsolate2(const v8::FunctionCallbackInfo<v8::Value>& args) {
       v8::Local<v8::Context>::New(isolate_2, context_2);
   v8::Context::Scope context_scope(context);
   reinterpret_cast<i::Isolate*>(isolate_2)->heap()->CollectAllGarbage(
-      i::Heap::kNoGCFlags, i::GarbageCollectionReason::kTesting,
-      v8::kGCCallbackFlagForced);
+      i::Heap::kForcedGC, i::GarbageCollectionReason::kTesting,
+      v8::kNoGCCallbackFlags);
   CompileRun("f2() //# sourceURL=isolate2b");
 }
 

test/wasm-api-tests/callbacks.cc

@@ -31,8 +31,8 @@ own<Trap> Stage4_GC(void* env, const Val args[], Val results[]) {
   printf("Stage4...\n");
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(env);
   isolate->heap()->PreciseCollectAllGarbage(
-      i::Heap::kNoGCFlags, i::GarbageCollectionReason::kTesting,
-      v8::kGCCallbackFlagForced);
+      i::Heap::kForcedGC, i::GarbageCollectionReason::kTesting,
+      v8::kNoGCCallbackFlags);
   results[0] = Val::i32(args[0].i32() + 1);
   return nullptr;
 }