heap: Add callback for incremental marking start

The callback is useful to embedders to trigger various actions such as
recomputing live set.

Bug: chromium:1056170
Change-Id: I7d80b9b768a728e23303f945e416df97fd9b7805
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2173358
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67532}

src/heap/heap.cc

@@ -3510,6 +3510,28 @@ void Heap::FinalizeIncrementalMarkingAtomically(
   CollectAllGarbage(current_gc_flags_, gc_reason, current_gc_callback_flags_);
 }
 
+void Heap::InvokeIncrementalMarkingPrologueCallbacks() {
+  GCCallbacksScope scope(this);
+  if (scope.CheckReenter()) {
+    AllowHeapAllocation allow_allocation;
+    TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_PROLOGUE);
+    VMState<EXTERNAL> state(isolate_);
+    HandleScope handle_scope(isolate_);
+    CallGCPrologueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
+  }
+}
+
+void Heap::InvokeIncrementalMarkingEpilogueCallbacks() {
+  GCCallbacksScope scope(this);
+  if (scope.CheckReenter()) {
+    AllowHeapAllocation allow_allocation;
+    TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_EPILOGUE);
+    VMState<EXTERNAL> state(isolate_);
+    HandleScope handle_scope(isolate_);
+    CallGCEpilogueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
+  }
+}
+
 void Heap::FinalizeIncrementalMarkingIncrementally(
     GarbageCollectionReason gc_reason) {
   if (FLAG_trace_incremental_marking) {
@@ -3526,27 +3548,9 @@ void Heap::FinalizeIncrementalMarkingIncrementally(
   TRACE_EVENT0("v8", "V8.GCIncrementalMarkingFinalize");
   TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_FINALIZE);
 
-  {
-    GCCallbacksScope scope(this);
-    if (scope.CheckReenter()) {
-      AllowHeapAllocation allow_allocation;
-      TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_PROLOGUE);
-      VMState<EXTERNAL> state(isolate_);
-      HandleScope handle_scope(isolate_);
-      CallGCPrologueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
-    }
-  }
+  InvokeIncrementalMarkingPrologueCallbacks();
   incremental_marking()->FinalizeIncrementally();
-  {
-    GCCallbacksScope scope(this);
-    if (scope.CheckReenter()) {
-      AllowHeapAllocation allow_allocation;
-      TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_EPILOGUE);
-      VMState<EXTERNAL> state(isolate_);
-      HandleScope handle_scope(isolate_);
-      CallGCEpilogueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
-    }
-  }
+  InvokeIncrementalMarkingEpilogueCallbacks();
 }
 
 void Heap::RegisterDeserializedObjectsForBlackAllocation(

src/heap/heap.h

@@ -1731,6 +1731,9 @@ class Heap {
   void FinalizeIncrementalMarkingIncrementally(
       GarbageCollectionReason gc_reason);
 
+  void InvokeIncrementalMarkingPrologueCallbacks();
+  void InvokeIncrementalMarkingEpilogueCallbacks();
+
   // Returns the timer used for a given GC type.
   // - GCScavenger: young generation GC
   // - GCCompactor: full GC

src/heap/incremental-marking.cc

@@ -322,6 +322,9 @@ void IncrementalMarking::StartMarking() {
     heap()->isolate()->PrintWithTimestamp(
         "[IncrementalMarking] Start marking\n");
   }
+
+  heap_->InvokeIncrementalMarkingPrologueCallbacks();
+
   is_compacting_ = !FLAG_never_compact && collector_->StartCompaction();
   collector_->StartMarking();
 
@@ -352,6 +355,8 @@ void IncrementalMarking::StartMarking() {
     heap_->local_embedder_heap_tracer()->TracePrologue(
         heap_->flags_for_embedder_tracer());
   }
+
+  heap_->InvokeIncrementalMarkingEpilogueCallbacks();
 }
 
 void IncrementalMarking::StartBlackAllocation() {