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

@@ -3510,23 +3510,7 @@ void Heap::FinalizeIncrementalMarkingAtomically(
   CollectAllGarbage(current_gc_flags_, gc_reason, current_gc_callback_flags_);
 }
 
-void Heap::FinalizeIncrementalMarkingIncrementally(
-    GarbageCollectionReason gc_reason) {
-  if (FLAG_trace_incremental_marking) {
-    isolate()->PrintWithTimestamp(
-        "[IncrementalMarking] (%s).\n",
-        Heap::GarbageCollectionReasonToString(gc_reason));
-  }
-
-  DevToolsTraceEventScope devtools_trace_event_scope(
-      this, "MajorGC", "incremental finalization step");
-
-  HistogramTimerScope incremental_marking_scope(
-      isolate()->counters()->gc_incremental_marking_finalize());
-  TRACE_EVENT0("v8", "V8.GCIncrementalMarkingFinalize");
-  TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_FINALIZE);
-
-  {
+void Heap::InvokeIncrementalMarkingPrologueCallbacks() {
   GCCallbacksScope scope(this);
   if (scope.CheckReenter()) {
     AllowHeapAllocation allow_allocation;
@@ -3535,9 +3519,9 @@ void Heap::FinalizeIncrementalMarkingIncrementally(
     HandleScope handle_scope(isolate_);
     CallGCPrologueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
   }
-  }
-  incremental_marking()->FinalizeIncrementally();
-  {
+}
+
+void Heap::InvokeIncrementalMarkingEpilogueCallbacks() {
   GCCallbacksScope scope(this);
   if (scope.CheckReenter()) {
     AllowHeapAllocation allow_allocation;
@@ -3546,7 +3530,27 @@ void Heap::FinalizeIncrementalMarkingIncrementally(
     HandleScope handle_scope(isolate_);
     CallGCEpilogueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
   }
+}
+
+void Heap::FinalizeIncrementalMarkingIncrementally(
+    GarbageCollectionReason gc_reason) {
+  if (FLAG_trace_incremental_marking) {
+    isolate()->PrintWithTimestamp(
+        "[IncrementalMarking] (%s).\n",
+        Heap::GarbageCollectionReasonToString(gc_reason));
   }
+
+  DevToolsTraceEventScope devtools_trace_event_scope(
+      this, "MajorGC", "incremental finalization step");
+
+  HistogramTimerScope incremental_marking_scope(
+      isolate()->counters()->gc_incremental_marking_finalize());
+  TRACE_EVENT0("v8", "V8.GCIncrementalMarkingFinalize");
+  TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_FINALIZE);
+
+  InvokeIncrementalMarkingPrologueCallbacks();
+  incremental_marking()->FinalizeIncrementally();
+  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() {