[heap] Add histogram counters to track GC reasons.

src/heap/incremental-marking.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/incremental-marking.h"
 
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/conversions.h"
 #include "src/heap/gc-idle-time-handler.h"
@@ skipping 497 matching lines @@
         "slack %dMB\n",
         Heap::GarbageCollectionReasonToString(gc_reason),
         old_generation_size_mb, old_generation_limit_mb,
         Max(0, old_generation_limit_mb - old_generation_size_mb));
   }
   DCHECK(FLAG_incremental_marking);
   DCHECK(state_ == STOPPED);
   DCHECK(heap_->gc_state() == Heap::NOT_IN_GC);
   DCHECK(!heap_->isolate()->serializer_enabled());
 
+  Counters* counters = heap_->isolate()->counters();
+
+  counters->incremental_marking_reason()->AddSample(
+      static_cast<int>(gc_reason));
   HistogramTimerScope incremental_marking_scope(
-      heap_->isolate()->counters()->gc_incremental_marking_start());
+      counters->gc_incremental_marking_start());
   TRACE_EVENT0("v8", "V8.GCIncrementalMarkingStart");
   ResetStepCounters();
   heap_->tracer()->NotifyIncrementalMarkingStart();
 
   was_activated_ = true;
 
   if (!heap_->mark_compact_collector()->sweeping_in_progress()) {
     StartMarking();
   } else {
     if (FLAG_trace_incremental_marking) {
@@ skipping 757 matching lines @@
   idle_marking_delay_counter_++;
 }
 
 
 void IncrementalMarking::ClearIdleMarkingDelayCounter() {
   idle_marking_delay_counter_ = 0;
 }
 
 }  // namespace internal
 }  // namespace v8