[heap] Be more restrictive about fast promotion mode

Exclude memory reducing scenarios and apply minimum percentage to absolute
sizes wrt. capacity rather then relative survival rate.

R=ulan@chromium.org
BUG=chromium:693413

Change-Id: I30cac99a2ce72227e269d72f52a459d4be6234a5
Reviewed-on: https://chromium-review.googlesource.com/445737Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43346}

src/heap/heap.cc

@@ -1800,13 +1800,16 @@ void Heap::Scavenge() {
 }
 
 void Heap::ComputeFastPromotionMode(double survival_rate) {
+  const size_t survived_in_new_space =
+      survived_last_scavenge_ * 100 / new_space_->Capacity();
   fast_promotion_mode_ =
       !FLAG_optimize_for_size && FLAG_fast_promotion_new_space &&
-      new_space_->IsAtMaximumCapacity() &&
-      survival_rate >= kMinPromotedPercentForFastPromotionMode;
+      !ShouldReduceMemory() && new_space_->IsAtMaximumCapacity() &&
+      survived_in_new_space >= kMinPromotedPercentForFastPromotionMode;
   if (FLAG_trace_gc_verbose) {
-    PrintIsolate(isolate(), "Fast promotion mode: %s survival rate: %f%%\n",
-                 fast_promotion_mode_ ? "true" : "false", survival_rate);
+    PrintIsolate(
+        isolate(), "Fast promotion mode: %s survival rate: %" PRIuS "%%\n",
+        fast_promotion_mode_ ? "true" : "false", survived_in_new_space);
   }
 }
 
@@ -5255,7 +5258,6 @@ const double Heap::kMaxHeapGrowingFactorMemoryConstrained = 2.0;
 const double Heap::kMaxHeapGrowingFactorIdle = 1.5;
 const double Heap::kConservativeHeapGrowingFactor = 1.3;
 const double Heap::kTargetMutatorUtilization = 0.97;
-const double Heap::kMinPromotedPercentForFastPromotionMode = 90;
 
 // Given GC speed in bytes per ms, the allocation throughput in bytes per ms
 // (mutator speed), this function returns the heap growing factor that will

src/heap/heap.h

@@ -648,7 +648,7 @@ class Heap {
   // The minimum size of a HeapObject on the heap.
   static const int kMinObjectSizeInWords = 2;
 
-  static const double kMinPromotedPercentForFastPromotionMode;
+  static const int kMinPromotedPercentForFastPromotionMode = 90;
 
   STATIC_ASSERT(kUndefinedValueRootIndex ==
                 Internals::kUndefinedValueRootIndex);