More efficient use of space in AllocationSite.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 1dc54b206e39cc72cf133d4b006d65352f09098f..f9e416a809d57cfb00079ccee01db7a4eb81f860 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8125,11 +8125,12 @@ class AllocationSite: public Struct {
   static const int kPretenureMinimumCreated = 100;
 
   // Values for pretenure decision field.
-  enum {
+  enum PretenureDecision {
     kUndecided = 0,
     kDontTenure = 1,
     kTenure = 2,
-    kZombie = 3
+    kZombie = 3,
+    kLastPretenureDecisionValue = kZombie
   };
 
   DECL_ACCESSORS(transition_info, Object)
@@ -8137,11 +8138,8 @@ class AllocationSite: public Struct {
   // walked in a particular order. So [[1, 2], 1, 2] will have one
   // nested_site, but [[1, 2], 3, [4]] will have a list of two.
   DECL_ACCESSORS(nested_site, Object)
-  DECL_ACCESSORS(memento_found_count, Smi)
-  DECL_ACCESSORS(memento_create_count, Smi)
-  // TODO(mvstanton): we don't need a whole integer to record pretenure
-  // decision. Consider sharing space with memento_found_count.
-  DECL_ACCESSORS(pretenure_decision, Smi)
+  DECL_ACCESSORS(pretenure_data, Smi)
+  DECL_ACCESSORS(pretenure_create_count, Smi)
   DECL_ACCESSORS(dependent_code, DependentCode)
   DECL_ACCESSORS(weak_next, Object)
 
@@ -8150,10 +8148,16 @@ class AllocationSite: public Struct {
   // This method is expensive, it should only be called for reporting.
   bool IsNestedSite();
 
+  // transition_info bitfields, for constructed array transition info.
   class ElementsKindBits:       public BitField<ElementsKind, 0,  15> {};
   class UnusedBits:             public BitField<int,          15, 14> {};
   class DoNotInlineBit:         public BitField<bool,         29,  1> {};
 
+  // Bitfields for pretenure_data
+  class MementoFoundCountBits:  public BitField<int,          0, 28> {};
+  class PretenureDecisionBits:  public BitField<PretenureDecision, 28, 2> {};
+  STATIC_ASSERT(PretenureDecisionBits::kMax >= kLastPretenureDecisionValue);
+
   // Increments the mementos found counter and returns true when the first
   // memento was found for a given allocation site.
   inline bool IncrementMementoFoundCount();
@@ -8164,12 +8168,39 @@ class AllocationSite: public Struct {
 
   void ResetPretenureDecision();
 
+  PretenureDecision pretenure_decision() {
+    int value = pretenure_data()->value();
+    return PretenureDecisionBits::decode(value);
+  }
+
+  void set_pretenure_decision(PretenureDecision decision) {
+    int value = pretenure_data()->value();
+    set_pretenure_data(
+        Smi::FromInt(PretenureDecisionBits::update(value, decision)),
+        SKIP_WRITE_BARRIER);
+  }
+
+  int memento_found_count() {
+    int value = pretenure_data()->value();
+    return MementoFoundCountBits::decode(value);
+  }
+
+  inline void set_memento_found_count(int count);
+
+  int memento_create_count() {
+    return pretenure_create_count()->value();
+  }
+
+  void set_memento_create_count(int count) {
+    set_pretenure_create_count(Smi::FromInt(count), SKIP_WRITE_BARRIER);
+  }
+
   // The pretenuring decision is made during gc, and the zombie state allows
   // us to recognize when an allocation site is just being kept alive because
   // a later traversal of new space may discover AllocationMementos that point
   // to this AllocationSite.
   bool IsZombie() {
-    return pretenure_decision()->value() == kZombie;
+    return pretenure_decision() == kZombie;
   }
 
   inline void MarkZombie();
@@ -8227,13 +8258,11 @@ class AllocationSite: public Struct {
 
   static const int kTransitionInfoOffset = HeapObject::kHeaderSize;
   static const int kNestedSiteOffset = kTransitionInfoOffset + kPointerSize;
-  static const int kMementoFoundCountOffset = kNestedSiteOffset + kPointerSize;
-  static const int kMementoCreateCountOffset =
-      kMementoFoundCountOffset + kPointerSize;
-  static const int kPretenureDecisionOffset =
-      kMementoCreateCountOffset + kPointerSize;
+  static const int kPretenureDataOffset = kNestedSiteOffset + kPointerSize;
+  static const int kPretenureCreateCountOffset =
+      kPretenureDataOffset + kPointerSize;
   static const int kDependentCodeOffset =
-      kPretenureDecisionOffset + kPointerSize;
+      kPretenureCreateCountOffset + kPointerSize;
   static const int kWeakNextOffset = kDependentCodeOffset + kPointerSize;
   static const int kSize = kWeakNextOffset + kPointerSize;
 
@@ -8250,7 +8279,7 @@ class AllocationSite: public Struct {
  private:
   inline DependentCode::DependencyGroup ToDependencyGroup(Reason reason);
   bool PretenuringDecisionMade() {
-    return pretenure_decision()->value() != kUndecided;
+    return pretenure_decision() != kUndecided;
   }
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(AllocationSite);