Basic array capacity feedback via allocation sites.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 103e93e8c4ac2547bea8167c190803000071ea6a..9e1003b713bda0d14457536bf004b0dc0f153f5a 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8152,7 +8152,8 @@ class AllocationSite: public Struct {
 
   // transition_info bitfields, for constructed array transition info.
   class ElementsKindBits:       public BitField<ElementsKind, 0,  15> {};
-  class UnusedBits:             public BitField<int,          15, 14> {};
+  class InitialElementsCapacity : public BitField<int, 15, 10> {};
+  class UnusedBits : public BitField<int, 25, 4> {};
   class DoNotInlineBit:         public BitField<bool,         29,  1> {};
 
   // Bitfields for pretenure_data
@@ -8242,6 +8243,11 @@ class AllocationSite: public Struct {
                         SKIP_WRITE_BARRIER);
   }
 
+  inline int GetInitialElementsCapacity();
+
+  // Note that the function accepts the new length, not capacity.
+  inline void DigestInitialElementsCapacityFeedback(int new_length);
+
   bool CanInlineCall() {
     int value = Smi::cast(transition_info())->value();
     return DoNotInlineBit::decode(value) == 0;
@@ -10101,7 +10107,12 @@ class JSArray: public JSObject {
   DECLARE_VERIFIER(JSArray)
 
   // Number of element slots to pre-allocate for an empty array.
-  static const int kPreallocatedArrayElements = 4;
+  static int PreallocatedArrayElements() {
+    if (FLAG_initial_capacity_mode == 1) return 4;
+    DCHECK(FLAG_initial_capacity_mode == 0 ||
+           FLAG_initial_capacity_mode == 2);
+    return 0;
+  }
 
   // Layout description.
   static const int kLengthOffset = JSObject::kHeaderSize;