Inobject slack tracking is done on a per-closure basis instead of per-shared info basis.

src/x64/builtins-x64.cc

Index: src/x64/builtins-x64.cc
diff --git a/src/x64/builtins-x64.cc b/src/x64/builtins-x64.cc
index 9e3b89ac61730cb9a5895636cd8ee49706278e1e..ab70f65faf3d4fb2ec9c72e421fc9342cf8d8ad3 100644
--- a/src/x64/builtins-x64.cc
+++ b/src/x64/builtins-x64.cc
@@ -101,7 +101,6 @@ void Builtins::Generate_InOptimizationQueue(MacroAssembler* masm) {
 
 static void Generate_JSConstructStubHelper(MacroAssembler* masm,
                                            bool is_api_function,
-                                           bool count_constructions,
                                            bool create_memento) {
   // ----------- S t a t e -------------
   //  -- rax: number of arguments
@@ -109,15 +108,9 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
   //  -- rbx: allocation site or undefined
   // -----------------------------------
 
-  // Should never count constructions for api objects.
-  ASSERT(!is_api_function || !count_constructions);\
-
   // Should never create mementos for api functions.
   ASSERT(!is_api_function || !create_memento);
 
-  // Should never create mementos before slack tracking is finished.
-  ASSERT(!count_constructions || !create_memento);
-
   // Enter a construct frame.
   {
     FrameScope scope(masm, StackFrame::CONSTRUCT);
@@ -166,23 +159,33 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
       __ CmpInstanceType(rax, JS_FUNCTION_TYPE);
       __ j(equal, &rt_call);
 
-      if (count_constructions) {
+      if (!is_api_function) {
+        // rsi: slack tracking counter
         Label allocate;
+        // The code below relies on these assumptions.
+        STATIC_ASSERT(JSFunction::kNoSlackTracking == 0);
+        STATIC_ASSERT(Map::ConstructionCount::kShift +
+                      Map::ConstructionCount::kSize == 32);
+        // Check if slack tracking is enabled.
+        __ movl(rsi, FieldOperand(rax, Map::kBitField3Offset));
+        __ shrl(rsi, Immediate(Map::ConstructionCount::kShift));
+        __ j(zero, &allocate);  // JSFunction::kNoSlackTracking
         // Decrease generous allocation count.
-        __ movp(rcx, FieldOperand(rdi, JSFunction::kSharedFunctionInfoOffset));
-        __ decb(FieldOperand(rcx,
-                             SharedFunctionInfo::kConstructionCountOffset));
-        __ j(not_zero, &allocate);
+        __ subl(FieldOperand(rax, Map::kBitField3Offset),
+                Immediate(1 << Map::ConstructionCount::kShift));
+
+        __ cmpl(rsi, Immediate(JSFunction::kFinishSlackTracking));
+        __ j(not_equal, &allocate);
 
         __ Push(rax);
         __ Push(rdi);
 
         __ Push(rdi);  // constructor
-        // The call will replace the stub, so the countdown is only done once.
         __ CallRuntime(Runtime::kHiddenFinalizeInstanceSize, 1);
 
         __ Pop(rdi);
         __ Pop(rax);
+        __ xorl(rsi, rsi);  // JSFunction::kNoSlackTracking
 
         __ bind(&allocate);
       }
@@ -213,9 +216,20 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
       // rax: initial map
       // rbx: JSObject
       // rdi: start of next object (including memento if create_memento)
+      // rsi: slack tracking counter (non-API function case)
       __ leap(rcx, Operand(rbx, JSObject::kHeaderSize));
       __ LoadRoot(rdx, Heap::kUndefinedValueRootIndex);
-      if (count_constructions) {
+      if (is_api_function) {
+        __ InitializeFieldsWithFiller(rcx, rdi, rdx);
+      } else {
+        Label no_inobject_slack_tracking, done_field_initialization;
+
+        // Check if slack tracking is enabled.
+        __ cmpl(rsi,
+                Immediate(static_cast<int32_t>(JSFunction::kNoSlackTracking)));
+        __ j(equal, &no_inobject_slack_tracking);
+
+        // Allocate object with a slack.
         __ movzxbp(rsi,
                    FieldOperand(rax, Map::kPreAllocatedPropertyFieldsOffset));
         __ leap(rsi,
@@ -229,21 +243,24 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
         __ InitializeFieldsWithFiller(rcx, rsi, rdx);
         __ LoadRoot(rdx, Heap::kOnePointerFillerMapRootIndex);
         __ InitializeFieldsWithFiller(rcx, rdi, rdx);
-      } else if (create_memento) {
-        __ leap(rsi, Operand(rdi, -AllocationMemento::kSize));
-        __ InitializeFieldsWithFiller(rcx, rsi, rdx);
-
-        // Fill in memento fields if necessary.
-        // rsi: points to the allocated but uninitialized memento.
-        Handle<Map> allocation_memento_map = factory->allocation_memento_map();
-        __ Move(Operand(rsi, AllocationMemento::kMapOffset),
-                allocation_memento_map);
-        // Get the cell or undefined.
-        __ movp(rdx, Operand(rsp, kPointerSize*2));
-        __ movp(Operand(rsi, AllocationMemento::kAllocationSiteOffset),
-                rdx);
-      } else {
-        __ InitializeFieldsWithFiller(rcx, rdi, rdx);
+        __ jmp(&done_field_initialization);
+
+        __ bind(&no_inobject_slack_tracking);
+        if (create_memento) {
+          __ leap(rsi, Operand(rdi, -AllocationMemento::kSize));
+          __ InitializeFieldsWithFiller(rcx, rsi, rdx);
+
+          // Fill in memento fields if necessary.
+          // rsi: points to the allocated but uninitialized memento.
+          __ Move(Operand(rsi, AllocationMemento::kMapOffset),
+                  factory->allocation_memento_map());
+          // Get the cell or undefined.
+          __ movp(rdx, Operand(rsp, kPointerSize*2));
+          __ movp(Operand(rsi, AllocationMemento::kAllocationSiteOffset), rdx);
+        } else {
+          __ InitializeFieldsWithFiller(rcx, rdi, rdx);
+        }
+        __ bind(&done_field_initialization);
       }
 
       // Add the object tag to make the JSObject real, so that we can continue
@@ -416,7 +433,7 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
     }
 
     // Store offset of return address for deoptimizer.
-    if (!is_api_function && !count_constructions) {
+    if (!is_api_function) {
       masm->isolate()->heap()->SetConstructStubDeoptPCOffset(masm->pc_offset());
     }
 
@@ -459,18 +476,13 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
 }
 
 
-void Builtins::Generate_JSConstructStubCountdown(MacroAssembler* masm) {
-  Generate_JSConstructStubHelper(masm, false, true, false);
-}
-
-
 void Builtins::Generate_JSConstructStubGeneric(MacroAssembler* masm) {
-  Generate_JSConstructStubHelper(masm, false, false, FLAG_pretenuring_call_new);
+  Generate_JSConstructStubHelper(masm, false, FLAG_pretenuring_call_new);
 }
 
 
 void Builtins::Generate_JSConstructStubApi(MacroAssembler* masm) {
-  Generate_JSConstructStubHelper(masm, true, false, false);
+  Generate_JSConstructStubHelper(masm, true, false);
 }