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

src/ia32/builtins-ia32.cc

Index: src/ia32/builtins-ia32.cc
diff --git a/src/ia32/builtins-ia32.cc b/src/ia32/builtins-ia32.cc
index 969aae19dff073bcb9919ca1894443a36fc0c278..9b04db128b77f2a166fb8a159b953c385e1e2165 100644
--- a/src/ia32/builtins-ia32.cc
+++ b/src/ia32/builtins-ia32.cc
@@ -102,7 +102,6 @@ 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 -------------
   //  -- eax: number of arguments
@@ -110,15 +109,9 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
   //  -- ebx: 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);
@@ -164,23 +157,32 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
       __ CmpInstanceType(eax, JS_FUNCTION_TYPE);
       __ j(equal, &rt_call);
 
-      if (count_constructions) {
+      if (!is_api_function) {
         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.
+        __ mov(esi, FieldOperand(eax, Map::kBitField3Offset));
+        __ shr(esi, Map::ConstructionCount::kShift);
+        __ j(zero, &allocate);  // JSFunction::kNoSlackTracking
         // Decrease generous allocation count.
-        __ mov(ecx, FieldOperand(edi, JSFunction::kSharedFunctionInfoOffset));
-        __ dec_b(FieldOperand(ecx,
-                              SharedFunctionInfo::kConstructionCountOffset));
-        __ j(not_zero, &allocate);
+        __ sub(FieldOperand(eax, Map::kBitField3Offset),
+               Immediate(1 << Map::ConstructionCount::kShift));
+
+        __ cmp(esi, JSFunction::kFinishSlackTracking);
+        __ j(not_equal, &allocate);
 
         __ push(eax);
         __ push(edi);
 
         __ push(edi);  // constructor
-        // The call will replace the stub, so the countdown is only done once.
         __ CallRuntime(Runtime::kHiddenFinalizeInstanceSize, 1);
 
         __ pop(edi);
         __ pop(eax);
+        __ xor_(esi, esi);  // JSFunction::kNoSlackTracking
 
         __ bind(&allocate);
       }
@@ -210,9 +212,17 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
       // eax: initial map
       // ebx: JSObject
       // edi: start of next object (including memento if create_memento)
-      __ lea(ecx, Operand(ebx, JSObject::kHeaderSize));
+      // esi: slack tracking counter (non-API function case)
       __ mov(edx, factory->undefined_value());
-      if (count_constructions) {
+      __ lea(ecx, Operand(ebx, JSObject::kHeaderSize));
+      if (!is_api_function) {
+        Label no_inobject_slack_tracking;
+
+        // Check if slack tracking is enabled.
+        __ cmp(esi, JSFunction::kNoSlackTracking);
+        __ j(equal, &no_inobject_slack_tracking);
+
+        // Allocate object with a slack.
         __ movzx_b(esi,
                    FieldOperand(eax, Map::kPreAllocatedPropertyFieldsOffset));
         __ lea(esi,
@@ -225,16 +235,19 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
         }
         __ InitializeFieldsWithFiller(ecx, esi, edx);
         __ mov(edx, factory->one_pointer_filler_map());
-        __ InitializeFieldsWithFiller(ecx, edi, edx);
-      } else if (create_memento) {
+        // Fill the remaining fields with one pointer filler map.
+
+        __ bind(&no_inobject_slack_tracking);
+      }
+
+      if (create_memento) {
         __ lea(esi, Operand(edi, -AllocationMemento::kSize));
         __ InitializeFieldsWithFiller(ecx, esi, edx);
 
         // Fill in memento fields if necessary.
         // esi: points to the allocated but uninitialized memento.
-        Handle<Map> allocation_memento_map = factory->allocation_memento_map();
         __ mov(Operand(esi, AllocationMemento::kMapOffset),
-               allocation_memento_map);
+               factory->allocation_memento_map());
         // Get the cell or undefined.
         __ mov(edx, Operand(esp, kPointerSize*2));
         __ mov(Operand(esi, AllocationMemento::kAllocationSiteOffset),
@@ -413,7 +426,7 @@ 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());
     }
 
@@ -455,18 +468,13 @@ 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);
 }