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

src/arm/builtins-arm.cc

Index: src/arm/builtins-arm.cc
diff --git a/src/arm/builtins-arm.cc b/src/arm/builtins-arm.cc
index 2e5cc7398c7e70bdba5d984e07435a358e317d0b..a73613f534007e8b015a5d9bca92b8be9a098a06 100644
--- a/src/arm/builtins-arm.cc
+++ b/src/arm/builtins-arm.cc
@@ -313,7 +313,6 @@ 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 -------------
   //  -- r0     : number of arguments
@@ -323,15 +322,9 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
   //  -- sp[...]: constructor arguments
   // -----------------------------------
 
-  // 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);
-
   Isolate* isolate = masm->isolate();
 
   // Enter a construct frame.
@@ -375,21 +368,24 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
       __ CompareInstanceType(r2, r3, JS_FUNCTION_TYPE);
       __ b(eq, &rt_call);
 
-      if (count_constructions) {
+      if (!is_api_function) {
         Label allocate;
+        MemOperand bit_field3 = FieldMemOperand(r2, Map::kBitField3Offset);
+        // Check if slack tracking is enabled.
+        __ ldr(r4, bit_field3);
+        __ DecodeField<Map::ConstructionCount>(r3, r4);
+        __ cmp(r3, Operand(static_cast<int32_t>(JSFunction::kNoSlackTracking)));
+        __ b(eq, &allocate);
         // Decrease generous allocation count.
-        __ ldr(r3, FieldMemOperand(r1, JSFunction::kSharedFunctionInfoOffset));
-        MemOperand constructor_count =
-            FieldMemOperand(r3, SharedFunctionInfo::kConstructionCountOffset);
-        __ ldrb(r4, constructor_count);
-        __ sub(r4, r4, Operand(1), SetCC);
-        __ strb(r4, constructor_count);
+        __ sub(r4, r4, Operand(1 << Map::ConstructionCount::kShift));
+        __ str(r4, bit_field3);
+        __ cmp(r3,
+               Operand(static_cast<int32_t>(JSFunction::kFinishSlackTracking)));
         __ b(ne, &allocate);
 
         __ push(r1);
 
         __ Push(r2, r1);  // r1 = constructor
-        // The call will replace the stub, so the countdown is only done once.
         __ CallRuntime(Runtime::kHiddenFinalizeInstanceSize, 1);
 
         __ pop(r2);
@@ -431,7 +427,20 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
       // r5: First in-object property of JSObject (not tagged)
       ASSERT_EQ(3 * kPointerSize, JSObject::kHeaderSize);
 
-      if (count_constructions) {
+      if (is_api_function) {
+        __ LoadRoot(r6, Heap::kUndefinedValueRootIndex);
+        __ add(r0, r4, Operand(r3, LSL, kPointerSizeLog2));  // End of object.
+        __ InitializeFieldsWithFiller(r5, r0, r6);
+      } else {
+        Label no_inobject_slack_tracking, done_field_initialization;
+
+        // Check if slack tracking is enabled.
+        __ ldr(r6, FieldMemOperand(r2, Map::kBitField3Offset));
+        __ DecodeField<Map::ConstructionCount>(r6);
+        __ cmp(r6, Operand(static_cast<int32_t>(JSFunction::kNoSlackTracking)));
+        __ b(eq, &no_inobject_slack_tracking);
+
+        // Allocate object with a slack.
         __ LoadRoot(r6, Heap::kUndefinedValueRootIndex);
         __ ldr(r0, FieldMemOperand(r2, Map::kInstanceSizesOffset));
         __ Ubfx(r0, r0, Map::kPreAllocatedPropertyFieldsByte * kBitsPerByte,
@@ -448,25 +457,31 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
         __ LoadRoot(r6, Heap::kOnePointerFillerMapRootIndex);
         __ add(r0, r4, Operand(r3, LSL, kPointerSizeLog2));  // End of object.
         __ InitializeFieldsWithFiller(r5, r0, r6);
-      } else if (create_memento) {
-        __ sub(r6, r3, Operand(AllocationMemento::kSize / kPointerSize));
-        __ add(r0, r4, Operand(r6, LSL, kPointerSizeLog2));  // End of object.
-        __ LoadRoot(r6, Heap::kUndefinedValueRootIndex);
-        __ InitializeFieldsWithFiller(r5, r0, r6);
-
-        // Fill in memento fields.
-        // r5: points to the allocated but uninitialized memento.
-        __ LoadRoot(r6, Heap::kAllocationMementoMapRootIndex);
-        ASSERT_EQ(0 * kPointerSize, AllocationMemento::kMapOffset);
-        __ str(r6, MemOperand(r5, kPointerSize, PostIndex));
-        // Load the AllocationSite
-        __ ldr(r6, MemOperand(sp, 2 * kPointerSize));
-        ASSERT_EQ(1 * kPointerSize, AllocationMemento::kAllocationSiteOffset);
-        __ str(r6, MemOperand(r5, kPointerSize, PostIndex));
-      } else {
-        __ LoadRoot(r6, Heap::kUndefinedValueRootIndex);
-        __ add(r0, r4, Operand(r3, LSL, kPointerSizeLog2));  // End of object.
-        __ InitializeFieldsWithFiller(r5, r0, r6);
+        __ b(&done_field_initialization);
+
+        __ bind(&no_inobject_slack_tracking);
+
+        if (create_memento) {
+          __ sub(r6, r3, Operand(AllocationMemento::kSize / kPointerSize));
+          __ add(r0, r4, Operand(r6, LSL, kPointerSizeLog2));  // End of object.
+          __ LoadRoot(r6, Heap::kUndefinedValueRootIndex);
+          __ InitializeFieldsWithFiller(r5, r0, r6);
+
+          // Fill in memento fields.
+          // r5: points to the allocated but uninitialized memento.
+          __ LoadRoot(r6, Heap::kAllocationMementoMapRootIndex);
+          ASSERT_EQ(0 * kPointerSize, AllocationMemento::kMapOffset);
+          __ str(r6, MemOperand(r5, kPointerSize, PostIndex));
+          // Load the AllocationSite
+          __ ldr(r6, MemOperand(sp, 2 * kPointerSize));
+          ASSERT_EQ(1 * kPointerSize, AllocationMemento::kAllocationSiteOffset);
+          __ str(r6, MemOperand(r5, kPointerSize, PostIndex));
+        } else {
+          __ LoadRoot(r6, Heap::kUndefinedValueRootIndex);
+          __ add(r0, r4, Operand(r3, LSL, kPointerSizeLog2));  // End of object.
+          __ InitializeFieldsWithFiller(r5, r0, r6);
+        }
+        __ bind(&done_field_initialization);
       }
 
       // Add the object tag to make the JSObject real, so that we can continue
@@ -655,7 +670,7 @@ 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());
     }
 
@@ -707,18 +722,13 @@ 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);
 }