Fix object initialization when slack tracking for it's map is still enabled.

src/ia32/builtins-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_IA32
 
 #include "src/code-factory.h"
 #include "src/codegen.h"
 #include "src/deoptimizer.h"
 #include "src/full-codegen/full-codegen.h"
@@ skipping 223 matching lines @@
         __ mov(edx, factory->undefined_value());
         __ lea(ecx, Operand(ebx, JSObject::kHeaderSize));
         if (!is_api_function) {
           Label no_inobject_slack_tracking;
 
           // Check if slack tracking is enabled.
           __ cmp(esi, Map::kSlackTrackingCounterEnd);
           __ j(less, &no_inobject_slack_tracking);
 
           // Allocate object with a slack.
-          __ movzx_b(
-              esi,
-              FieldOperand(
-                  eax,
-                  Map::kInObjectPropertiesOrConstructorFunctionIndexOffset));
-          __ movzx_b(eax, FieldOperand(eax, Map::kUnusedPropertyFieldsOffset));
-          __ sub(esi, eax);
-          __ lea(esi,
-                 Operand(ebx, esi, times_pointer_size, JSObject::kHeaderSize));
+          // See comment on JSObject::InitializeBody() for details.
+          __ movzx_b(esi, FieldOperand(eax, Map::kUnusedPropertyFieldsOffset));
+          __ neg(esi);
+          __ lea(esi, Operand(edi, esi, times_pointer_size, 0));
           // esi: offset of first field after pre-allocated fields
           if (FLAG_debug_code) {
-            __ cmp(esi, edi);
+            __ cmp(ecx, esi);
             __ Assert(less_equal,
                       kUnexpectedNumberOfPreAllocatedPropertyFields);
           }
           __ InitializeFieldsWithFiller(ecx, esi, edx);
           __ mov(edx, factory->one_pointer_filler_map());
           // Fill the remaining fields with one pointer filler map.
 
           __ bind(&no_inobject_slack_tracking);
         }
 
@@ skipping 1590 matching lines @@
 
   __ bind(&ok);
   __ ret(0);
 }
 
 #undef __
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_IA32