Provide a tagged allocation top pointer.

src/x64/code-stubs-x64.cc

 // Copyright 2013 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_X64
 
 #include "src/code-stubs.h"
 #include "src/api-arguments.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 4186 matching lines @@
   // Fall back to runtime if the target differs from the new target's
   // initial map constructor.
   __ cmpp(rdi, FieldOperand(rcx, Map::kConstructorOrBackPointerOffset));
   __ j(not_equal, &new_object);
 
   // Allocate the JSObject on the heap.
   Label allocate, done_allocate;
   __ movzxbl(rbx, FieldOperand(rcx, Map::kInstanceSizeOffset));
   __ leal(rbx, Operand(rbx, times_pointer_size, 0));
   __ Allocate(rbx, rax, rdi, no_reg, &allocate, NO_ALLOCATION_FLAGS);
+  __ decp(rdi);  // Untag result end.
   __ bind(&done_allocate);
 
   // Initialize the JSObject fields.
   __ movp(FieldOperand(rax, JSObject::kMapOffset), rcx);
   __ LoadRoot(rbx, Heap::kEmptyFixedArrayRootIndex);
   __ movp(FieldOperand(rax, JSObject::kPropertiesOffset), rbx);
   __ movp(FieldOperand(rax, JSObject::kElementsOffset), rbx);
   STATIC_ASSERT(JSObject::kHeaderSize == 3 * kPointerSize);
   __ leap(rbx, FieldOperand(rax, JSObject::kHeaderSize));
 
@@ skipping 128 matching lines @@
   __ bind(&no_rest_parameters);
   {
     // ----------- S t a t e -------------
     //  -- rsi    : context
     //  -- rsp[0] : return address
     // -----------------------------------
 
     // Allocate an empty rest parameter array.
     Label allocate, done_allocate;
     __ Allocate(JSArray::kSize, rax, rdx, rcx, &allocate, NO_ALLOCATION_FLAGS);
+    __ decp(rdx);  // Untag result end.
     __ bind(&done_allocate);
 
     // Setup the rest parameter array in rax.
     __ LoadNativeContextSlot(Context::JS_ARRAY_FAST_ELEMENTS_MAP_INDEX, rcx);
     __ movp(FieldOperand(rax, JSArray::kMapOffset), rcx);
     __ LoadRoot(rcx, Heap::kEmptyFixedArrayRootIndex);
     __ movp(FieldOperand(rax, JSArray::kPropertiesOffset), rcx);
     __ movp(FieldOperand(rax, JSArray::kElementsOffset), rcx);
     __ movp(FieldOperand(rax, JSArray::kLengthOffset), Immediate(0));
     STATIC_ASSERT(JSArray::kSize == 4 * kPointerSize);
@@ skipping 996 matching lines @@
                            kStackUnwindSpace, nullptr, return_value_operand,
                            NULL);
 }
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64