Provide a tagged allocation top pointer.

src/crankshaft/arm/lithium-codegen-arm.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.
 
 #include "src/crankshaft/arm/lithium-codegen-arm.h"
 
 #include "src/base/bits.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/crankshaft/arm/lithium-gap-resolver-arm.h"
@@ skipping 5144 matching lines @@
     if (instr->hydrogen()->IsOldSpaceAllocation()) {
       DCHECK(!instr->hydrogen()->IsNewSpaceAllocation());
       allocation_flags = static_cast<AllocationFlags>(flags | PRETENURE);
     }
     // If the allocation folding dominator allocate triggered a GC, allocation
     // happend in the runtime. We have to reset the top pointer to virtually
     // undo the allocation.
     ExternalReference allocation_top =
         AllocationUtils::GetAllocationTopReference(isolate(), allocation_flags);
     Register top_address = scratch0();
-    __ sub(r0, r0, Operand(kHeapObjectTag));
     __ mov(top_address, Operand(allocation_top));
     __ str(r0, MemOperand(top_address));
-    __ add(r0, r0, Operand(kHeapObjectTag));
   }
 }
 
 void LCodeGen::DoFastAllocate(LFastAllocate* instr) {
   DCHECK(instr->hydrogen()->IsAllocationFolded());
   DCHECK(!instr->hydrogen()->IsAllocationFoldingDominator());
   Register result = ToRegister(instr->result());
   Register scratch1 = ToRegister(instr->temp1());
   Register scratch2 = ToRegister(instr->temp2());
 
@@ skipping 371 matching lines @@
   __ ldr(result, FieldMemOperand(scratch,
                                  FixedArray::kHeaderSize - kPointerSize));
   __ bind(deferred->exit());
   __ bind(&done);
 }
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8