Provide a tagged allocation top pointer.

src/crankshaft/arm64/lithium-codegen-arm64.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.
 
 #include "src/crankshaft/arm64/lithium-codegen-arm64.h"
 
 #include "src/arm64/frames-arm64.h"
 #include "src/base/bits.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
@@ skipping 1475 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 = x10;
-    __ Sub(x0, x0, Operand(kHeapObjectTag));
     __ Mov(top_address, Operand(allocation_top));
     __ Str(x0, MemOperand(top_address));
-    __ Add(x0, x0, 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 4229 matching lines @@
   // Index is equal to negated out of object property index plus 1.
   __ Sub(result, result, Operand::UntagSmiAndScale(index, kPointerSizeLog2));
   __ Ldr(result, FieldMemOperand(result,
                                  FixedArray::kHeaderSize - kPointerSize));
   __ Bind(deferred->exit());
   __ Bind(&done);
 }
 
 }  // namespace internal
 }  // namespace v8