Provide a tagged allocation top pointer.

src/crankshaft/mips/lithium-codegen-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.7
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5112 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();
-    __ Subu(v0, v0, Operand(kHeapObjectTag));
     __ li(top_address, Operand(allocation_top));
     __ sw(v0, MemOperand(top_address));
-    __ Addu(v0, v0, 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 400 matching lines @@
   __ lw(result, FieldMemOperand(scratch,
                                 FixedArray::kHeaderSize - kPointerSize));
   __ bind(deferred->exit());
   __ bind(&done);
 }
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8