MIPS: Improve code aging sequence.

src/mips/lithium-codegen-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // 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 141 matching lines @@
   if (NeedsEagerFrame()) {
     if (info()->IsStub()) {
       __ Push(ra, fp, cp);
       __ Push(Smi::FromInt(StackFrame::STUB));
       // Adjust FP to point to saved FP.
       __ Addu(fp, sp, Operand(2 * kPointerSize));
     } else {
       // The following three instructions must remain together and unmodified
       // for code aging to work properly.
       __ Push(ra, fp, cp, a1);
-      // Add unused load of ip to ensure prologue sequence is identical for
+      // Add unused nop to ensure prologue sequence is identical for
       // full-codegen and lithium-codegen.
-      __ LoadRoot(at, Heap::kUndefinedValueRootIndex);
+      __ nop(Assembler::CODE_AGE_SEQUENCE_NOP);
       // Adj. FP to point to saved FP.
       __ Addu(fp, sp, Operand(2 * kPointerSize));
     }
     frame_is_built_ = true;
     info_->AddNoFrameRange(0, masm_->pc_offset());
   }
 
   // Reserve space for the stack slots needed by the code.
   int slots = GetStackSlotCount();
   if (slots > 0) {
@@ skipping 5632 matching lines @@
   __ Subu(scratch, result, scratch);
   __ lw(result, FieldMemOperand(scratch,
                                 FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal