MIPS: Fix and simplify code aging.

src/mips/macro-assembler-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 4508 matching lines @@
     Addu(fp, sp, Operand(StandardFrameConstants::kFixedFrameSizeFromFp));
   } else {
     PredictableCodeSizeScope predictible_code_size_scope(
       this, kNoCodeAgeSequenceLength * Assembler::kInstrSize);
     // The following three instructions must remain together and unmodified
     // for code aging to work properly.
     if (isolate()->IsCodePreAgingActive()) {
       // Pre-age the code.
       Code* stub = Code::GetPreAgedCodeAgeStub(isolate());
       nop(Assembler::CODE_AGE_MARKER_NOP);
-      // Save the function's original return address
-      // (it will be clobbered by Call(t9)).
-      mov(at, ra);
-      // Load the stub address to t9 and call it.
+      // Load the stub address to t9 and call it,
+      // GetCodeAgeAndParity() extracts the stub address from this instruction.
       li(t9,
-         Operand(reinterpret_cast<uint32_t>(stub->instruction_start())));
-      Call(t9);
-      // Record the stub address in the empty space for GetCodeAgeAndParity().
-      emit_code_stub_address(stub);
+         Operand(reinterpret_cast<uint32_t>(stub->instruction_start())),
+         CONSTANT_SIZE);
+      nop();  // Prevent jalr to jal optimization.
+      jalr(t9, a0);
+      nop();  // Branch delay slot nop.
+      nop();  // Pad the empty space.
     } else {
       Push(ra, fp, cp, a1);
       nop(Assembler::CODE_AGE_SEQUENCE_NOP);
       // Adjust fp to point to caller's fp.
       Addu(fp, sp, Operand(StandardFrameConstants::kFixedFrameSizeFromFp));
     }
   }
 }
 
 
@@ skipping 1201 matching lines @@
        opcode == BGTZL);
   opcode = (cond == eq) ? BEQ : BNE;
   instr = (instr & ~kOpcodeMask) | opcode;
   masm_.emit(instr);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS