MIPS: Fix and simplify code aging.

src/mips/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 624 matching lines @@
   return result;
 }
 
 
 void Code::GetCodeAgeAndParity(byte* sequence, Age* age,
                                MarkingParity* parity) {
   if (IsYoungSequence(sequence)) {
     *age = kNoAgeCodeAge;
     *parity = NO_MARKING_PARITY;
   } else {
-    Address target_address = Memory::Address_at(
-        sequence + Assembler::kInstrSize * (kNoCodeAgeSequenceLength - 1));
+    Address target_address = Assembler::target_address_at(
+        sequence + Assembler::kInstrSize);
     Code* stub = GetCodeFromTargetAddress(target_address);
     GetCodeAgeAndParity(stub, age, parity);
   }
 }
 
 
 void Code::PatchPlatformCodeAge(Isolate* isolate,
                                 byte* sequence,
                                 Code::Age age,
                                 MarkingParity parity) {
   uint32_t young_length;
   byte* young_sequence = GetNoCodeAgeSequence(&young_length);
   if (age == kNoAgeCodeAge) {
     CopyBytes(sequence, young_sequence, young_length);
     CPU::FlushICache(sequence, young_length);
   } else {
     Code* stub = GetCodeAgeStub(isolate, age, parity);
     CodePatcher patcher(sequence, young_length / Assembler::kInstrSize);
-    // Mark this code sequence for FindPlatformCodeAgeSequence()
+    // Mark this code sequence for FindPlatformCodeAgeSequence().
     patcher.masm()->nop(Assembler::CODE_AGE_MARKER_NOP);
-    // Save the function's original return address
-    // (it will be clobbered by Call(t9))
-    patcher.masm()->mov(at, ra);
-    // Load the stub address to t9 and call it
-    patcher.masm()->li(t9,
-        Operand(reinterpret_cast<uint32_t>(stub->instruction_start())));
-    patcher.masm()->Call(t9);
-    // Record the stub address in the empty space for GetCodeAgeAndParity()
-    patcher.masm()->emit_code_stub_address(stub);
+    // Load the stub address to t9 and call it,
+    // GetCodeAgeAndParity() extracts the stub address from this instruction.
+    patcher.masm()->li(
+        t9,
+        Operand(reinterpret_cast<uint32_t>(stub->instruction_start())),
+        CONSTANT_SIZE);
+    patcher.masm()->nop();  // Prevent jalr to jal optimization.
+    patcher.masm()->jalr(t9, a0);
+    patcher.masm()->nop();  // Branch delay slot nop.
+    patcher.masm()->nop();  // Pad the empty space.
   }
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS