MIPS: Avoid use of a0 as return address in jalr.

src/mips/codegen-mips.cc

 // Copyright 2012 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/v8.h"
 
 #if V8_TARGET_ARCH_MIPS
 
 #include "src/codegen.h"
 #include "src/macro-assembler.h"
@@ skipping 1234 matching lines @@
     Code* stub = GetCodeAgeStub(isolate, age, parity);
     CodePatcher patcher(sequence, young_length / Assembler::kInstrSize);
     // Mark this code sequence for FindPlatformCodeAgeSequence().
     patcher.masm()->nop(Assembler::CODE_AGE_MARKER_NOP);
     // 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);
+    // Save the function's original return address.
+    patcher.masm()->mov(at, ra);  // This also prevents jalr->jal optimization.
+    patcher.masm()->jalr(t9);
     patcher.masm()->nop();  // Branch delay slot nop.
     patcher.masm()->nop();  // Pad the empty space.
   }
 }
 
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_MIPS