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

src/mips/macro-assembler-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 <limits.h>  // For LONG_MIN, LONG_MAX.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_MIPS
 
@@ skipping 4874 matching lines @@
   // for code aging to work properly.
   if (code_pre_aging) {
     // Pre-age the code.
     Code* stub = Code::GetPreAgedCodeAgeStub(isolate());
     nop(Assembler::CODE_AGE_MARKER_NOP);
     // 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())),
        CONSTANT_SIZE);
-    nop();  // Prevent jalr to jal optimization.
-    jalr(t9, a0);
+    // Save the function's original return address.
+    mov(at, ra);  // This also prevents jalr->jal optimization.
+    jalr(t9);
     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 1219 matching lines @@
   if (mag.shift > 0) sra(result, result, mag.shift);
   srl(at, dividend, 31);
   Addu(result, result, Operand(at));
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_MIPS