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

src/mips/codegen-mips.cc

Index: src/mips/codegen-mips.cc
diff --git a/src/mips/codegen-mips.cc b/src/mips/codegen-mips.cc
index 23780d8b2597679c747b7eefa78bbcd34e4685de..956407bc578b7cc3633aa9282e0b8ba504e0dae3 100644
--- a/src/mips/codegen-mips.cc
+++ b/src/mips/codegen-mips.cc
@@ -1252,8 +1252,9 @@ void Code::PatchPlatformCodeAge(Isolate* isolate,
         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.
   }