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

src/mips/macro-assembler-mips.cc

Index: src/mips/macro-assembler-mips.cc
diff --git a/src/mips/macro-assembler-mips.cc b/src/mips/macro-assembler-mips.cc
index f554b0c1efc84757d5edc7bd2923272dc1522598..8ca73e60e6cc9ab26736ffd5484e3e553a441853 100644
--- a/src/mips/macro-assembler-mips.cc
+++ b/src/mips/macro-assembler-mips.cc
@@ -4892,8 +4892,9 @@ void MacroAssembler::Prologue(bool code_pre_aging) {
     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 {