MIPS: Fix --new-string-add feature.

src/mips/full-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 3716 matching lines @@
 
 
 void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   ASSERT_EQ(2, args->length());
   if (FLAG_new_string_add) {
     VisitForStackValue(args->at(0));
     VisitForAccumulatorValue(args->at(1));
 
     __ pop(a1);
+    __ mov(a0, result_register());  // NewStringAddStub requires args in a0, a1.
     NewStringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
     __ CallStub(&stub);
   } else {
     VisitForStackValue(args->at(0));
     VisitForStackValue(args->at(1));
 
     StringAddStub stub(STRING_ADD_CHECK_BOTH);
     __ CallStub(&stub);
   }
   context()->Plug(v0);
@@ skipping 1228 matching lines @@
       Assembler::target_address_at(pc_immediate_load_address)) ==
          reinterpret_cast<uint32_t>(
              isolate->builtins()->OsrAfterStackCheck()->entry()));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS