X64: Implement StringCharAtStub which is used from lithium codegen.

This fixes issue 1116 and enables the stub in lithium codegen.

Review URL: http://codereview.chromium.org/6588113

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7013 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/code-stubs-x64.cc

@@ -4833,6 +4833,61 @@ void StringCompareStub::Generate(MacroAssembler* masm) {
   __ TailCallRuntime(Runtime::kStringCompare, 2, 1);
 }
 
+
+void StringCharAtStub::Generate(MacroAssembler* masm) {
+  // Expects two arguments (object, index) on the stack:
+
+  // Stack frame on entry.
+  //  rsp[0]: return address
+  //  rsp[8]: index
+  //  rsp[16]: object
+
+  Register object = rbx;
+  Register index = rax;
+  Register scratch1 = rcx;
+  Register scratch2 = rdx;
+  Register result = rax;
+
+  __ pop(scratch1);  // Return address.
+  __ pop(index);
+  __ pop(object);
+  __ push(scratch1);
+
+  Label need_conversion;
+  Label index_out_of_range;
+  Label done;
+  StringCharAtGenerator generator(object,
+                                  index,
+                                  scratch1,
+                                  scratch2,
+                                  result,
+                                  &need_conversion,
+                                  &need_conversion,
+                                  &index_out_of_range,
+                                  STRING_INDEX_IS_NUMBER);
+  generator.GenerateFast(masm);
+  __ jmp(&done);
+
+  __ bind(&index_out_of_range);
+  // When the index is out of range, the spec requires us to return
+  // the empty string.
+  __ Move(result, Factory::empty_string());
+  __ jmp(&done);
+
+  __ bind(&need_conversion);
+  // Move smi zero into the result register, which will trigger
+  // conversion.
+  __ Move(result, Smi::FromInt(0));
+  __ jmp(&done);
+
+  StubRuntimeCallHelper call_helper;
+  generator.GenerateSlow(masm, call_helper);
+
+  __ bind(&done);
+  __ ret(0);
+}
+
+
 void ICCompareStub::GenerateSmis(MacroAssembler* masm) {
   ASSERT(state_ == CompareIC::SMIS);
   NearLabel miss;

src/x64/lithium-codegen-x64.cc

@@ -705,8 +705,8 @@ void LCodeGen::DoCallStub(LCallStub* instr) {
       break;
     }
     case CodeStub::StringCharAt: {
-      // TODO(1116): Add StringCharAt stub to x64.
-      Abort("Unimplemented: %s", "StringCharAt Stub");
+      StringCharAtStub stub;
+      CallCode(stub.GetCode(), RelocInfo::CODE_TARGET, instr);
       break;
     }
     case CodeStub::MathPow: {