Get rid of unused STRING_ADD_ERECT_FRAME flag.

src/x64/code-stubs-x64.cc

 // Copyright 2013 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 3757 matching lines @@
   StringHelper::GenerateCopyCharacters(masm, rbx, rcx, r14, false);
   // rbx: next character of result
   // rdx: first char of second string
   // r15: length of second string
   StringHelper::GenerateCopyCharacters(masm, rbx, rdx, r15, false);
   __ IncrementCounter(counters->string_add_native(), 1);
   __ ret(2 * kPointerSize);
 
   // Just jump to runtime to add the two strings.
   __ bind(&call_runtime);
-
-  if ((flags_ & STRING_ADD_ERECT_FRAME) != 0) {
-    GenerateRegisterArgsPop(masm, rcx);
-    // Build a frame
-    {
-      FrameScope scope(masm, StackFrame::INTERNAL);
-      GenerateRegisterArgsPush(masm);
-      __ CallRuntime(Runtime::kStringAdd, 2);
-    }
-    __ Ret();
-  } else {
-    __ TailCallRuntime(Runtime::kStringAdd, 2, 1);
-  }
+  __ TailCallRuntime(Runtime::kStringAdd, 2, 1);
 
   if (call_builtin.is_linked()) {
     __ bind(&call_builtin);
-    if ((flags_ & STRING_ADD_ERECT_FRAME) != 0) {
-      GenerateRegisterArgsPop(masm, rcx);
-      // Build a frame
-      {
-        FrameScope scope(masm, StackFrame::INTERNAL);
-        GenerateRegisterArgsPush(masm);
-        __ InvokeBuiltin(builtin_id, CALL_FUNCTION);
-      }
-      __ Ret();
-    } else {
-      __ InvokeBuiltin(builtin_id, JUMP_FUNCTION);
-    }
+    __ InvokeBuiltin(builtin_id, JUMP_FUNCTION);
   }
 }
 
 
 void StringAddStub::GenerateRegisterArgsPush(MacroAssembler* masm) {
   __ push(rax);
   __ push(rdx);
 }
 
 
@@ skipping 2022 matching lines @@
   __ bind(&fast_elements_case);
   GenerateCase(masm, FAST_ELEMENTS);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64