Add a TurboFan skeleton for StringAddStub.

src/compiler/x64/code-generator-x64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
 #include "src/scopes.h"
@@ skipping 1433 matching lines @@
         __ pushq(Register::from_code(i));
         register_save_area_size += kPointerSize;
       }
       frame()->SetRegisterSaveAreaSize(register_save_area_size);
     }
   } else if (descriptor->IsJSFunctionCall()) {
     CompilationInfo* info = this->info();
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
   }
 
   if (info()->is_osr()) {
     // TurboFan OSR-compiled functions cannot be entered directly.
     __ Abort(kShouldNotDirectlyEnterOsrFunction);
 
     // Unoptimized code jumps directly to this entrypoint while the unoptimized
@@ skipping 32 matching lines @@
         }
       }
       __ popq(rbp);  // Pop caller's frame pointer.
       __ ret(0);
     } else {
       // No saved registers.
       __ movq(rsp, rbp);  // Move stack pointer back to frame pointer.
       __ popq(rbp);       // Pop caller's frame pointer.
       __ ret(0);
     }
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     __ movq(rsp, rbp);  // Move stack pointer back to frame pointer.
     __ popq(rbp);       // Pop caller's frame pointer.
     int pop_count = descriptor->IsJSFunctionCall()
                         ? static_cast<int>(descriptor->JSParameterCount())
                         : 0;
     __ Ret(pop_count * kPointerSize, rbx);
   } else {
     __ ret(0);
   }
 }
@@ skipping 187 matching lines @@
     }
   }
   MarkLazyDeoptSite();
 }
 
 #undef __
 
 }  // namespace internal
 }  // namespace compiler
 }  // namespace v8