Add a TurboFan skeleton for StringAddStub.

src/compiler/arm64/code-generator-arm64.cc

 // Copyright 2014 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/arm64/macro-assembler-arm64.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 1060 matching lines @@
     __ Mov(fp, csp);
     // TODO(dcarney): correct callee saved registers.
     __ PushCalleeSavedRegisters();
     frame()->SetRegisterSaveAreaSize(20 * kPointerSize);
   } else if (descriptor->IsJSFunctionCall()) {
     CompilationInfo* info = this->info();
     __ SetStackPointer(jssp);
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ SetStackPointer(jssp);
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
   }
 
   if (info()->is_osr()) {
     // TurboFan OSR-compiled functions cannot be entered directly.
     __ Abort(kShouldNotDirectlyEnterOsrFunction);
 
@@ skipping 28 matching lines @@
       if (stack_slots > 0) {
         __ Add(csp, csp, AlignedStackSlots(stack_slots) * kPointerSize);
       }
       // Restore registers.
       // TODO(dcarney): correct callee saved registers.
       __ PopCalleeSavedRegisters();
     }
     __ Mov(csp, fp);
     __ Pop(fp, lr);
     __ Ret();
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     __ Mov(jssp, fp);
     __ Pop(fp, lr);
     int pop_count = descriptor->IsJSFunctionCall()
                         ? static_cast<int>(descriptor->JSParameterCount())
                         : 0;
     __ Drop(pop_count);
     __ Ret();
   } else {
     __ Ret();
   }
@@ skipping 181 matching lines @@
     }
   }
   MarkLazyDeoptSite();
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8