[csa] Migrate String.prototype.concat to TurboFan builtin.

src/builtins/builtins-string-gen.cc

 // Copyright 2017 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/builtins/builtins-regexp-gen.h"
 #include "src/builtins/builtins-utils-gen.h"
 #include "src/builtins/builtins.h"
 #include "src/code-factory.h"
 #include "src/code-stub-assembler.h"
 #include "src/objects.h"
@@ skipping 644 matching lines @@
 
     Bind(&if_positioninbounds);
   }
 
   // Load the character at the {position} from the {receiver}.
   Node* value = StringCharCodeAt(receiver, position);
   Node* result = SmiFromWord32(value);
   Return(result);
 }
 
+// ES6 String.prototype.concat(...args)
+// #sec-string.prototype.concat
+TF_BUILTIN(StringPrototypeConcat, CodeStubAssembler) {
+  CodeStubArguments arguments(
+      this, ChangeInt32ToIntPtr(Parameter(BuiltinDescriptor::kArgumentsCount)));
+  Node* receiver = arguments.GetReceiver();
+  Node* context = Parameter(BuiltinDescriptor::kContext);
+
+  // Check that {receiver} is coercible to Object and convert it to a String.
+  receiver = ToThisString(context, receiver, "String.prototype.concat");
+
+  // Concatenate all the arguments passed to this builtin.
+  Variable var_result(this, MachineRepresentation::kTagged);
+  var_result.Bind(receiver);
+  arguments.ForEach(
+      CodeStubAssembler::VariableList({&var_result}, zone()),
+      [this, context, &var_result](Node* arg) {
+        arg = CallStub(CodeFactory::ToString(isolate()), context, arg);
+        var_result.Bind(CallStub(CodeFactory::StringAdd(isolate()), context,
+                                 var_result.value(), arg));
+      });
+  arguments.PopAndReturn(var_result.value());
+}
+
 void StringBuiltinsAssembler::StringIndexOf(
     Node* receiver, Node* instance_type, Node* search_string,
     Node* search_string_instance_type, Node* position,
     std::function<void(Node*)> f_return) {
   CSA_ASSERT(this, IsString(receiver));
   CSA_ASSERT(this, IsString(search_string));
   CSA_ASSERT(this, TaggedIsSmi(position));
 
   Label zero_length_needle(this),
       call_runtime_unchecked(this, Label::kDeferred), return_minus_1(this),
@@ skipping 935 matching lines @@
     CallRuntime(Runtime::kThrowIncompatibleMethodReceiver, context,
                 HeapConstant(factory()->NewStringFromAsciiChecked(
                     "String Iterator.prototype.next", TENURED)),
                 iterator);
     Unreachable();
   }
 }
 
 }  // namespace internal
 }  // namespace v8