[stubs,interpreter] Optimise SMI loading for 64-bit targets.

src/builtins/builtins-string.cc

 // Copyright 2016 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.h"
 #include "src/builtins/builtins-utils.h"
 
 #include "src/code-factory.h"
 
 namespace v8 {
@@ skipping 33 matching lines @@
     Node* code32 = assembler->TruncateTaggedToWord32(context, code);
     Node* code16 = assembler->Word32And(
         code32, assembler->Int32Constant(String::kMaxUtf16CodeUnit));
     Node* result = assembler->StringFromCharCode(code16);
     assembler->Return(result);
   }
 
   assembler->Bind(&if_notoneargument);
   {
     // Determine the resulting string length.
-    Node* parent_frame_length =
-        assembler->Load(MachineType::Pointer(), parent_frame_pointer,
-                        assembler->IntPtrConstant(
-                            ArgumentsAdaptorFrameConstants::kLengthOffset));
-    Node* length = assembler->SmiToWord(parent_frame_length);
+    Node* length = assembler->LoadAndUntagSmi(
+        parent_frame_pointer, ArgumentsAdaptorFrameConstants::kLengthOffset);
 
     // Assume that the resulting string contains only one-byte characters.
     Node* result = assembler->AllocateSeqOneByteString(context, length);
 
     // Truncate all input parameters and append them to the resulting string.
     Variable var_offset(assembler, MachineType::PointerRepresentation());
     Label loop(assembler, &var_offset), done_loop(assembler);
     var_offset.Bind(assembler->IntPtrConstant(0));
     assembler->Goto(&loop);
     assembler->Bind(&loop);
@@ skipping 451 matching lines @@
   Node* receiver = assembler->Parameter(0);
   Node* context = assembler->Parameter(3);
 
   Node* result = assembler->ToThisValue(
       context, receiver, PrimitiveType::kString, "String.prototype.valueOf");
   assembler->Return(result);
 }
 
 }  // namespace internal
 }  // namespace v8