[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/js-native-context-specialization.cc

 // Copyright 2015 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/js-native-context-specialization.h"
 
 #include "src/accessors.h"
 #include "src/code-factory.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
@@ skipping 555 matching lines @@
 
     // Determine the {receiver} length.
     Node* length = effect = graph()->NewNode(
         simplified()->LoadField(AccessBuilder::ForStringLength()), receiver,
         effect, control);
 
     // Ensure that {index} is less than {receiver} length.
     index = effect = graph()->NewNode(simplified()->CheckBounds(), index,
                                       length, effect, control);
 
-    // Load the character from the {receiver}.
-    value = graph()->NewNode(simplified()->StringCharCodeAt(), receiver, index,
+    // Return the character from the {receiver} as single character string.
+    value = graph()->NewNode(simplified()->StringCharAt(), receiver, index,
                              control);
-
-    // Return it as a single character string.
-    value = graph()->NewNode(simplified()->StringFromCharCode(), value);
   } else {
     // Retrieve the native context from the given {node}.
     // Compute element access infos for the receiver maps.
     AccessInfoFactory access_info_factory(dependencies(), native_context(),
                                           graph()->zone());
     ZoneVector<ElementAccessInfo> access_infos(zone());
     if (!access_info_factory.ComputeElementAccessInfos(
             receiver_maps, access_mode, &access_infos)) {
       return NoChange();
     }
@@ skipping 232 matching lines @@
             factory()->LookupSingleCharacterStringFromCode(
                 string->Get(static_cast<int>(mindex.Value()))));
         ReplaceWithValue(node, value, effect, control);
         return Replace(value);
       } else if (flags() & kDeoptimizationEnabled) {
         // Ensure that {index} is less than {receiver} length.
         Node* length = jsgraph()->Constant(string->length());
         index = effect = graph()->NewNode(simplified()->CheckBounds(), index,
                                           length, effect, control);
 
-        // Load the character from the {receiver}.
-        value = graph()->NewNode(simplified()->StringCharCodeAt(), receiver,
-                                 index, control);
-
-        // Return it as a single character string.
-        value = graph()->NewNode(simplified()->StringFromCharCode(), value);
+        // Return the character from the {receiver} as single character string.
+        value = graph()->NewNode(simplified()->StringCharAt(), receiver, index,
+                                 control);
         ReplaceWithValue(node, value, effect, control);
         return Replace(value);
       }
     }
   }
 
   // Check if the {nexus} reports type feedback for the IC.
   if (nexus.IsUninitialized()) {
     if ((flags() & kDeoptimizationEnabled) &&
         (flags() & kBailoutOnUninitialized)) {
@@ skipping 979 matching lines @@
   return jsgraph()->javascript();
 }
 
 SimplifiedOperatorBuilder* JSNativeContextSpecialization::simplified() const {
   return jsgraph()->simplified();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8