Narrow String::CharAt from int32_t to uint16_t.

runtime/lib/string.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/bootstrap_natives.h"
 
 #include "include/dart_api.h"
 #include "vm/exceptions.h"
 #include "vm/dart_api_impl.h"
 #include "vm/isolate.h"
@@ skipping 200 matching lines @@
   return Smi::New(hash_val);
 }
 
 
 DEFINE_NATIVE_ENTRY(String_getLength, 1) {
   const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   return Smi::New(receiver.Length());
 }
 
 
-static int32_t StringValueAt(const String& str, const Integer& index) {
+static uint16_t StringValueAt(const String& str, const Integer& index) {
   if (index.IsSmi()) {
     const intptr_t index_value = Smi::Cast(index).Value();
     if ((0 <= index_value) && (index_value < str.Length())) {
       return str.CharAt(index_value);
     }
   }
 
   // An index larger than Smi is always illegal.
   Exceptions::ThrowRangeError("index", index, 0, str.Length());
   return 0;
 }
 
 
 DEFINE_NATIVE_ENTRY(String_charAt, 2) {
   const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Integer, index, arguments->NativeArgAt(1));
-  uint32_t value = StringValueAt(receiver, index);
-  ASSERT(value <= 0x10FFFF);
-  return Symbols::FromCharCode(value);
+  uint16_t value = StringValueAt(receiver, index);
+  return Symbols::FromCharCode(static_cast<int32_t>(value));
 }
 
 
+// Returns the 16-bit UTF-16 code unit at the given index.
 DEFINE_NATIVE_ENTRY(String_codeUnitAt, 2) {
   const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Integer, index, arguments->NativeArgAt(1));
-
-  int32_t value = StringValueAt(receiver, index);
-  ASSERT(value >= 0);
-  ASSERT(value <= 0xFFFF);
-  return Smi::New(value);
+  uint16_t value = StringValueAt(receiver, index);
+  return Smi::New(static_cast<intptr_t>(value));
 }
 
 
 DEFINE_NATIVE_ENTRY(String_concat, 2) {
   const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(String, b, arguments->NativeArgAt(1));
   return String::Concat(receiver, b);
 }
 
 
@@ skipping 60 matching lines @@
       ? String::Handle(OneByteString::New(length_value, Heap::kNew))
       : String::Handle(TwoByteString::New(length_value, Heap::kNew));
   NoGCScope no_gc;
 
   uint16_t* data_position = reinterpret_cast<uint16_t*>(codeUnits.DataAddr(0));
   String::Copy(result, 0, data_position, length_value);
   return result.raw();
 }
 
 }  // namespace dart