Add two-byte string support in codeunitAt intrinsic.

runtime/vm/intrinsifier_ia32.cc

 // Copyright (c) 2013, 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.
 //
 // The intrinsic code below is executed before a method has built its frame.
 // The return address is on the stack and the arguments below it.
 // Registers EDX (arguments descriptor) and ECX (function) must be preserved.
 // Each intrinsification method returns true if the corresponding
 // Dart method was intrinsified.
 
@@ skipping 1492 matching lines @@
 
 
 bool Intrinsifier::String_getLength(Assembler* assembler) {
   __ movl(EAX, Address(ESP, + 1 * kWordSize));  // String object.
   __ movl(EAX, FieldAddress(EAX, String::length_offset()));
   __ ret();
   return true;
 }
 
 
-// TODO(srdjan): Implement for two and four byte strings as well.
 bool Intrinsifier::String_codeUnitAt(Assembler* assembler) {
-  Label fall_through;
+  Label fall_through, try_two_byte_string;
   __ movl(EBX, Address(ESP, + 1 * kWordSize));  // Index.
   __ movl(EAX, Address(ESP, + 2 * kWordSize));  // String.
   __ testl(EBX, Immediate(kSmiTagMask));
   __ j(NOT_ZERO, &fall_through, Assembler::kNearJump);  // Non-smi index.
   // Range check.
   __ cmpl(EBX, FieldAddress(EAX, String::length_offset()));
   // Runtime throws exception.
   __ j(ABOVE_EQUAL, &fall_through, Assembler::kNearJump);
   __ CompareClassId(EAX, kOneByteStringCid, EDI);
-  __ j(NOT_EQUAL, &fall_through);
+  __ j(NOT_EQUAL, &try_two_byte_string, Assembler::kNearJump);
   __ SmiUntag(EBX);
   __ movzxb(EAX, FieldAddress(EAX, EBX, TIMES_1, OneByteString::data_offset()));
   __ SmiTag(EAX);
   __ ret();
+
+  __ Bind(&try_two_byte_string);
+  __ CompareClassId(EAX, kTwoByteStringCid, EDI);
+  __ j(NOT_EQUAL, &fall_through, Assembler::kNearJump);
+  ASSERT(kSmiTagShift == 1);
+  __ movzxw(EAX, FieldAddress(EAX, EBX, TIMES_1, OneByteString::data_offset()));
+  __ SmiTag(EAX);
+  __ ret();
+
   __ Bind(&fall_through);
   return false;
 }
 
 
 bool Intrinsifier::String_getIsEmpty(Assembler* assembler) {
   Label is_true;
   // Get length.
   __ movl(EAX, Address(ESP, + 1 * kWordSize));  // String object.
   __ movl(EAX, FieldAddress(EAX, String::length_offset()));
@@ skipping 221 matching lines @@
   __ ret();
 
   __ Bind(&fall_through);
   return false;
 }
 
 #undef __
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32