Add two-byte string support in codeunitAt intrinsic.

runtime/vm/intrinsifier_mips.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.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intrinsifier.h"
 
 #include "vm/assembler.h"
@@ skipping 1490 matching lines @@
 
 
 bool Intrinsifier::String_getLength(Assembler* assembler) {
   __ lw(T0, Address(SP, 0 * kWordSize));
   __ Ret();
   __ delay_slot()->lw(V0, FieldAddress(T0, String::length_offset()));
   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;
 
   __ lw(T1, Address(SP, 0 * kWordSize));  // Index.
   __ lw(T0, Address(SP, 1 * kWordSize));  // String.
 
   // Checks.
   __ andi(CMPRES, T1, Immediate(kSmiTagMask));
   __ bne(T1, ZR, &fall_through);  // Index is not a Smi.
   __ lw(T2, FieldAddress(T0, String::length_offset()));  // Range check.
   // Runtime throws exception.
   __ BranchUnsignedGreaterEqual(T1, T2, &fall_through);
   __ LoadClassId(TMP1, T0);  // Class ID check.
-  __ BranchNotEqual(TMP1, kOneByteStringCid, &fall_through);
+  __ BranchNotEqual(TMP1, kOneByteStringCid, &try_two_byte_string);
 
   // Grab byte and return.
   __ SmiUntag(T1);
   __ addu(T2, T0, T1);
   __ lbu(V0, FieldAddress(T2, OneByteString::data_offset()));
   __ Ret();
   __ delay_slot()->SmiTag(V0);
+
+  __ Bind(&try_two_byte_string);
+  __ BranchNotEqual(TMP1, kTwoByteStringCid, &fall_through);
+  ASSERT(kSmiTagShift == 1);
+  __ addu(T2, T0, T1);
+  __ lhu(V0, FieldAddress(T2, OneByteString::data_offset()));
+  __ Ret();
+  __ delay_slot()->SmiTag(V0);
+
   __ Bind(&fall_through);
   return false;
 }
 
 
 bool Intrinsifier::String_getIsEmpty(Assembler* assembler) {
   Label is_true;
 
   __ lw(T0, Address(SP, 0 * kWordSize));
   __ lw(T0, FieldAddress(T0, String::length_offset()));
@@ skipping 226 matching lines @@
   __ Bind(&ok);
   __ Ret();
 
   __ Bind(&fall_through);
   return false;
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS