Use rep_movsb for copying one-byte strings in substring intrinsic.

runtime/vm/assembler_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.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_IA32)
 
 #include "vm/assembler.h"
 #include "vm/code_generator.h"
 #include "vm/heap.h"
@@ skipping 312 matching lines @@
 
 
 void Assembler::cmovns(Register dst, Register src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0x0F);
   EmitUint8(0x49);
   EmitRegisterOperand(dst, src);
 }
 
 
+void Assembler::rep_movsb() {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0xF3);
+  EmitUint8(0xA4);
+}
+
+
 void Assembler::movss(XmmRegister dst, const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0xF3);
   EmitUint8(0x0F);
   EmitUint8(0x10);
   EmitOperand(dst, src);
 }
 
 
 void Assembler::movss(const Address& dst, XmmRegister src) {
@@ skipping 2027 matching lines @@
 
 const char* Assembler::FpuRegisterName(FpuRegister reg) {
   ASSERT((0 <= reg) && (reg < kNumberOfXmmRegisters));
   return xmm_reg_names[reg];
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32