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

runtime/vm/assembler_x64.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_X64)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 372 matching lines @@
 
 
 void Assembler::movsxd(Register dst, const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitOperandREX(dst, src, REX_W);
   EmitUint8(0x63);
   EmitOperand(dst & 7, src);
 }
 
 
+void Assembler::rep_movsb() {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0xF3);
+  EmitUint8(0xA4);
+}
+
+
 void Assembler::leaq(Register dst, const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitOperandREX(dst, src, REX_W);
   EmitUint8(0x8D);
   EmitOperand(dst & 7, src);
 }
 
 
 void Assembler::movss(XmmRegister dst, const Address& src) {
   ASSERT(dst <= XMM15);
@@ skipping 2110 matching lines @@
 
 const char* Assembler::FpuRegisterName(FpuRegister reg) {
   ASSERT((0 <= reg) && (reg < kNumberOfXmmRegisters));
   return xmm_reg_names[reg];
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64