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

runtime/vm/assembler_x64_test.cc

 // Copyright (c) 2012, 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/os.h"
 #include "vm/unit_test.h"
@@ skipping 2288 matching lines @@
   __ ret();
 }
 
 
 ASSEMBLER_TEST_RUN(TestSetCC, test) {
   typedef uword (*TestSetCC)();
   uword res = reinterpret_cast<TestSetCC>(test->entry())();
   EXPECT_EQ(0xFFFFFF00, res);
 }
 
+
+ASSEMBLER_TEST_GENERATE(TestRepMovsBytes, assembler) {
+  // Save incoming arguments.
+  __ pushq(RDI);  // Arg0, from.
+  __ pushq(RSI);  // Arg1, to.
+  __ pushq(RDX);  // Arg2, count.
+  __ movq(RSI, Address(RSP, 2 * kWordSize));  // from.
+  __ movq(RDI, Address(RSP, 1 * kWordSize));  // to.
+  __ movq(RCX, Address(RSP, 0 * kWordSize));  // count.
+  __ rep_movsb();
+  // Remove saved arguments.
+  __ popq(RAX);
+  __ popq(RAX);
+  __ popq(RAX);
+  __ ret();
+}
+
+
+ASSEMBLER_TEST_RUN(TestRepMovsBytes, test) {
+  const char* from = "0123456789";
+  const char* to = new char[10];
+  typedef void (*TestRepMovsBytes)(const char* from, const char* to, int count);
+  reinterpret_cast<TestRepMovsBytes>(test->entry())(from, to, 10);
+  EXPECT_EQ(to[0], '0');
+  for (int i = 0; i < 10; i++) {
+    EXPECT_EQ(from[i], to[i]);
+  }
+  delete [] to;
+}
+
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64