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

runtime/vm/assembler_x64_test.cc

Index: runtime/vm/assembler_x64_test.cc
===================================================================
--- runtime/vm/assembler_x64_test.cc	(revision 21290)
+++ runtime/vm/assembler_x64_test.cc	(working copy)
@@ -2306,6 +2306,36 @@
   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