Ported SubStringStub to X64.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index 4ac39339c90cc6fc27efc89b60709d434bac4349..8a3e869de71badd672af477fee4711cc33771169 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -1537,6 +1537,40 @@ void Assembler::movzxwl(Register dst, const Operand& src) {
 }
 
 
+void Assembler::repmovsb() {
+  EnsureSpace ensure_space(this);
+  last_pc_ = pc_;
+  emit(0xF3);
+  emit(0xA4);
+}
+
+
+void Assembler::repmovsw() {
+  EnsureSpace ensure_space(this);
+  last_pc_ = pc_;
+  emit(0x66);  // Operand size override.
+  emit(0xF3);
+  emit(0xA4);
+}
+
+
+void Assembler::repmovsl() {
+  EnsureSpace ensure_space(this);
+  last_pc_ = pc_;
+  emit(0xF3);
+  emit(0xA5);
+}
+
+
+void Assembler::repmovsq() {
+  EnsureSpace ensure_space(this);
+  last_pc_ = pc_;
+  emit(0xF3);
+  emit_rex_64();
+  emit(0xA5);
+}
+
+
 void Assembler::mul(Register src) {
   EnsureSpace ensure_space(this);
   last_pc_ = pc_;