Ported SubStringStub to X64.

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 556 matching lines @@
 
   void movsxbq(Register dst, const Operand& src);
   void movsxwq(Register dst, const Operand& src);
   void movsxlq(Register dst, Register src);
   void movsxlq(Register dst, const Operand& src);
   void movzxbq(Register dst, const Operand& src);
   void movzxbl(Register dst, const Operand& src);
   void movzxwq(Register dst, const Operand& src);
   void movzxwl(Register dst, const Operand& src);
 
+  // Repeated moves.
+
+  void repmovsb();
+  void repmovsw();
+  void repmovsl();
+  void repmovsq();
+
   // New x64 instruction to load from an immediate 64-bit pointer into RAX.
   void load_rax(void* ptr, RelocInfo::Mode rmode);
   void load_rax(ExternalReference ext);
 
   // Conditional moves.
   void cmovq(Condition cc, Register dst, Register src);
   void cmovq(Condition cc, Register dst, const Operand& src);
   void cmovl(Condition cc, Register dst, Register src);
   void cmovl(Condition cc, Register dst, const Operand& src);
 
@@ skipping 808 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_