Clean up multi byte nop support on x64 to more closely match IA32.

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 618 matching lines @@
   // - Instructions on 32-bit (doubleword) operands/registers use 'l'.
   // - Instructions on 64-bit (quadword) operands/registers use 'q'.
   //
   // Some mnemonics, such as "and", are the same as C++ keywords.
   // Naming conflicts with C++ keywords are resolved by adding a trailing '_'.
 
   // Insert the smallest number of nop instructions
   // possible to align the pc offset to a multiple
   // of m, where m must be a power of 2.
   void Align(int m);
+  void Nop(int bytes = 1);
   // Aligns code to something that's optimal for a jump target for the platform.
   void CodeTargetAlign();
 
   // Stack
   void pushfq();
   void popfq();
 
   void push(Immediate value);
   // Push a 32 bit integer, and guarantee that it is actually pushed as a
   // 32 bit value, the normal push will optimize the 8 bit case.
@@ skipping 498 matching lines @@
   void bt(const Operand& dst, Register src);
   void bts(const Operand& dst, Register src);
 
   // Miscellaneous
   void clc();
   void cld();
   void cpuid();
   void hlt();
   void int3();
   void nop();
-  void nop(int n);
   void rdtsc();
   void ret(int imm16);
   void setcc(Condition cc, Register reg);
 
   // Label operations & relative jumps (PPUM Appendix D)
   //
   // Takes a branch opcode (cc) and a label (L) and generates
   // either a backward branch or a forward branch and links it
   // to the label fixup chain. Usage:
   //
@@ skipping 223 matching lines @@
   // an instruction or relocation information.
   inline bool buffer_overflow() const {
     return pc_ >= reloc_info_writer.pos() - kGap;
   }
 
   // Get the number of bytes available in the buffer.
   inline int available_space() const {
     return static_cast<int>(reloc_info_writer.pos() - pc_);
   }
 
-  static bool IsNop(Address addr) { return *addr == 0x90; }
+  static bool IsNop(Address addr);
 
   // Avoid overflows for displacements etc.
   static const int kMaximalBufferSize = 512*MB;
   static const int kMinimalBufferSize = 4*KB;
 
   byte byte_at(int pos)  { return buffer_[pos]; }
   void set_byte_at(int pos, byte value) { buffer_[pos] = value; }
 
  protected:
   bool emit_debug_code() const { return emit_debug_code_; }
@@ skipping 229 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_