Fix potential overwriting of debug jumps of following code.

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 535 matching lines @@
   // Should equal Debug::kX64JSReturnSequenceLength - kCallTargetAddressOffset;
   static const int kPatchReturnSequenceAddressOffset = 13 - 4;
   // Distance between start of patched debug break slot and where the
   // 32-bit displacement of a near call would be, relative to the pushed
   // return address.  TODO: Use return sequence length instead.
   // Should equal Debug::kX64JSReturnSequenceLength - kCallTargetAddressOffset;
   static const int kPatchDebugBreakSlotAddressOffset = 13 - 4;
   // TODO(X64): Rename this, removing the "Real", after changing the above.
   static const int kRealPatchReturnSequenceAddressOffset = 2;
 
-  // The x64 JS return sequence is padded with int3 to make it large
-  // enough to hold a call instruction when the debugger patches it.
+  // Some x64 JS code is padded with int3 to make it large
+  // enough to hold an instruction when the debugger patches it.
+  static const int kJumpInstructionLength = 13;
   static const int kCallInstructionLength = 13;
   static const int kJSReturnSequenceLength = 13;
 
   // The debug break slot must be able to contain a call instruction.
   static const int kDebugBreakSlotLength = kCallInstructionLength;
 
   // One byte opcode for test eax,0xXXXXXXXX.
   static const byte kTestEaxByte = 0xA9;
 
   // ---------------------------------------------------------------------------
   // Code generation
   //
   // Function names correspond one-to-one to x64 instruction mnemonics.
   // Unless specified otherwise, instructions operate on 64-bit operands.
   //
   // If we need versions of an assembly instruction that operate on different
   // width arguments, we add a single-letter suffix specifying the width.
   // This is done for the following instructions: mov, cmp, inc, dec,
   // add, sub, and test.
   // There are no versions of these instructions without the suffix.
   // - Instructions on 8-bit (byte) operands/registers have a trailing 'b'.
   // - Instructions on 16-bit (word) operands/registers have a trailing 'w'.
   // - 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 '_'.
 
+  // Inserts nop instructions taking up n bytes.
+  void Pad(int n);
   // Insert the smallest number of nop instructions
   // possible to align the pc offset to a multiple
   // of m. m must be a power of 2.
   void Align(int m);
   // Aligns code to something that's optimal for a jump target for the platform.
   void CodeTargetAlign();
 
   // Stack
   void pushfq();
   void popfq();
@@ skipping 508 matching lines @@
   // but it may be bound only once.
 
   void bind(Label* L);  // binds an unbound label L to the current code position
   void bind(NearLabel* L);
 
   // Calls
   // Call near relative 32-bit displacement, relative to next instruction.
   void call(Label* L);
   void call(Handle<Code> target, RelocInfo::Mode rmode);
 
+  // Calls directly to the given address using a relative offset.
+  // Should only ever be used in Code objects for calls within the
+  // same Code object. Should not be used when generating new code (use labels),
+  // but only when patching existing code.
+  void call(Address target);
+
   // Call near absolute indirect, address in register
   void call(Register adr);
 
   // Call near indirect
   void call(const Operand& operand);
 
   // Jumps
   // Jump short or near relative.
   // Use a 32-bit signed displacement.
   void jmp(Label* L);  // unconditional jump to L
@@ skipping 411 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_