X64: Optimize MOD operator by a constant power of 2.

src/x64/codegen-x64.h

 // Copyright 2009 the V8 project authors. 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 583 matching lines @@
   friend class JumpTarget;
   friend class Reference;
   friend class Result;
 
   friend class CodeGeneratorPatcher;  // Used in test-log-stack-tracer.cc
 
   DISALLOW_COPY_AND_ASSIGN(CodeGenerator);
 };
 
 
+// -------------------------------------------------------------------------
+// Code stubs
+//
+// These independent code objects are created once, and used multiple
+// times by generated code to perform common tasks, often the slow
+// case of a JavaScript operation.  They are all subclasses of CodeStub,
+// which is declared in code-stubs.h.
+
+
 // Flag that indicates whether or not the code that handles smi arguments
 // should be placed in the stub, inlined, or omitted entirely.
 enum GenericBinaryFlags {
   SMI_CODE_IN_STUB,
   SMI_CODE_INLINED
 };
 
 
 class GenericBinaryOpStub: public CodeStub {
  public:
@@ skipping 34 matching lines @@
            | ModeBits::encode(mode_)
            | FlagBits::encode(flags_);
   }
   void Generate(MacroAssembler* masm);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_CODEGEN_X64_H_