X64: Convert smis to holding 32 bits of payload.

src/utils.h

 // Copyright 2006-2008 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 18 matching lines @@
 #define V8_UTILS_H_
 
 #include <stdlib.h>
 
 namespace v8 {
 namespace internal {
 
 // ----------------------------------------------------------------------------
 // General helper functions
 
-// Returns true iff x is a power of 2.  Does not work for zero.
+// Returns true iff x is a power of 2 (or zero). Cannot be used with the
+// maximally negative value of the type T (the -1 overflows).
 template <typename T>
 static inline bool IsPowerOf2(T x) {
   return (x & (x - 1)) == 0;
 }
 
 
 // The C++ standard leaves the semantics of '>>' undefined for
 // negative signed operands. Most implementations do the right thing,
 // though.
 static inline int ArithmeticShiftRight(int x, int s) {
@@ skipping 522 matching lines @@
 #endif
   while (dest < limit) {
     *dest++ = static_cast<sinkchar>(*src++);
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_UTILS_H_