Avoid pointer underflow in CopyCharsUnsigned.

src/v8utils.h

 // Copyright 2012 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 231 matching lines @@
 void CopyCharsUnsigned(sinkchar* dest, const sourcechar* src, int chars) {
   sinkchar* limit = dest + chars;
 #ifdef V8_HOST_CAN_READ_UNALIGNED
   if (sizeof(*dest) == sizeof(*src)) {
     if (chars >= static_cast<int>(OS::kMinComplexMemCopy / sizeof(*dest))) {
       OS::MemCopy(dest, src, chars * sizeof(*dest));
       return;
     }
     // Number of characters in a uintptr_t.
     static const int kStepSize = sizeof(uintptr_t) / sizeof(*dest);  // NOLINT
-    while (dest <= limit - kStepSize) {
+    ASSERT(dest + kStepSize > dest);  // Check for overflow.
+    while (dest + kStepSize <= limit) {
       *reinterpret_cast<uintptr_t*>(dest) =
           *reinterpret_cast<const uintptr_t*>(src);
       dest += kStepSize;
       src += kStepSize;
     }
   }
 #endif
   while (dest < limit) {
     *dest++ = static_cast<sinkchar>(*src++);
   }
@@ skipping 41 matching lines @@
 
   // Add formatted contents like printf based on a va_list.
   void AddFormattedList(const char* format, va_list list);
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(StringBuilder);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_V8UTILS_H_