Fix bug in bignum implementation.

src/bignum.cc

 // Copyright 2011 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 717 matching lines @@
     carry = new_carry;
   }
   if (carry != 0) {
     bigits_[used_digits_] = carry;
     used_digits_++;
   }
 }
 
 
 void Bignum::SubtractTimes(const Bignum& other, int factor) {
+#ifdef DEBUG
+  Bignum a, b;
+  a.AssignBignum(*this);
+  b.AssignBignum(other);
+  b.MultiplyByUInt32(factor);
+  a.SubtractBignum(b);
+#endif
   ASSERT(exponent_ <= other.exponent_);
   if (factor < 3) {
     for (int i = 0; i < factor; ++i) {
       SubtractBignum(other);
     }
     return;
   }
   Chunk borrow = 0;
   int exponent_diff = other.exponent_ - exponent_;
   for (int i = 0; i < other.used_digits_; ++i) {
     DoubleChunk product = static_cast<DoubleChunk>(factor) * other.bigits_[i];
     DoubleChunk remove = borrow + product;
     Chunk difference =
         bigits_[i + exponent_diff] - static_cast<Chunk>(remove & kBigitMask);
     bigits_[i + exponent_diff] = difference & kBigitMask;
     borrow = static_cast<Chunk>((difference >> (kChunkSize - 1)) +
                                 (remove >> kBigitSize));
   }
   for (int i = other.used_digits_ + exponent_diff; i < used_digits_; ++i) {
     if (borrow == 0) return;
     Chunk difference = bigits_[i] - borrow;
     bigits_[i] = difference & kBigitMask;
     borrow = difference >> (kChunkSize - 1);
-    ++i;
   }
   Clamp();
+  ASSERT(Bignum::Equal(a, *this));
 }
 
 
 } }  // namespace v8::internal