[bigint] Small bug caused by clang/llvm optimizations

[Zachary Kurmas]

Your bigint library has a bug when compiled on a mac using the -O2 flag with Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn).    In particular,  Line 115 of BigInteger::convertToSignedPrimitive()

X x = -X(b);

doesn’t behave as expected when compiled at the -O2 level.

Compile the attached C++ sample on a mac using clang/llvm to see exactly what goes wrong.


I fixed this bug and added 64-bit tests.  You will find the fixes/improvements here:  https://github.com/kurmasz/bigint

Zack Kurmas

--
Zachary Kurmas
Assistant Professor of Computer Science
Grand Valley State University
http://www.cis.gvsu.edu/~kurmasz


——

#include <iostream>

template<class Out> Out negate(unsigned long in) {
  // This line casts an unsigned long to a signed long.
  //  2^63 as an unsigned long is 0x8000000000000000
  //  Casting 2^63 from unsigned long to signed long should
  //  produce -2^63 (which is also 0x8000000000000000).
  //  The twos complement of -2^63 just happens to be -2^63,
  //  so, (long)(2^63) should always be -2^63, correct?

  // This code works as expected when compiled with gcc 4.4.7; but,
  // when compiled on a mac with -O2 using LLVM 5.1 / clang-503.0.40,
  // it produces the wrong answer.  (The source of the problem is
  // clearly visible in the generated assembly code.)

  Out out = -Out(in);
   if (out > 0) {
    std::cout << "Oops!" << std::endl;
  } else {
    std::cout << "Correct." << std::endl;
  }

  return out;
}


int main() {
  unsigned long in =  9223372036854775808UL;

  std::cout << "Attempt 3: " << std::endl;
  negate<long>(in);
}

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