src/common/safe_math.h

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// safe_math.h: Helpful math functions.
#ifndef SAFE_MATH_H__
#define SAFE_MATH_H__

#include <utility>

namespace google_breakpad {

// Adds `a` and `b`, returning a pair of:
// - The result after any truncation.
// - Whether an overflow/underflow occurred.
template <typename T>
std::pair<T, bool> AddWithOverflowCheck(T a, T b) {
#ifdef _WIN32
  // Since C++11, unsigned overflow is well-defined; do everything unsigned,
  // assuming 2's complement.
  if (std::is_unsigned<T>::value) {
    T result = a + b;
    // Since we're adding two values >= 0, having a smaller value implies
    // overflow.
    bool overflow = result < a;
    return {result, overflow};
  }

  using TUnsigned = typename std::make_unsigned<T>::type;
  T result = TUnsigned(a) + TUnsigned(b);
  bool overflow;
  if ((a >= 0) == (b >= 0)) {
    if (a >= 0) {
      overflow = result < a;
    } else {
      overflow = result > a;
    }
  } else {
    // If signs are different, it's impossible for overflow to happen.
    overflow = false;
  }
  return {result, overflow};
#else
  T result;
  bool overflow = __builtin_add_overflow(a, b, &result);
  return {result, overflow};
#endif
}

template <typename T>
T AddIgnoringOverflow(T a, T b) {
  return AddWithOverflowCheck(a, b).first;
}

}  // namespace google_breakpad

#endif  // SAFE_MATH_H__