Function rounds_to_nearest

Source/external/fast_float.h:3179–3236

* Returns true if the floating-pointing rounding mode is to 'nearest'. * It is the default on most system. This function is meant to be inexpensive. * Credit : @mwalcott3 */

 * Credit : @mwalcott3
 */
fastfloat_really_inline bool rounds_to_nearest() noexcept {
  // https://lemire.me/blog/2020/06/26/gcc-not-nearest/
#if (FLT_EVAL_METHOD != 1) && (FLT_EVAL_METHOD != 0)
  return false;
#endif
  // See
  // A fast function to check your floating-point rounding mode
  // https://lemire.me/blog/2022/11/16/a-fast-function-to-check-your-floating-point-rounding-mode/
  //
  // This function is meant to be equivalent to :
  // prior: #include <cfenv>
  //  return fegetround() == FE_TONEAREST;
  // However, it is expected to be much faster than the fegetround()
  // function call.
  //
  // The volatile keywoard prevents the compiler from computing the function
  // at compile-time.
  // There might be other ways to prevent compile-time optimizations (e.g., asm).
  // The value does not need to be std::numeric_limits<float>::min(), any small
  // value so that 1 + x should round to 1 would do (after accounting for excess
  // precision, as in 387 instructions).
  static volatile float fmin = std::numeric_limits<float>::min();
  float fmini = fmin; // we copy it so that it gets loaded at most once.
  //
  // Explanation:
  // Only when fegetround() == FE_TONEAREST do we have that
  // fmin + 1.0f == 1.0f - fmin.
  //
  // FE_UPWARD:
  //  fmin + 1.0f > 1
  //  1.0f - fmin == 1
  //
  // FE_DOWNWARD or  FE_TOWARDZERO:
  //  fmin + 1.0f == 1
  //  1.0f - fmin < 1
  //
  // Note: This may fail to be accurate if fast-math has been
  // enabled, as rounding conventions may not apply.
  #if FASTFLOAT_VISUAL_STUDIO
  #   pragma warning(push)
  //  todo: is there a VS warning?
  //  see https://stackoverflow.com/questions/46079446/is-there-a-warning-for-floating-point-equality-checking-in-visual-studio-2013
  #elif defined(__clang__)
  #   pragma clang diagnostic push
  #   pragma clang diagnostic ignored "-Wfloat-equal"
  #elif defined(__GNUC__)
  #   pragma GCC diagnostic push
  #   pragma GCC diagnostic ignored "-Wfloat-equal"
  #endif
  return (fmini + 1.0f == 1.0f - fmini);
  #if FASTFLOAT_VISUAL_STUDIO
  #   pragma warning(pop)
  #elif defined(__clang__)
  #   pragma clang diagnostic pop
  #elif defined(__GNUC__)
  #   pragma GCC diagnostic pop
  #endif
}