Home/ Questions/Q 1036117
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-16T14:39:01+00:00

While writing some test cases, and some of the tests check for the result

  • 0

While writing some test cases, and some of the tests check for the result of a NaN.

I tried using std::isnan but the assert failes:

Assertion `std::isnan(x)' failed.

After printing the value of x, it turned out it’s negative NaN (-nan) which is totally acceptable in my case.

After trying to use the fact that NaN != NaN and using assert(x == x), the compiler does me a ‘favor’ and optimises the assert away.

Making my own isNaN function is being optimised away as well.

How can I check for both equality of NaN and -NaN?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    This is embarrassing.

    The reason the compiler (GCC in this case) was optimising away the comparison and isnan returned false was because someone in my team had turned on -ffast-math.

    From the docs:

    -ffast-math
    Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math,
    -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and
    fcx-limited-range.

    This option causes the preprocessor macro __FAST_MATH__ to be defined.

    This option should never be turned on by any -O option since it can result in
    incorrect output for programs which depend on an exact implementation of IEEE
    or ISO rules/specifications for math functions.

    Notice the ending sentence – -ffast-math is unsafe.

    • 0