Negative NaN is not a NaN?

c++ nan fast-math
LiraNuna picture LiraNuna · Aug 29, 2010 · Viewed 11.4k times · Source

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?

Answer

LiraNuna picture LiraNuna · Aug 29, 2010

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.

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