Be careful though when compiling your code with -ffinite-math-only (which is part of -ffast-math and -Ofast). The compiler will likely replace your function with a constant expression returning false, since it assumes that NaNs cannot occur in your code. I found that the only safe way to actually check for NaNs when using these flags is to memcpy the float into a byte array (to avoid aliasing issues) and manually compare against the NaN bitmask.