This only work if it's a leaf function that will throw away the result. If you feed the result of your --fast-math function into other working code you risk breaking it.
`-ffast-math` is fully local, asides from GCC's unexpected `crtfastmath.o` linkage which is global.
Functions with `-ffast-math` enabled still return fp values via usual registers and in usual formats. If some function `f` is expected to return -1.0 to 1.0 for particluar inputs, `-ffast-math` can only make it to return 1.001 or NaN instead. If another function without `-ffast-math` expects and doesn't verify f's return value, it will surely misbehave, but only because the original analysis of f no longer holds.
`-ffast-math` the compiler option is bad because this effect is not evident from the code. Anything visible in the code should be okay.
