[swatcoder]

You're mistaken.

You can/would just use it in the translation units where you want it; usually for numerical code where you want certain optimizations or behaviors and know that the tradeoffs are irrelevant.

It's mostly harmless for everday application math anyway, and so enabling it for your whole application isn't a catastrophe, but it's not what people who know what they're doing would usually do. It's usually used for a specific file or perhaps a specific support library.

[leni536]

-ffast-math affects other translation units too, because it introduces a global initialization that affects some CPU flags. You can't really contain it to a single TU.

[senkora]

It seems like this depends on whether you specify -ffast-math when linking or only when compiling: https://stackoverflow.com/a/68938551

My understanding is that if you don't specify -ffast-math when linking then you shouldn't get crtfastmath.o linked in.

[adgjlsfhk1]

so this is complicated by the fact that GCC has a bug until last year where it would set the subnormal truncation flag when it wasn't supposed to.