Is this equivalent to --ffast-math?

From what I know of -ffast-math and can read from the docs for *_fast. I am not convinced that the *_fast intrinsics do _everything_ -ffast-math allows. They seem focused around algebraic equivalence (a/b is equivalent to a*(1/b) ) and assumptions of finite math. There's a few other things that -ffast-math allows like ignoring certain errors, ignoring the existence of signed zero, ignoring signalling NaN handling, ignoring SIGFPE handling, etc...

Yes, because many of the traditional "fast math" assumptions are definitely not something that should be hidden behind an attractive option like that. In particular assuming the nonexistence of NaNs is essentially never anything but a ticket to the UB land.