__rcp __sqrt undefined ?

hello all,

i’ve read that i should use e.g. __sinf instead of sinf to have a faster approximation when high accuracy is not required (but speed).
it works fine, but it also states the same thing for
1/x -> __rcp(x)
sqrtf(x) -> __sqrtf(x)

however the latter two are “undefined” when trying to compile. is there a cure for this or do i need faith in the compiler that it gets it and optimizes accourdingly ? i’m a little sceptic on this…

ideas anyone ? :)