[Bug tree-optimization/55016] request for specific builtins for rcp and rsqrt

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55016

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> 2012-10-23 06:12:30 UTC ---
(In reply to comment #2)
> For the application I have in mind a global flag will such as -ffaster-math
> will not be suitable
> as it would affect also places where full "single precision" is still required.

Flags can be used per function, but that doesn't work very well indeed.

> I would like just to profit of the rcp and rsqrt instructions for cases where
> their low "precision" is enough.

Some math libraries (AIX for instance) provide a correctly rounded rsqrt, so it
would have to be called vaguely_estimate_rsqrt or something (still not to be
confused with the fast-math version that adds one refinement round to reach
almost full precision).