On Sat, Nov 10, 2018 at 6:36 AM Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > On Fri, Nov 09, 2018 at 01:03:55PM -0700, Jeff Law wrote: > > >> And signed zeroes. Yeah. I think it would have to be > > >> flag_unsafe_math_optimizations + some more. > > > > > > Indeed. > > So we need to give Giuliano some clear guidance on guarding. This is > > out of my area of expertise, so looking to y'all to help here. > > IMO, it needs flag_unsafe_optimizations, as above; and it needs to be > investigated which (if any) options like flag_signed_zeros it needs in > addition to that. It needs an option like that whenever the new expression > can give a zero with a different sign than the original expression, etc. > Although it could be said that flag_unsafe_optimizations supercedes all > of that. It isn't clear.
It indeed isn't clear whether at least some of the other flags make no sense with -funsafe-math-optimizations. Still at least for documentation purposes please use !flag_siged_zeros && flag_unsafe_math_optimizations && ... flag_unsafe_math_optimizations is generally used when there's extra rounding involved. Some specific kind of transforms have individual flags and do not require flag_unsafe_math_optimizations (re-association and contraction for example). I'm not sure I would require flag_unsafe_math_optimizations for a 2ulp error though. Richard. > > Segher
