Hi, On Mon, 5 Nov 2018, Jeff Law wrote:
> >> Don't we have a flag specific to honoring nans? Would that be better > >> to use than flag_unsafe_math_optimizations? As Uli mentioned, > >> there's > > > > That's only relevant for the comparison optimization, of course. > > > > Converting erfc to 1-erf is dubious, since the whole point of erfc is > > for cases where 1-erf is inaccurate. (Conversion in the other > > direction also needs flag_unsafe_math_optimizations.) > > > Understood. Thanks for clarifying. It seems like > unsafe-math-optimization is a better fit than the nan specific flag. But still we should consider general usefullness, even with unsafe-math. In this case we would remove a usage of a slow function that the user specifically used to deal with inaccuracies with an equally slow function (plus a little arithmetic that is shadows by the functions slowness) that now exacly produces the inaccuracies the user wanted to avoid. I.e. the speed gain is zero. The "canonicalization gain" referred to in the PR might be real, but it comes at the cost of introducing definite catastrophic cancellation. IMHO that's not a sensible transformation to do, under any flags. Ciao, Michael.
