Hi, On Tue, 7 Nov 2017, Richard Biener wrote:
> > With FMA however the situation is different becuase there are rounding > > differences. Why we can convert multiplicatoin+add into FMA without > > -ffast-math at first place? > > We do with -ffp-contract=fast which is the default for C. But note that the reverse transformation can't be done without changing rounding behaviour detrimentally. I.e. the patch as is should be conditional on a suboption of fast-math, if the user wrote an FMA he quite surely wants the single-rounding and the splitting would break this. I.e. like Marc said, only split FMAs that were generated by the compiler. At which point it indeed seems a bit nicer to not even generate them in the first place. It's basically a pattern match on the defs/uses of the potential FMA. Before commiting to create the FMA the pass could just as well ask the backend before doing it, i.e. all at gimple time. Would introduce some more arch dependencies into GIMPLE, but I don't think that's a problem here. Ciao, Michael.
