On Sat, Sep 3, 2011 at 5:42 PM, Michael Matz <m...@suse.de> wrote: >> > I've decided to not use four new bits from target_flags, and instead >> > created a new mask (recip_mask). Four bits would have fit in target >> > bits right now, but in the future we might want to add more >> > specialization, like modes for which the reciprocals are active. >> > >> > What do you think? >> >> These new flags looks like a nice addition, but I wonder, why we need >> separate options to handle vector recip. A vector rsqrt or rdiv is >> generated automatically in the same way as scalar rsqrt or rdiv is >> generated, so IMO, -mrecip-sqrt and -mrecip-div should be enough. > > No, the difference does matter. Using reciprocal estimates for scalar > divs often results in errors in benchmarks because those sometimes are > used to feed integer conversions for either index calculations or > printouts. The small rounding errors with the reciprocals lead to > incorrect outputs then. Context where the div can be vectorized often > don't have this problem (they're then used purely for calculations over > arrays of float data). For instance spec2006 and polyhedron break with > -mrecip purely because of the scalar reciprocals, but work with only > vectorized ones. I.e. users really want to differ between both.
I agree with your analysis. > Also, when this patch goes in I plan to submit another one that activates > vectorized rcp/rsqrt under -ffast-math already (that's what ICC happens to > do too). Great! In the past, we tried to use -mrecip with -ffast-math. IIRC, polyhedron broke on scalar rdiv and spec2006 broke on rsqrt. Taking into account your analysis above, using separate options and activating vectorized ones for -ffast-math makes much sense. >> For the future - could rs6000 and x86 use the same compile options to >> handle reciprocals? > > I'd guess so. rs6000 uses a hand-written comma-splitter, which we could > reuse. Perhaps rs6000 could adopt our approach in addition to its comma-splitter? OTOH, whatever is more convenient, I don't care that much. I have CC'd rs6000 maintainer for his opinion. Uros.
