On Wed, Nov 6, 2019 at 3:01 PM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Richard Biener <richard.guent...@gmail.com> writes: > > On Tue, Nov 5, 2019 at 3:29 PM Richard Sandiford > > <richard.sandif...@arm.com> wrote: > >> > >> This patch adds a mode in which the vectoriser tries each available > >> base vector mode and picks the one with the lowest cost. For now > >> the behaviour is behind a default-off --param, but a later patch > >> enables it by default for SVE. > >> > >> The patch keeps the current behaviour of preferring a VF of > >> loop->simdlen over any larger or smaller VF, regardless of costs > >> or target preferences. > > > > Can you avoid using a --param for this? Instead I'd suggest to > > amend the vectorize_modes target hook to return some > > flags like VECT_FIRST_MODE_WINS. We'd eventually want > > to make the target able to say do-not-vectorize-epiloges-of-MODE > > (I think we may not want to vectorize SSE vectorized loop > > epilogues with MMX-with-SSE or GPRs for example). I guess > > for the latter we'd use a new target hook. > > The reason for using a --param was that I wanted a way of turning > this on and off on the command line, so that users can experiment > with it if necessary. E.g. enabling the --param could be a viable > alternative to -mprefix-* in some cases. Disabling it would be > a way of working around a bad cost model decision without going > all the way to -fno-vect-cost-model. > > These kinds of --params can become useful workarounds until an > optimisation bug is fixed.
I'm arguing that the default depends on the actual ISAs so there isn't a one-fits all and given we have OMP SIMD and target cloning for multiple ISAs this looks like a wrong approach. For sure the target can use its own switches to override defaults here, or alternatively we might want to have a #pragma GCC simdlen mimicing OMP behavior here. Richard. > > Thanks, > Richard
