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. Thanks, Richard
