[4/6] Optionally pick the cheapest loop_vec_info

[Richard Biener]

On Wed, Nov 6, 2019 at 3:01 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Richard Biener <richard.guenther@gmail.com> writes:
> > On Tue, Nov 5, 2019 at 3:29 PM Richard Sandiford
> > <Richard.Sandiford@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