Re: [PATCH] Add a bit dislike for separate mem alternative when op is REG_P.

[Hongtao Liu <crazylht@gmail.com>]

On Wed, Jun 1, 2022 at 12:40 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Vladimir Makarov via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > On 2022-05-29 23:05, Hongtao Liu wrote:
> >> On Fri, May 27, 2022 at 5:12 AM Vladimir Makarov via Gcc-patches
> >> <gcc-patches@gcc.gnu.org> wrote:
> >>>
> >>> On 2022-05-24 23:39, liuhongt wrote:
> >>>> Rigt now, mem_cost for separate mem alternative is 1 * frequency which
> >>>> is pretty small and caused the unnecessary SSE spill in the PR, I've tried
> >>>> to rework backend cost model, but RA still not happy with that(regress
> >>>> somewhere else). I think the root cause of this is cost for separate 'm'
> >>>> alternative cost is too small, especially considering that the mov cost
> >>>> of gpr are 2(default for REGISTER_MOVE_COST). So this patch increase mem_cost
> >>>> to 2*frequency, also increase 1 for reg_class cost when m alternative.
> >>>>
> >>>>
> >>>> Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}.
> >>>> Ok for trunk?
> >>> Thank you for addressing this problem. And sorry I can not approve this
> >>> patch at least w/o your additional work on benchmarking this change.
> >>>
> >>> This code is very old.  It is coming from older RA (former file
> >>> regclass.c) and existed practically since GCC day 1.  People tried many
> >>> times to improve this code.  The code also affects many targets.
> >> Yes, that's why I increased it as low as possible, so it won't regress
> >> #c6 in the PR.
> >>> I can approve this patch if you show that there is no regression at
> >>> least on x86-64 on some credible benchmark, e.g. SPEC2006 or SPEC2017.
> >>>
> >> I've tested the patch for SPEC2017 with both  -march=cascadelake
> >> -Ofast -flto and -O2 -mtune=generic.
> >> No obvious regression is observed, the binaries are all different from
> >> before, so I looked at 2 of them, the difference mainly comes from
> >> different choices of registers(xmm13 -> xmm12).
> >> Ok for trunk then?
> >
> > OK.
> >
> > Thank you for checking SPEC2017.
> >
> > I hope it will not create troubles for other targets.
>
> Can we hold off for a bit?  Like Alexander says, there seem to be
> some inconsistencies in the target patterns, so I think we should
> first rule out any changes being needed there.
Yes, i'm also testing another patch.
>
> Thanks,
> Richard



-- 
BR,
Hongtao