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

[Richard Sandiford]

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.

Thanks,
Richard