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

[Vladimir Makarov <vmakarov@redhat.com>]

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.