Re: Enable the vectorizer at -O2 for GCC 12

["Kewen.Lin" <linkw@linux.ibm.com>]

on 2021/8/30 下午10:11, Bill Schmidt wrote:
> On 8/30/21 8:04 AM, Florian Weimer wrote:
>> There has been a discussion, both off-list and on the gcc-help mailing
>> list (“Why vectorization didn't turn on by -O2”, spread across several
>> months), about enabling the auto-vectorizer at -O2, similar to what
>> Clang does.
>>
>> I think the review concluded that the very cheap cost model should be
>> used for that.
>>
>> Are there any remaining blockers?
> 
> Hi Florian,
> 
> I don't think I'd characterize it as having blockers, but we are continuing to investigate small performance issues that arise with very-cheap, including some things that regressed in GCC 12.  Kewen Lin is leading that effort.  Kewen, do you feel we have any major remaining concerns with this plan?
> 

Hi Florian & Bill,

There are some small performance issues like PR101944 and PR102054, and
still two degraded bmks (P9 520.omnetpp_r -2.41% and P8 526.blender_r
-1.31%) to be investigated/clarified, but since their performance numbers
with separated loop and slp vectorization options look neutral, they are
very likely noises.  IMHO I don't think they are/will be blockers.  

So I think it's good to turn this on by default for Power.

BR,
Kewen