| From: | Nathan Bossart <nathandbossart(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | "Devulapalli, Raghuveer" <raghuveer(dot)devulapalli(at)intel(dot)com>, "Shankaran, Akash" <akash(dot)shankaran(at)intel(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Rowley <dgrowleyml(at)gmail(dot)com>, Ants Aasma <ants(dot)aasma(at)cybertec(dot)at>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, "Amonson, Paul D" <paul(dot)d(dot)amonson(at)intel(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Popcount optimization using AVX512 |
| Date: | 2024-07-31 02:01:31 |
| Message-ID: | Zqma-_guYhQmA9_A@nathan |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
On Tue, Jul 30, 2024 at 06:46:51PM -0700, Andres Freund wrote:
> On 2024-07-30 20:20:34 -0500, Nathan Bossart wrote:
>> On Tue, Jul 30, 2024 at 05:49:59PM -0700, Andres Freund wrote:
>> > Why are we actually checking for xsave? We're not using xsave itself and I
>> > couldn't find a comment in 792752af4eb5 explaining what we're using it as a
>> > proxy for? Is that just to know if _xgetbv() exists? Is it actually possible
>> > that xsave isn't available when avx512 is?
>>
>> Yes, it's to verify we have XGETBV, which IIUC requires support from both
>> the processor and the OS (see 598e011 and upthread discussion). AFAIK the
>> way we are detecting AVX-512 support is quite literally by-the-book unless
>> I've gotten something wrong.
>
> I'm basically wondering whether we need to check for compiler (not OS support)
> support for xsave if we also check for -mavx512vpopcntdq -mavx512bw
> support. Afaict the latter implies support for xsave.
The main purpose of the XSAVE compiler check is to determine whether we
need to add -mxsave in order to use _xgetbv() [0]. If that wasn't a
factor, we could probably skip it. Earlier versions of the patch used
inline assembly in the non-MSVC path to call XGETBV, which I was trying to
avoid.
[0] https://postgr.es/m/20240330032209.GA2018686%40nathanxps13
--
nathan
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Zhijie Hou (Fujitsu) | 2024-07-31 02:10:24 | RE: Conflict detection and logging in logical replication |
| Previous Message | Andres Freund | 2024-07-31 01:46:51 | Re: Popcount optimization using AVX512 |