Date: Tue, 30 Jul 2024 21:01:31 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com>,
	"Shankaran, Akash" <akash.shankaran@intel.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, David Rowley <dgrowleyml@gmail.com>,
	Ants Aasma <ants.aasma@cybertec.at>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
	"Amonson, Paul D" <paul.d.amonson@intel.com>,
	Noah Misch <noah@leadboat.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Message-ID: <Zqma-_guYhQmA9_A@nathan>
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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