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From: Noah Misch <[email protected]>
To: Tom Lane <[email protected]>
Cc: Nathan Bossart <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: Amonson, Paul D <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Shankaran, Akash <[email protected]>
Subject: Re: Popcount optimization using AVX512
Date: Mon, 6 Nov 2023 19:15:01 -0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <BL1PR11MB5304097DF7EA81D04C33F3D1DCA6A@BL1PR11MB5304.namprd11.prod.outlook.com>
	<CAEze2WjaFLhp7=Eo-mbSaHsoeq7ZEs00yZ2+FpSpijH+KN_hbA@mail.gmail.com>
	<20231107022240.GA729644@nathanxps13>
	<[email protected]>

On Mon, Nov 06, 2023 at 09:52:58PM -0500, Tom Lane wrote:
> Nathan Bossart <[email protected]> writes:
> > Like I said, I don't have any proposals yet, but assuming we do want to
> > support newer intrinsics, either open-coded or via auto-vectorization, I
> > suspect we'll need to gather consensus for a new policy/strategy.
> 
> Yeah.  The function-pointer solution kind of sucks, because for the
> sort of operation we're considering here, adding a call and return
> is probably order-of-100% overhead.  Worse, it adds similar overhead
> for everyone who doesn't get the benefit of the optimization.

The glibc/gcc "ifunc" mechanism was designed to solve this problem of choosing
a function implementation based on the runtime CPU, without incurring function
pointer overhead.  I would not attempt to use AVX512 on non-glibc systems, and
I would use ifunc to select the desired popcount implementation on glibc:
https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Function-Attributes.html

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