From: Nathan Bossart <nathandbossart@gmail.com>
To: Amonson, Paul D <paul.d.amonson@intel.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Shankaran, Akash <akash.shankaran@intel.com>
Cc: Noah Misch <noah@leadboat.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Date: Thu, 28 Mar 2024 16:51:36 -0500
Message-ID: <20240328215136.GA918358@nathanxps13> (raw)
In-Reply-To: <20240328213854.GA916732@nathanxps13>
References: <CAApHDvrf-oKJ-mCrFZ_g-fZpRrCXJrKcZobwtUgzi0R5aXtVxQ@mail.gmail.com>
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<20240328213854.GA916732@nathanxps13>
On Thu, Mar 28, 2024 at 04:38:54PM -0500, Nathan Bossart wrote:
> Here is a v14 of the patch that I think is beginning to approach something
> committable. Besides general review and testing, there are two things that
> I'd like to bring up:
>
> * The latest patch set from Paul Amonson appeared to support MSVC in the
> meson build, but not the autoconf one. I don't have much expertise here,
> so the v14 patch doesn't have any autoconf/meson support for MSVC, which
> I thought might be okay for now. IIUC we assume that 64-bit/MSVC builds
> can always compile the x86_64 popcount code, but I don't know whether
> that's safe for AVX512.
>
> * I think we need to verify there isn't a huge performance regression for
> smaller arrays. IIUC those will still require an AVX512 instruction or
> two as well as a function call, which might add some noticeable overhead.
I forgot to mention that I also want to understand whether we can actually
assume availability of XGETBV when CPUID says we support AVX512:
> + /*
> + * We also need to check that the OS has enabled support for the ZMM
> + * registers.
> + */
> +#ifdef _MSC_VER
> + return (_xgetbv(0) & 0xe0) != 0;
> +#else
> + uint64 xcr = 0;
> + uint32 high;
> + uint32 low;
> +
> +__asm__ __volatile__(" xgetbv\n":"=a"(low), "=d"(high):"c"(xcr));
> + return (low & 0xe0) != 0;
> +#endif
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
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To: pgsql-hackers@postgresql.org
Cc: nathandbossart@gmail.com, paul.d.amonson@intel.com, tgl@sss.pgh.pa.us, dgrowleyml@gmail.com, andres@anarazel.de, alvherre@alvh.no-ip.org, akash.shankaran@intel.com, noah@leadboat.com, boekewurm+postgres@gmail.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Popcount optimization using AVX512
In-Reply-To: <20240328215136.GA918358@nathanxps13>
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