Feedback-ID: ia2694551:Fastmail
Date: Mon, 1 Apr 2024 13:06:12 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: "Amonson, Paul D" <paul.d.amonson@intel.com>, 
	Tom Lane <tgl@sss.pgh.pa.us>, David Rowley <dgrowleyml@gmail.com>, 
	Andres Freund <andres@anarazel.de>, "Shankaran, Akash" <akash.shankaran@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: <202404011106.y4fci35kzdqt@alvherre.pgsql>
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On 2024-Mar-31, Nathan Bossart wrote:

> +uint64
> +pg_popcount_avx512(const char *buf, int bytes)
> +{
> +	uint64		popcnt;
> +	__m512i		accum = _mm512_setzero_si512();
> +
> +	for (; bytes >= sizeof(__m512i); bytes -= sizeof(__m512i))
> +	{
> +		const		__m512i val = _mm512_loadu_si512((const __m512i *) buf);
> +		const		__m512i cnt = _mm512_popcnt_epi64(val);
> +
> +		accum = _mm512_add_epi64(accum, cnt);
> +		buf += sizeof(__m512i);
> +	}
> +
> +	popcnt = _mm512_reduce_add_epi64(accum);
> +	return popcnt + pg_popcount_fast(buf, bytes);
> +}

Hmm, doesn't this arrangement cause an extra function call to
pg_popcount_fast to be used here?  Given the level of micro-optimization
being used by this code, I would have thought that you'd have tried to
avoid that.  (At least, maybe avoid the call if bytes is 0, no?)

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"El Maquinismo fue proscrito so pena de cosquilleo hasta la muerte"
(Ijon Tichy en Viajes, Stanislaw Lem)