Date: Tue, 2 Apr 2024 13:40:21 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
	Ants Aasma <ants.aasma@cybertec.at>,
	"Amonson, Paul D" <paul.d.amonson@intel.com>,
	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: <20240402184021.GA2802541@nathanxps13>
References: <202404021734.635tuluozzn3@alvherre.pgsql>
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On Tue, Apr 02, 2024 at 01:43:48PM -0400, Tom Lane wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> On 2024-Apr-02, Nathan Bossart wrote:
>>> Another idea I had is to turn pg_popcount() into a macro that just uses the
>>> pg_number_of_ones array when called for few bytes:
>>> 
>>> 	static inline uint64
>>> 	pg_popcount_inline(const char *buf, int bytes)
>>> 	{
>>> 		uint64		popcnt = 0;
>>> 
>>> 		while (bytes--)
>>> 			popcnt += pg_number_of_ones[(unsigned char) *buf++];
>>> 
>>> 		return popcnt;
>>> 	}
>>> 
>>> 	#define pg_popcount(buf, bytes) \
>>> 		((bytes < 64) ? \
>>> 		 pg_popcount_inline(buf, bytes) : \
>>> 		 pg_popcount_optimized(buf, bytes))
>>> 
>>> But again, I'm not sure this is really worth it for the current use-cases.
> 
>> Eh, that seems simple enough, and then you can forget about that case.
> 
> I don't like the double evaluation of the macro argument.  Seems like
> you could get the same results more safely with
> 
> 	static inline uint64
> 	pg_popcount(const char *buf, int bytes)
> 	{
> 		if (bytes < 64)
> 		{
> 			uint64		popcnt = 0;
> 
> 			while (bytes--)
> 				popcnt += pg_number_of_ones[(unsigned char) *buf++];
> 
> 			return popcnt;
> 		}
> 		return pg_popcount_optimized(buf, bytes);
> 	}

Yeah, I like that better.  I'll do some testing to see what the threshold
really should be before posting an actual patch.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com