Feedback-ID: i0fe9450f:Fastmail
Date: Tue, 5 Nov 2024 13:12:03 +0900
From: Michael Paquier <michael@paquier.xyz>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Ranier Vilela <ranier.vf@gmail.com>,
	Bertrand Drouvot <bertranddrouvot.pg@gmail.com>,
	Peter Smith <smithpb2250@gmail.com>,
	Peter Eisentraut <peter@eisentraut.org>,
	Heikki Linnakangas <hlinnaka@iki.fi>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: define pg_structiszero(addr, s, r)
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On Tue, Nov 05, 2024 at 04:23:34PM +1300, David Rowley wrote:
> I tried your optimisation in the attached allzeros.c and here are my resu=
lts:
>=20
> # My version
> $ gcc allzeros.c -O2 -o allzeros && for i in {1..3}; do ./allzeros; done
> char: done in 1543600 nanoseconds
> size_t: done in 196300 nanoseconds (7.86347 times faster than char)
>=20
> # Ranier's optimization
> $ gcc allzeros.c -O2 -D RANIERS_OPTIMIZATION -o allzeros && for i in
> size_t: done in 531700 nanoseconds (3.6545 times faster than char)
> char: done in 1957200 nanoseconds

I am not seeing numbers as good as yours, but the winner is clear as
well here:

$ gcc allzeros.c -O2 -o allzeros && for i in {1..3}; do
=2E/allzeros; done
char: done in 6578995 nanoseconds
size_t: done in 829916 nanoseconds (7.9273 times faster than char)
char: done in 6581465 nanoseconds
size_t: done in 829948 nanoseconds (7.92997 times faster than char)
char: done in 6585748 nanoseconds
size_t: done in 834929 nanoseconds (7.88779 times faster than char)

$ gcc allzeros.c -O2 -D RANIERS_OPTIMIZATION -o allzeros && for i in
{1..3}; do ./allzeros;
done char: done in 6591803 nanoseconds
size_t: done in 1236102 nanoseconds (5.33273 times faster than char)
char: done in 6606219 nanoseconds
size_t: done in 1235979 nanoseconds (5.34493 times faster than char)
char: done in 6594413 nanoseconds
size_t: done in 1238770 nanoseconds (5.32336 times faster than char)

I'm surprised to see that assigning aligned_end at these two different
locations has this much effect once the compiler optimizes the
surroundings, but well.
--
Michael

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