Date: Fri, 9 Feb 2024 15:27:57 -0800
From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
	"Shankaran, Akash" <akash.shankaran@intel.com>,
	Nathan Bossart <nathandbossart@gmail.com>,
	"Amonson, Paul D" <paul.d.amonson@intel.com>,
	Tom Lane <tgl@sss.pgh.pa.us>,
	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: <20240209232757.5d@rfd.leadboat.com>
References: <202401250949.xrwkd46bgbpl@alvherre.pgsql>
 <202401260642.3rxzgb2xrhkl@alvherre.pgsql>
 <20240209182432.7c2peq5cgbdl43um@awork3.anarazel.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240209182432.7c2peq5cgbdl43um@awork3.anarazel.de>
User-Agent: Mutt/2.2.12 (2023-09-09)
Archived-At: 
 <https://www.postgresql.org/message-id/20240209232757.5d%40rfd.leadboat.com>
Precedence: bulk

On Fri, Feb 09, 2024 at 10:24:32AM -0800, Andres Freund wrote:
> On 2024-01-26 07:42:33 +0100, Alvaro Herrera wrote:
> > This suggests that finding a way to make the ifunc stuff work (with good
> > performance) is critical to this work.
> 
> Ifuncs are effectively implemented as a function call via a pointer, they're
> not magic, unfortunately. The sole trick they provide is that you don't
> manually have to use the function pointer.

The IFUNC creators introduced it so glibc could use arch-specific memcpy with
the instruction sequence of a non-pointer, extern function call, not the
instruction sequence of a function pointer call.  I don't know why the
upthread ifunc_test.patch benchmark found ifunc performing worse than function
pointers.  However, it would be odd if toolchains have replaced the original
IFUNC with something equivalent to or slower than function pointers.