Re: Popcount optimization using AVX512

On Mon, Nov 06, 2023 at 09:52:58PM -0500, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
> > Like I said, I don't have any proposals yet, but assuming we do want to
> > support newer intrinsics, either open-coded or via auto-vectorization, I
> > suspect we'll need to gather consensus for a new policy/strategy.
> 
> Yeah.  The function-pointer solution kind of sucks, because for the
> sort of operation we're considering here, adding a call and return
> is probably order-of-100% overhead.  Worse, it adds similar overhead
> for everyone who doesn't get the benefit of the optimization.

The glibc/gcc "ifunc" mechanism was designed to solve this problem of choosing
a function implementation based on the runtime CPU, without incurring function
pointer overhead.  I would not attempt to use AVX512 on non-glibc systems, and
I would use ifunc to select the desired popcount implementation on glibc:
https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Function-Attributes.html