Re: Popcount optimization using AVX512

Here is a v19 of the patch set. I moved out the refactoring of the
function pointer selection code to 0001. I think this is a good change
independent of $SUBJECT, and I plan to commit this soon. In 0002, I
changed the syslogger.c usage of pg_popcount() to use pg_number_of_ones
instead. This is standard practice elsewhere where the popcount functions
are unlikely to win. I'll probably commit this one soon, too, as it's even
more trivial than 0001.

0003 is the AVX512 POPCNT patch. Besides refactoring out 0001, there are
no changes from v18. 0004 is an early proof-of-concept for using AVX512
for the visibility map code. The code is missing comments, and I haven't
performed any benchmarking yet, but I figured I'd post it because it
demonstrates how it's possible to build upon 0003 in other areas.

AFAICT the main open question is the function call overhead in 0003 that
Alvaro brought up earlier. After 0002 is committed, I believe the only
in-tree caller of pg_popcount() with very few bytes is bit_count(), and I'm
not sure it's worth expending too much energy to make sure there are
absolutely no regressions there. However, I'm happy to do so if folks feel
that it is necessary, and I'd be grateful for thoughts on how to proceed on
this one.

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Nathan Bossart
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