Re: Popcount optimization using AVX512

On Mon, Mar 18, 2024 at 11:20:18AM -0500, Nathan Bossart wrote:
> I don't think David was suggesting that we need to remove the runtime
> checks for AVX512.  IIUC he was pointing out that most of the performance
> gain is from removing the function call overhead, which your v8-0002 patch
> already does for the proposed AVX512 code.  We can apply a similar
> optimization for systems without AVX512 by inlining the code for
> pg_popcount64() and pg_popcount32().

Here is a more fleshed-out version of what I believe David is proposing.
On my machine, the gains aren't quite as impressive (~8.8s to ~5.2s for the
test_popcount benchmark).  I assume this is because this patch turns
pg_popcount() into a function pointer, which is what the AVX512 patches do,
too.  I left out the 32-bit section from pg_popcount_fast(), but I'll admit
that I'm not yet 100% sure that we can assume we're on a 64-bit system
there.

IMHO this work is arguably a prerequisite for the AVX512 work, as turning
pg_popcount() into a function pointer will likely regress performance for
folks on systems without AVX512 otherwise.

-- 
Nathan Bossart
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