Re: Improve CRC32C performance on SSE4.2

On Wed, Feb 12, 2025 at 09:48:57PM +0000, Devulapalli, Raghuveer wrote:
>> I think the idea behind USE_SSE42_CRC32C is to avoid the function pointer
>> overhead if possible.  I looked at switching to always using runtime checks for this
>> stuff, and we concluded that we'd better not [0].
> 
> Does that mean we want this feature for the new PCLMUL (and AVX-512) crc32c implementations too? The code for that
willlook a little ugly, I might need to think about a cleaner way to do this. 
 

Well, I suspect the AVX-512 version will pretty much always need the
runtime check given that its not available on a lot of newer hardware and
requires a bunch of extra runtime checks (see pg_popcount_avx512.c).  But
it might be worth doing for PCLMUL.  Otherwise, I think we'd have to leave
out the PCLMUL optimizations if built with -msse4.2 -mpclmul because we
don't want to regress existing -msse4.2 users with a runtime check.

-- 
nathan