Re: Improve CRC32C performance on SSE4.2

On Wed, Feb 12, 2025 at 10:12:20PM +0000, Devulapalli, Raghuveer wrote:
>> 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.
> 
> Sounds good to me. Although, users building with just -msse4.2 will now encounter an 
> an additional pclmul runtime check. That would be a regression unless they update to 
> building with both -msse4.2 and -mpclmul. 

My thinking was that building with just -msse4.2 would cause the existing
SSE 4.2 implementation to be used (without the function pointer).  That's
admittedly a bit goofy because they'd miss out on the PCLMUL optimization,
but things at least don't get any worse for them.

-- 
nathan