Re: Improve CRC32C performance on SSE4.2

On Mon, Feb 17, 2025 at 05:58:01PM +0700, John Naylor wrote:
> I tried using branching for the runtime check, and this looks like the
> way to go:
> - Existing -msse4.2 builders will still call directly, but inside the
> function there is a length check and only for long input will it do a
> runtime check for pclmul.
> - This smooths the way for -msse4.2 (and the equivalent on Arm) to
> inline calls with short constant input (e.g. WAL insert lock),
> although I've not done that here.
> - This can be a simple starting point for consolidating runtime
> checks, as was proposed for popcount in the AVX-512 CRC thread, but
> with branching my model was Andres' sketch here:
> 
> https://www.postgresql.org/message-id/20240731023918.ixsfbeuub6e76one%40awork3.anarazel.de

Oh, nifty.  IIUC this should help avoid quite a bit of overhead even before
adding the PCLMUL stuff since it removes the function pointers for the
runtime-check builds.

While this needn't block this patch set, I do find the dispatch code to be
pretty complicated.  Maybe we can improve that in the future by using
macros to auto-generate much of it.  My concern here is less about this
particular patch set and more about the long term maintainability as we add
more and more stuff like it, each with its own tangled web of build and
dispatch rules.

-- 
nathan