Re: always use runtime checks for CRC-32C instructions

Nathan Bossart <nathandbossart(at)gmail(dot)com> writes:
> Okay. With that in mind, I think the path forward for new instructions is
> as follows:

> * If the special CRC instructions can be used with the default compiler
> flags, we can only use newer instructions if they can also be used with
> the default compiler flags. (My M2 machine appears to add +crypto by
> default, so I bet your buildfarm animals would fall into this bucket.)
> * Otherwise, if the CRC instructions can be used with added flags (i.e.,
> the runtime check path), we can do a runtime check for the new
> instructions as well. (Most other buildfarm animals would fall into this
> bucket.)

This seems like a reasonable proposal.

> Any platform that can use the CRC instructions with default compiler flags
> but not the new instructions wouldn't be able to take advantage of the
> proposed optimization, but it also wouldn't be subject to the small
> performance regression.

Check. For now I think that's fine. If we get to a place where this
policy is really leaving a lot of performance on the table, we can
revisit it ... but that situation is hypothetical and may remain so.

(It's worth noting also that a package builder can move the goalposts
at will, since our idea of "default flags" is really whatever the user
says to use.)

regards, tom lane