On Fri, Nov 03, 2023 at 10:46:57AM +0000, Xiang Gao wrote: > On Date: Thu, 2 Nov 2023 09:35:50AM -0500, Nathan Bossart wrote: >> The idea is that we don't want to start forcing runtime checks on builds >> where we aren't already doing runtime checks. IOW if the compiler can use >> the ARMv8 CRC instructions with the default compiler flags, we should only >> use vmull_p64() if it can also be used with the default compiler flags. > > This is the newest patch, I think the code for which crc algorithm to > choose is a bit complicated. Maybe we can just use USE_ARMV8_VMULL only, > and do runtime checks on the vmull_p64 instruction at all times. This > will not affect the existing builds, because this is a new instruction > and new logic. In addition, it can also reduce the complexity of the > code.
I don't think we can. AFAICT a runtime check necessitates a function pointer or a branch, both of which incurred an impact on performance in my tests. It looks like this latest patch still does the runtime check even for the USE_ARMV8_CRC32C case. -- Nathan Bossart Amazon Web Services: https://aws.amazon.com
