RE: CRC32C Parallel Computation Optimization on ARM

On Mon, 6 Nov 2023 13:16:13PM -0600, 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.

I think I understand what you mean, this is the latest patch. Thank you!
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