RE: CRC32C Parallel Computation Optimization on ARM

On Tue, 31 Oct 2023 15:48:21PM -0500, Nathan Bossart wrote:
>> Thanks. I went ahead and split this prerequisite part out to a separate
>> thread [0] since it's sort-of unrelated to your proposal here. It's not
>> really a prerequisite, but I do think it will simplify things a bit.

>Per the other thread [0], we should try to avoid the runtime check when
>possible, as it seems to produce a small regression. This means that if
>the ARMv8 CRC instructions are found with the default compiler flags, we
>can only use vmull_p64() if it can also be used with the default flags.
>Otherwise, we can just do the runtime check.

>[0] https://postgr.es/m/2620794.1698783160%40sss.pgh.pa.us

After reading the discussion, I understand that in order to avoid performance
regression in some instances, we need to try our best to avoid runtime checks.
I don't know if I understand it correctly.
if so, we need to check whether to use the ARM CRC32C and VMULL instruction
directly or with runtime check. There will be many scenarios here and the code
will be more complicated.
Could you please give me some suggestions about how to refine this patch?
Thanks very much!

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