Re: CRC32C Parallel Computation Optimization on ARM

On Mon, Oct 30, 2023 at 11:21:43AM -0500, Nathan Bossart wrote:
> On Fri, Oct 27, 2023 at 07:01:10AM +0000, Xiang Gao wrote:
>> On Thu, 26 Oct, 2023 11:37:52AM -0500, Nathan Bossart wrote:
>>>> We consider that a runtime check needs to be done in any scenario.
>>>> Here we only confirm that the compilation can be successful.
>>> >A runtime check will be done when choosing which algorithm.
>>> >You can think of us as merging USE_ARMV8_VMULL and USE_ARMV8_VMULL_WITH_RUNTIME_CHECK into USE_ARMV8_VMULL.
>>
>>>Oh. Looking again, I see that we are using a runtime check for ARM in all
>>>cases with this patch. If so, maybe we should just remove
>>>USE_ARV8_CRC32C_WITH_RUNTIME_CHECK in a prerequisite patch (and have
>>>USE_ARMV8_CRC32C always do the runtime check). I suspect there are other
>>>opportunities to simplify things, too.
>>
>> Yes, I have been removed USE_ARMV8_CRC32C_WITH_RUNTIME_CHECK in this patch.
>
> 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

--
Nathan Bossart
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