RE: Popcount optimization using AVX512

> -----Original Message-----
> From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
> Sent: Thursday, March 28, 2024 2:39 PM
> To: Amonson, Paul D <paul(dot)d(dot)amonson(at)intel(dot)com>
>
> * The latest patch set from Paul Amonson appeared to support MSVC in the
> meson build, but not the autoconf one. I don't have much expertise here,
> so the v14 patch doesn't have any autoconf/meson support for MSVC, which
> I thought might be okay for now. IIUC we assume that 64-bit/MSVC builds
> can always compile the x86_64 popcount code, but I don't know whether
> that's safe for AVX512.

I also do not know how to integrate MSVC+Autoconf, the CI uses MSVC+Meson+Ninja so I stuck with that.

> * I think we need to verify there isn't a huge performance regression for
> smaller arrays. IIUC those will still require an AVX512 instruction or
> two as well as a function call, which might add some noticeable overhead.

Not considering your changes, I had already tested small buffers. At less than 512 bytes there was no measurable regression (there was one extra condition check) and for 512+ bytes it moved from no regression to some gains between 512 and 4096 bytes. Assuming you introduced no extra function calls, it should be the same.

> I forgot to mention that I also want to understand whether we can actually assume availability of XGETBV when CPUID says we support AVX512:

You cannot assume as there are edge cases where AVX-512 was found on system one during compile but it's not actually available in a kernel on a second system at runtime despite the CPU actually having the hardware feature.

I will review the new patch to see if there are anything that jumps out at me.

Thanks,
Paul