Re: Popcount optimization using AVX512

On Thu, Apr 18, 2024 at 06:12:22PM +0000, Shankaran, Akash wrote:
> Good find. I confirmed after speaking with an intel expert, and from the intel AVX-512 manual [0] section 14.3, which recommends to check bit27. From the manual:
>
> "Prior to using Intel AVX, the application must identify that the operating system supports the XGETBV instruction,
> the YMM register state, in addition to processor's support for YMM state management using XSAVE/XRSTOR and
> AVX instructions. The following simplified sequence accomplishes both and is strongly recommended.
> 1) Detect CPUID.1:ECX.OSXSAVE[bit 27] = 1 (XGETBV enabled for application use1).
> 2) Issue XGETBV and verify that XCR0[2:1] = '11b' (XMM state and YMM state are enabled by OS).
> 3) detect CPUID.1:ECX.AVX[bit 28] = 1 (AVX instructions supported).
> (Step 3 can be done in any order relative to 1 and 2.)"

Thanks for confirming. IIUC my patch should be sufficient, then.

> It also seems that step 1 and step 2 need to be done prior to the CPUID OSXSAVE check in the popcount code.

This seems to contradict the note about doing step 3 at any point, and
given step 1 is the OSXSAVE check, I'm not following what this means,
anyway.

I'm also wondering if we need to check that (_xgetbv(0) & 0xe6) == 0xe6
instead of just (_xgetbv(0) & 0xe0) != 0, as the status of the lower half
of some of the ZMM registers is stored in the SSE and AVX state [0]. I
don't know how likely it is that 0xe0 would succeed but 0xe6 wouldn't, but
we might as well make it correct.

[0] https://en.wikipedia.org/wiki/Control_register#cite_ref-23

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Nathan Bossart
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