Horton

Re: Improve CRC32C performance on SSE4.2

John Naylor <johncnaylorls@gmail.com>

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From: John Naylor <johncnaylorls@gmail.com>
To: Andy Fan <zhihuifan1213@163.com>
Cc: "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com>, Nathan Bossart <nathandbossart@gmail.com>, Jesper Pedersen <jesperpedersen.db@gmail.com>, Tomas Vondra <tomas@vondra.me>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, "Shankaran, Akash" <akash.shankaran@intel.com>
Date: 2025-06-17T08:55:06Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Include _mm512_zextsi128_si512() in AVX-512 configure probes.

  2. Properly fix AVX-512 CRC calculation bug

  3. Workaround code generation bug in clang

  4. Compute CRC32C using AVX-512 instructions where available

  5. Inline CRC computation for small fixed-length input on x86

  6. Be more paranoid in configure's checks for CRC and POPCNT intrinsics.

Attachments

On Tue, Jun 17, 2025 at 6:40 AM Andy Fan <zhihuifan1213@163.com> wrote:
>
> "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com> writes:
>
> > Great catch! From the intrinsic manual:
> >
> > Cast vector of type __m128i to type __m512i; the upper 384 bits of the
> > result are undefined.

Thanks Raghuveer and Nathan, for the diagnosis!

> Just be curious, what kind of optimization (like what -O2 does) could
> mask this issue?

In case Andy is asking about "how" rather than "under what
circumstances", my guess is: -O1+  may have just chosen instructions
that also happen to zero-extend, which are common. -O0 doesn't
represent the naive straightforward structure of what the programmer
wrote, it's more like an "exploded" representation suitable for later
optimization passes. That's why it always looks goofy.

> > Replacing that with _mm512_zextsi128_si512 fixes the problem.

Here's a patch for testing, which also reverts the previous
workaround. Help welcome, but I still promise to test it in the near
future regardless.

--
John Naylor
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