Great catch! From the intrinsic manual:
Cast vector of type __m128i to type __m512i; the upper 384 bits of the result are undefined.
Replacing that with _mm512_zextsi128_si512 fixes the problem.
> -----Original Message-----
> From: Nathan Bossart <nathandbossart@gmail.com>
> Sent: Monday, June 16, 2025 3:14 PM
> To: Devulapalli, Raghuveer <raghuveer.devulapalli@intel.com>
> Cc: John Naylor <johncnaylorls@gmail.com>; Andy Fan
> <zhihuifan1213@163.com>; Jesper Pedersen <jesperpedersen.db@gmail.com>;
> Tomas Vondra <tomas@vondra.me>; pgsql-hackers@lists.postgresql.org;
> Shankaran, Akash <akash.shankaran@intel.com>
> Subject: Re: Improve CRC32C performance on SSE4.2
>
> On Mon, Jun 16, 2025 at 06:31:11PM +0000, Devulapalli, Raghuveer wrote:
> > Attached is a simple reproducer. It passes with clang v16 -O0, but
> > fails with 17 and 18 only when built with -O0..
>
> I've just started looking into this, but the difference in code generated for
> _mm512_castsi128_si512() between gcc, clang 16, and clang 17 looks interesting.
>
> --
> nathan
