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 <[email protected]> > Sent: Monday, June 16, 2025 3:14 PM > To: Devulapalli, Raghuveer <[email protected]> > Cc: John Naylor <[email protected]>; Andy Fan > <[email protected]>; Jesper Pedersen <[email protected]>; > Tomas Vondra <[email protected]>; [email protected]; > Shankaran, Akash <[email protected]> > 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
