Re: vectorized CRC on ARM64

On Tue, Mar 17, 2026 at 11:52 PM John Naylor <johncnaylorls@gmail.com>
wrote:

> On Wed, Mar 18, 2026 at 10:34 AM Haibo Yan <tristan.yim@gmail.com> wrote:
> >
> > Hi John
> >
> > Thank yo for working on this. I had one question about the mixed use of
> intrinsics and inline asm here.
>
> > Since the implementation already uses NEON intrinsics such as vld1q_u64,
> I was wondering why the pmull / pmull2 + eor helpers still need to be
> inline asm rather than intrinsics.
> >
> > Is that due to compiler/toolchain support, or because the
> intrinsic-based version produced noticeably worse code?
>
> I answered that in the email you replied to, re-quoted here:
>
> > To follow-up for curiosity's sake, [1] says that Apple chips can issue
> > PMULL + EOR as a single uop if they are next to each other in the
> > instruction stream.
> > [1] https://dougallj.github.io/applecpu/firestorm.html
>
> I don't know if that's relevant for current server hardware, so it
> could be pointless. I'm personally not a fan of inline assembly, but I
> also didn't yet want to put in the effort to alter generated code. I
> don't think it would be very hard to do, however.
>

Thanks, that makes sense as an explanation for why the inline asm is there
today. But it also sounds like this is more of a temporary implementation
choice than a conclusion that intrinsics are unsuitable. If so, I wonder
whether it would be better to treat an intrinsics-based version as the
preferred end state unless benchmarks show a clear regression.
Regards
Haibo