Re: Question about the Implementation of vector32_is_highbit_set on ARM

On Wed, Nov 8, 2023 at 2:44 PM Xiang Gao <Xiang(dot)Gao(at)arm(dot)com> wrote:
> * function. We could instead adopt the behavior of Arm's vmaxvq_u32(), i.e.
> * check each 32-bit element, but that would require an additional mask
> * operation on x86.
> */

> But I still don't understand why the vmaxvq_u32 intrinsic is not used on the arm platform.

The current use case expects all 1's or all 0's in a 32-bit lane. If
anyone tried using it for arbitrary values, vmaxvq_u32 could give a
different answer than on x86 using _mm_movemask_epi8, so I think
that's the origin of that comment. But it's still a maintenance hazard
as is, since x86 wouldn't work for arbitrary values. It seems the path
forward is to rename this function to vector32_is_any_lane_set(), as
in the attached (untested on Arm). That would allow each
implementation to use the most efficient path, whether it's by 8- or
32-bit lanes. If we someday needed to look at only the high bits, we
would need a new function that performed the necessary masking on x86.

It's possible this method could shave cycles on Arm in some 8-bit lane
cases where we don't actually care about the high bit specifically,
since the movemask equivalent is slow on that platform, but I haven't
looked yet.