Re: use ARM intrinsics in pg_lfind32() where available

On Mon, Aug 22, 2022 at 11:50:35AM +0700, John Naylor wrote:
> On Sat, Aug 20, 2022 at 5:28 AM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>> Thanks for the pointer. GCC, Clang, and the Arm compiler all seem to
>> define __ARM_NEON, so here is a patch that uses that instead.
>
> Is this also ever defined on 32-bit? If so, is it safe, meaning the
> compiler will not emit these instructions without additional flags?
> I'm wondering if __aarch64__ would be clearer on that, and if we get
> windows-on-arm support as has been proposed, could also add _M_ARM64.

I haven't been able to enable __ARM_NEON on 32-bit, but if it is somehow
possible, we should probably add an __aarch64__ check since functions like
vmaxvq_u32() do not appear to be available on 32-bit. I have been able to
compile for __aarch64__ without __ARM_NEON, so it might still be a good
idea to check for __ARM_NEON. So, to be safe, perhaps we should use
something like the following:

#if (defined(__aarch64__) || defined(__aarch64)) && defined(__ARM_NEON)

> I also see #if defined(__aarch64__) || defined(__aarch64) in our
> codebase already, but I'm not sure what recognizes the latter.

I'm not sure what uses the latter, either.

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com