Re: use ARM intrinsics in pg_lfind32() where available

On Tue, Aug 23, 2022 at 4:15 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
>
> On Mon, Aug 22, 2022 at 11:50:35AM +0700, John Naylor wrote:

> > 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.

The important thing is: if we compile with __aarch64__ as a target:
- Will the compiler emit the intended instructions from the intrinsics
without extra flags?
- Can a user on ARM64 ever get a runtime fault if the machine attempts
to execute NEON instructions? "I have been able to compile for
__aarch64__ without __ARM_NEON" doesn't really answer that question --
what exactly did this entail?

> > 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.

I took a quick look around at Debian code search, *BSD, Apple, and a
few other places, and I can't find it. Then, I looked at the
discussions around commit 5c7603c318872a42e "Add ARM64 (aarch64)
support to s_lock.h", and the proposed patch [1] only had __aarch64__
. When it was committed, the platform was vaporware and I suppose we
included "__aarch64" as a prophylactic measure because no other reason
was given. It doesn't seem to exist anywhere, so unless someone can
demonstrate otherwise, I'm going to rip it out soon.

[1] https://www.postgresql.org/message-id/flat/1368448758.23422.12.camel%40t520.redhat.com

-- 
John Naylor
EDB: http://www.enterprisedb.com