Thread

Re: [PATCH] Fix ARM64/MSVC atomic memory ordering issues on Win11 by adding explicit DMB ​barriers

Thomas Munro <thomas.munro@gmail.com> — 2025-11-23T20:32:39Z

On Mon, Nov 24, 2025 at 4:55 AM Andres Freund <andres@anarazel.de> wrote:
> > -  if cc.links(prog, name: '__crc32cb, __crc32ch, __crc32cw, and __crc32cd without -march=armv8-a+crc',
> > +  if cc.get_id() == 'msvc'
> > +    cdata.set('USE_ARMV8_CRC32C', 1)
> > +    have_optimized_crc = true
>
> Should have a comment explaining why we can do this unconditionally...

I was wondering about that in light of your revelation that it must be
using /arch:armv8.0 by default.  If we only support Windows/ARM on
armv8.1 hardware (like Windows 11 itself), then I think that means
that we'd want /arch:armv8.1 here:

+  # Add ARM64 architecture flag for Windows 11 ARM64 for correct intrensics
+  if host_machine.system() == 'windows' and host_machine.cpu_family()
== 'aarch64'
+    add_project_arguments('/arch:armv9.4', language: ['c', 'cpp'])
+  endif

We can't impose our own random high ISA requirement like that, or some
machines will choke on illegal instructions.

I wonder if the same applies to Visual Studio on x86.  The OS now
requires x86-64-v2 (like RHEL9, and many other Linux distros except
Debian?), but Visual Studio might not know that... but then we might
say that's an issue for the EDB packaging crew to think about, not us.

In that case we might want to say "OK you choose the ARM version, but
we're not going to write the runtime test for CRC on armv8, we'll do a
compile-time test only, because it would be stupid to waste time
writing code for armv8.0".