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

On Tue, Dec 16, 2025, at 8:57 PM, Thomas Munro wrote:
> On Wed, Dec 17, 2025 at 10:40 AM Nathan Bossart
> <nathandbossart(at)gmail(dot)com> wrote:

Hey Nathan, Thomas, thanks for your continued investment of time in this patch. More thoughts below, but I wonder if the thing to do is to commit this and then for me to follow-on with a few more targeted changes to round out this platform combo?

>> Hm. I think the USE_LLVM_BACKPORT_SECTION_MEMORY_MANAGER thing might need
>> work, too. We don't have any Windows buildfarm machines with LLVM enabled,
>> but IIUC it should be possible. Perhaps we can add that to unicorn.
>
> The LLVM code has never run on Windows and will likely need
> patching... I know of at least one change required and will write
> about that.

I think it would be amazing to support this eventually, but as I'm not a MSVC or Win11 guru this feels like a longer term patch. I'm still struggling with simpler things at the moment. The platform is not well established so some libraries are missing or out of date. There's no ARM64-native Perl either.

>> > Also, while the patch
>> > is targeting Windows 11 (IIUC), there are some notes in the docs that give
>> > the impression Windows 10 is supported, too [0]. I could easily change it
>> > to say that AArch64 requires Windows 11, but I don't know what to do with
>> > the references to specific versions of Visual Studio and the Windows SDK.

When I get a chance I'll post a docs patch for debate/review.

>> Actually, I'm not sure there's anything specific to Windows 11 in this
>> patch, besides perhaps the choice to set USE_ARMV8_CRC32C unconditionally.
>> I don't know how likely it is that someone will try to run Postgres on
>> Windows on an AArch64 machine without CRC extension support, though.
>
> Assuming you haven't blocked OS updates, Windows stopped booting on
> pre-ARMv8.1 hardware a while back. RPi4's Broadcom chip and the
> Snapdragon 835 (found in the oldest Windows laptops, according to a
> quick Google search) are ARMv8-A only, but did actually have the CRC32
> instructions, so it would actually work anyway. They also have the
> optional NEON SIMD stuff, which we use unconditionally:
>
> /*
> * We use the Neon instructions if the compiler provides access to them (as
> * indicated by __ARM_NEON) and we are on aarch64. While Neon support is
> * technically optional for aarch64, it appears that all available 64-bit
> * hardware does have it. Neon exists in some 32-bit hardware too, but we
> * could not realistically use it there without a run-time check, which seems
> * not worth the trouble for now.
> */
>
> A single chip in this report lacked FEAT_CRC32, the X-Gene 1 from 2012:
>
> https://gpages.juszkiewicz.com.pl/arm-socs-table/arm-socs.html
>
> So I don't think it's worth worrying about, I was just mentioning the
> "Windows 11 requires CRC32, Windows 10 is dead" thing to avoid Greg
> being forced to waste time researching the missing feature test code
> :-) The reason Windows can't boot on old ARM chips probably has more
> to do with the modern atomics needed for decent lock performance,
> which every kernel wants.

I think there are a few things to offer up in the next few weeks:
* docs
* popcnt
* SIMD
* wiki page with HOWTO info for the next "brave" soul to give this a go
* what else?

What this patch does is get "unicorn" off the dead list, which would be nice.

-greg