GCC used to store pointers in FP registers on aarch64

[Attila Szegedi]

On Mon, Feb 24, 2025 at 1:21 PM Florian Weimer <fw@deneb.enyo.de> wrote:

> * Attila Szegedi:
>
> >> That seems … quite unlikely.  GCC 8 has seen extensive use on
> >> AArch64, on a variety of implementations, and I don't recall
> >> problems in this area.  I don't follow AArch64 *that* closely,
> >> admittedly, but I expect it would have caused quite a ruckus.
> >>
> >
> > Yeah. The lack of discussion also led me to believe that even if this is
> an
> > issue, it's definitely not a widely encountered one. (It's also possible
> > that it's a red herring, although, well, as I said, forcing general regs
> > only did fix it.)
>
> Is it non-deterministic?  It might be a context switching issue in the
> kernel/hypervisor/firmware.  I usually don't notice fixes for those
> because they do not lead to questions whether it's necessary to
> rebuild the whole distribution.  These bugs do happen from time to time:
>
>   [PATCH v3 0/8] KVM: arm64: FPSIMD/SVE/SME fixes
>   <
> https://lore.kernel.org/linux-arm-kernel/20250210195226.1215254-1-mark.rutland@arm.com/
> >
>

Huh. That is interesting. Yes, it is non-deterministic. And it does occur
solely in containerized environments, so it's eminently possible it's a
hypervisor issue.

(Still, a bit amusing that nothing came up with regard to how I don't see
GCC 12 schedule pointers onto aarch64 FP registers anymore :-).  I
understood from Kyrylo's post that it's probably because no such explicit
decision was made.)

Attila.