[PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y

[Marco Elver]

 On Wed, 4 Feb 2026 at 15:15, Will Deacon <will at kernel.org> wrote:
>
> On Wed, Feb 04, 2026 at 02:14:00PM +0100, Peter Zijlstra wrote:
> > On Wed, Feb 04, 2026 at 11:46:02AM +0100, Marco Elver wrote:
> > > On Tue, 3 Feb 2026 at 12:47, Will Deacon <will at kernel.org> wrote:
> > > [...]
> > > > > > What does GCC do with this? :/
> > > > >
> > > > > GCC currently doesn't see it, LTO is clang only.
> > > >
> > > > LTO is just one way that a compiler could end up breaking dependency
> > > > chains, so I really want to maintain the option to enable this path for
> > > > GCC in case we run into problems caused by other optimisations in future.
> > >
> > > It will work for GCC, but only from GCC 11. Before that __auto_type
> > > does not drop qualifiers:
> > > https://godbolt.org/z/sc5bcnzKd (switch to GCC 11 to see it compile)
> > >
> > > So to summarize, all supported Clang versions deal with __auto_type
> > > correctly for the fallback; GCC from version 11 does (kernel currently
> > > supports GCC 8 and above). From GCC 14 and Clang 19 we have
> > > __typeof_unqual__.
> > >
> > > I really don't see another way forward; there's no other good way to
> > > solve this issue. I would advise against pessimizing new compilers and
> > > features because maybe one day we might still want to enable this
> > > version of READ_ONCE() for GCC 8-10.
> > >
> > > Should we one day choose to enable this READ_ONCE() version for GCC,
> > > we will (a) either have bumped the minimum GCC version to 11+, or (b)
> > > we can only do so from GCC 11. At this point GCC 11 was released 5
> > > years ago!
> >
> > There is, from this thread:
> >
> >   https://lkml.kernel.org/r/20260111182010.GH3634291@ZenIV
> >
> > another trick to strip qualifiers:
> >
> >   #define unqual_non_array(T) __typeof__(((T(*)(void))0)())
> >
> > which will work from GCC-8.4 onwards. Arguably, it should be possible to
> > raise the minimum from 8 to 8.4 (IMO).

That looks like an interesting option.

> That sounds reasonable to me but I'm not usually the one to push back
> on raising the minimum compiler version!
>
> > But yes; in general I think it is fine to have 'old' compilers generate
> > suboptimal code.
>
> I'm absolutely fine with the codegen being terrible for ancient
> toolchains as long as it's correct.

>From that discussion a month ago and this one, it seems we need
something to fix __unqual_scalar_typeof().

What's the way forward?

1. Bump minimum GCC version to 8.4. Replace __unqual_scalar_typeof()
for old compilers with the better unqual_non_array hack?

2. Leave __unqual_scalar_typeof() as-is. The patch "compiler: Use
__typeof_unqual__() for __unqual_scalar_typeof()" will fix the codegen
issues for new compilers. Doesn't fix not dropping 'const' for old
compilers for non-scalar types, and requires localized workarounds
(like this patch here).

Either way we need a fix for this arm64 LTO version to fix the
context-analysis "see through" the inline asm (how this patch series
started).

Option #1 needs a lot more due-diligence and testing that it all works
for all compilers and configs (opening Pandora's Box :-)). For option
#2 we just need these patches here to at least fix the acute issue
with this arm64 LTO version.