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

[Peter Zijlstra]

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).

But yes; in general I think it is fine to have 'old' compilers generate
suboptimal code.