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

[Marco Elver]

On Fri, 6 Feb 2026 at 19:26, David Laight <david.laight.linux at gmail.com> wrote:
> On Fri, 6 Feb 2026 16:09:35 +0100
> Marco Elver <elver at google.com> wrote:
>
> >  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.
>
> Option 3.
>
> Look are where/why they are used and change the code to do it differently.
> Don't forget the similar __unsigned_scalar_typeof() in bitfield.h.
> (I posted a patch that nuked that one not long ago - used sizeof instead.)
>
> The one in minmax_array (in minmax.h) is particularly pointless.
> The value 'suffers' integer promotion as soon as it is used, nothing
> wrong with 'auto _x = x + 0' there.
> That will work elsewhere.

Agreed that getting rid of __unqual_scalar_typeof() in favor of 'auto'
where possible is the way to go.

Unfortunately I spent the last week occasionally glancing at this
arm64 READ_ONCE problem, and could not come up with something that
avoids using typeof_unqual() or __unqual_scalar_typeof(). I'm inclined
to go with the unqual_non_array hack, but not make this available as a
macro for general use - we have too many of these horrid macros, don't
want to add more to this hack pile.