linux-kernel - Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse: incorrect type in initializer (different address spaces)

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Date: Tue, 2 Apr 2024 13:43:00 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, kernel test robot <lkp@...el.com>, 
	oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Arjan van de Ven <arjan@...ux.intel.com>, x86@...nel.org, 
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>, 
	Sparse Mailing-list <linux-sparse@...r.kernel.org>, "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse:
 incorrect type in initializer (different address spaces)

On Mon, Mar 4, 2024 at 8:07 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Mon, Mar 04 2024 at 06:42, Uros Bizjak wrote:
>
> > On Mon, Mar 4, 2024 at 12:49 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >>
> >> On Sun, Mar 03 2024 at 21:24, Uros Bizjak wrote:
> >> > On Sun, Mar 3, 2024 at 9:21 PM Uros Bizjak <ubizjak@...il.com> wrote:
> >> >> On Sun, Mar 3, 2024 at 9:10 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >> >> > That's so sad because it would provide us compiler based __percpu
> >> >> > validation.
> >> >>
> >> >> Unfortunately, the c compiler can't strip qualifiers, so typeof() is
> >> >> of limited use also when const and volatile qualifiers are used.
> >> >> Perhaps some extension could be introduced to c standard to provide an
> >> >> unqualified type, e.g. typeof_unqual().
> >> >
> >> > Oh, there is one in C23 [1].
> >>
> >> Yes. I found it right after ranting.
> >>
> >> gcc >= 14 and clang >= 16 have support for it of course only when adding
> >> -std=c2x to the command line.
> >>
> >> Sigh. The name space qualifiers are non standard and then the thing
> >> which makes them more useful is hidden behind a standard.
> >
> > With GCC, you can use __typeof_unqual__ (please note underscores)
> > without -std=c2x [1]:
> >
> > "... Alternate spelling __typeof_unqual__ is available in all C modes
> > and provides non-atomic unqualified version of what __typeof__
> > operator returns..."
> >
> > Please also see the example in my last post. It can be compiled without -std=...
>
> With gcc >= 14. Not so with clang...

Please note that clang-17.0.6 currently fails to compile kernel with
named address spaces [1]. So perhaps kernel can use __typeof_unqual__
(available without -std=c2x) in the hope that clang implements
__typeof_unqual__ in one of its next releases, following the examples
of GCC [2] and MSVC[3].

[1] https://lore.kernel.org/lkml/20240320173758.GA3017166@dev-arch.thelio-3990X/
[2] https://gcc.gnu.org/onlinedocs/gcc/Typeof.html
[3] https://learn.microsoft.com/en-us/cpp/c-language/typeof-unqual-c?view=msvc-170

Uros.