linux-kernel - Re: [PATCH] sparse: use the _Generic() version of __unqual_scalar

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Date:   Fri, 19 Jun 2020 16:12:37 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] sparse: use the _Generic() version of
 __unqual_scalar_typeof()

On Fri, Jun 19, 2020 at 11:51:05AM +0200, Marco Elver wrote:
> On Fri, Jun 19, 2020 at 12:26AM +0200, Luc Van Oostenryck wrote:
> > If the file is being checked with sparse, use the version of
> > __unqual_scalar_typeof() using _Generic(), leaving the unoptimized
> > version only for the oldest versions of GCC.
> > 
> > This reverts commit
> >   b398ace5d2ea ("compiler_types.h: Use unoptimized __unqual_scalar_typeof for sparse")
> > 
> > Note: a recent version of sparse will be needed (minimum v0.6.2-rc2
> >        or later than 2020-05-28).
> > 
> > Cc: Marco Elver <elver@...gle.com>
> > Cc: Borislav Petkov <bp@...e.de>
> > Cc: Will Deacon <will@...nel.org>
> > Link: https://marc.info/?l=linux-sparse&m=159233481816454
> > Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com>
> 
> Definitely support this change, so in principle:
> 
> 	Acked-by: Marco Elver <elver@...gle.com>
> 
> But, I think sparse still isn't entirely happy with all legal uses of
> _Generic. Running latest sparse on:

Indeed.

> Whereas GCC or Clang accept this as expected. I can't find the
> standardese right now, but in [1] we have:
> 
> 	"[...] The conversion is performed in type domain only: it
> 	discards the top-level cvr-qualifiers and atomicity and applies
> 	array-to-pointer/function-to-pointer transformations to the type
> 	of the controlling expression [...]"
> 
> [1] https://en.cppreference.com/w/c/language/generic

Yes, these are the rules following the resolution of DR481, which
are now used for C17 and also by gcc & clang for C11 but were not
present the C11 standard.

This should be fixed now but I'm waiting for the tests results.

Thanks for reporting this.
-- Luc