Re: Add __auto_type C extension, use it in <stdatomic.h>

[Basile Starynkevitch <basile at starynkevitch dot net>]

On Wed, 2013-11-13 at 11:39 +0100, Richard Biener wrote:
> On Wed, Nov 13, 2013 at 1:39 AM, Joseph S. Myers
> <joseph@codesourcery.com> wrote:
> > <stdatomic.h> contains what C11 describes as "generic functions".
> > Although DR#419 makes clear that users cannot #undef these macros (or
> > otherwise suppress use of a macro definition) and expect to find an
> > underlying function, they still need to behave like functions as
> > regards evaluating their arguments exactly once (see C11 7.1.4).
> >
> > I noted when adding <stdatomic.h> to mainline that some of the macro
> > definitions there failed that requirement in the case where the
> > pointer argument had variably modified type, because then typeof
> > evaluates its argument and so that argument would be evaluated twice.
> > Avoiding such double evaluation requires defining the type of a
> > temporary variable, and initializing it with the pointer argument,
> > with a single evaluation.  To achieve this, this patch adds a new GNU
> > C extension __auto_type, essentially a restricted version of C++11
> > auto, and uses it in <stdatomic.h>.
> 
> I suppose you didn't use '__auto' because that's much more likely
> used elsewhere than '__auto_type'?

I have no idea, but does anyone knows if other free compilers (notably
Clang/LLVM) are adding a similar feature?

If they do, perhaps (if it is not too painful) we should use the same
keyword (i.e. __auto_type) and a similar semantics.

And I also like that feature, but it should be documented outside of the
support of <stdatomic.h> since it is genuinely useful by itself (e.g. as
an alternative to typeof).

Regards.
-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
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