Hi!
On Wed, Apr 22, 2020 at 12:25:03PM +0200, Rasmus Villemoes wrote:
> On 21/04/2020 17.15, Will Deacon wrote:
> > Unfortunately, dropping pointer qualifiers inside the macro poses quite
> > a challenge, especially since the pointed-to type is permitted to be an
> > aggregate, and this is relied upon by mm/ code accessing things like
> > 'pmd_t'. Based on numerous hacks and discussions on the mailing list,
> > this is the best I've managed to come up with.
>
> Hm, maybe this can be brought to work, only very lightly tested. It
> basically abuses what -Wignored-qualifiers points out:
>
> warning: type qualifiers ignored on function return type
>
> Example showing the idea:
>
> const int c(void);
> volatile int v(void);
>
> int hack(int x, int y)
> {
> typeof(c()) a = x;
> typeof(v()) b = y;
>
> a += b;
> b += a;
> a += b;
> return a;
> }
Nasty. I like it :-)
> Since that compiles, a cannot be const-qualified, and the generated code
> certainly suggests that b is not volatile-qualified. So something like
>
> #define unqual_type(x) _unqual_type(x, unique_id_dance)
> #define _unqual_type(x, id) typeof( ({
> typeof(x) id(void);
> id();
> }) )
>
> and perhaps some _Pragma("GCC diagnostic push")/_Pragma("GCC diagnostic
> ignored -Wignored-qualifiers")/_Pragma("GCC diagnostic pop") could
> prevent the warning (which is in -Wextra, so I don't think it would
> appear in a normal build anyway).
>
> No idea how well any of this would work across gcc versions or with clang.
https://gcc.gnu.org/legacy-ml/gcc-patches/2016-05/msg01054.html
This is defined to work this way in ISO C since C11.
But, it doesn't work with GCC before GCC 7 :-(
Segher
