Messages in this thread | | | Date | Wed, 22 Apr 2020 06:48:07 -0500 | From | Segher Boessenkool <> | Subject | Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading from scalar types |
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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
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