Messages in this thread |  | | From | Michal Nazarewicz <> | Subject | Re: [PATCH net-next] compiler: Document behavior compiling with -O0 | Date | Fri, 25 Aug 2017 13:45:28 +0200 |
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On Thu, Aug 24 2017, Joe Stringer wrote: > Recent changes[0] to make use of __compiletime_assert() from container_of() > increased the scope of this macro, resulting in a wider set of > situations where developers cannot compile their code using "-O0". I > noticed this when making use of the macro in my own development, and > spent more time than I'd like to admit tracking the problem down. This > patch documents the behavior in lieu of a compile-time assertion > implementation that does not rely on optimizations. > > Example compilation failure: > > ./include/linux/compiler.h:547:38: error: call to ‘__compiletime_assert_94’ declared with attribute error: pointer type mismatch in container_of() > _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__) > ^ > ./include/linux/compiler.h:530:4: note: in definition of macro ‘__compiletime_assert’ > prefix ## suffix(); \ > ^~~~~~ > ./include/linux/compiler.h:547:2: note: in expansion of macro ‘_compiletime_assert’ > _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__) > ^~~~~~~~~~~~~~~~~~~ > ./include/linux/build_bug.h:46:37: note: in expansion of macro ‘compiletime_assert’ > #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg) > ^~~~~~~~~~~~~~~~~~ > ./include/linux/kernel.h:860:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’ > BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \ > ^~~~~~~~~~~~~~~~ > > [0] http://lkml.kernel.org/r/20170525120316.24473-7-abbotti@mev.co.uk > > Signed-off-by: Joe Stringer <joe@ovn.org> > --- > include/linux/compiler.h | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index eca8ad75e28b..bb640167fdac 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -517,6 +517,11 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s > # define __compiletime_error_fallback(condition) do { } while (0) > #endif > > +/* > + * __compiletime_assert() relies on compiler optimizations to remove the check > + * against '__cond' if 'condition' is false. As a result, compiling with -O0 > + * will cause compilation errors here regardless of the value of 'condition'. > + */ > #define __compiletime_assert(condition, msg, prefix, suffix) \ > do { \ > bool __cond = !(condition); \
Could __builtin_choose_expr help here? Something like:
#define __compiletime_assert(condition, msg, prefix, suffix) \ do { \ bool __cond = !(condition); \ extern int prefix ## suffix(void) __compiletime_error(msg); \ __builting_choose_expr(cond, prefix ## suffix(), 0); \ __compiletime_error_fallback(__cond); \ } while (0)
Or better still, _Static_assert?
-- Best regards ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ «If at first you don’t succeed, give up skydiving»
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