On 25 August 2017 at 04:45, Michal Nazarewicz <[email protected]> wrote: > 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/[email protected] >> >> Signed-off-by: Joe Stringer <[email protected]> >> --- >> 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?
I tried both of the above, and they both complain that "condition" isn't a constant.
