Hi Daniel, 2018-08-21 17:11 GMT+09:00 Daniel Santos <daniel.santos@xxxxxxxxx>: > On 08/19/2018 03:25 PM, Nick Desaulniers wrote: >> + gbiv who wrote this cool paste (showing alternatives to >> _Static_assert, which is supported by both compilers in -std=gnu89, >> but not until gcc 4.6): https://godbolt.org/g/DuLsxu >> >> I can't help but think that BUILD_BUG_ON_MSG should use >> _Static_assert, then have fallbacks for gcc < 4.6. > > Unfortunately _Static_assert is a woefully inadequate replacement > because it requires a C constant expression. Example: > > int a = 1; > _Static_assert(a == 1, "a != 1"); > > results in "error: expression in static assertion is not constant." You are right. I tried diagnose_if from Clang: static inline void assert_d(int i) __attribute__((diagnose_if(!i, "oh no", "error"))) { } Clang is silent about int a = 1; assert_d(a); But, if (0) assert_d(0); is error. Hence, it cannot be used for BUILD_BUG(). Anyway, I will just try to rebase this patch and send it to Linus. > Language standards tend to shy away from defining implementation details > like optimizations, but we need to have completed a good data flow > analysis and constant propagation in order to do BUILD_BUG_ON_MSG, et. > al.; this is why they only work when optimizations are enabled. As the > optimizer improves, new expressions can be used with BUILD_BUG_ON*. I > did an analysis of this back in 2012 of how various types of variables > could be resolved to constants at compile-time and how that evolved from > gcc 3.4 to 4.7: > > https://drive.google.com/open?id=1cQRAAOzjFy6Aw7CDc4QauHvd_spVkd5a > > This changed again when -findirect-inline was added -- i.e., > BUILD_BUG_ON could be used on parameters of inline functions even when > called by pointer, although the caller needed __flatten in some cases -- > a bit messy. > > Daniel -- Best Regards Masahiro Yamada
