Re: [PATCH] utilize _Static_assert() for BUILD_BUG_ON() when the compiler supports it

[Rusty Russell]

Jan Beulich <JBeulich@xxxxxxxx> writes:
> This makes the resulting diagnostics quite a bit more useful.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Nice.  I'm a bit disappointed we can't just treat _Static_assert() as
void, like:

#define BUILD_BUG_ON_ZERO(e) (_Static_assert(!(e), "!(" #e ")"), 0)

> @@ -54,6 +61,15 @@ struct pt_regs;
>   */
>  #ifndef __OPTIMIZE__
>  #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> +#elif __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)
> +#define __build_bug_on_failed(n) __build_bug_on_##n##_failed
> +#define _BUILD_BUG_ON(n, condition)				\
> +	do {							\
> +		extern void __compiletime_error(#condition)	\
> +		__build_bug_on_failed(n)(void);			\
> +		if (condition) __build_bug_on_failed(n)();	\
> +	} while(0)
> +#define BUILD_BUG_ON(condition...) _BUILD_BUG_ON(__COUNTER__, ##condition)
>  #else
>  extern int __build_bug_on_failed;
>  #define BUILD_BUG_ON(condition)					\

Why does this depend on gcc version?  Looks like residue from an attempt
to use _Static_assert here?

Thanks,
Rusty.
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