linux-kernel - Re: [PATCH] utilize _Static_assert() for BUILD_BUG

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Date:	Mon, 05 Nov 2012 08:46:12 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Rusty Russell" <rusty@...tcorp.com.au>
Cc:	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] utilize _Static_assert() for BUILD_BUG_ON() when
 the compiler supports it

>>> On 05.11.12 at 03:19, Rusty Russell <rusty@...tcorp.com.au> wrote:
> Jan Beulich <JBeulich@...e.com> writes:
>> @@ -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?

That was the original hope, yes, but didn't work because of
certain uses BUILD_BUG_ON() in inline functions. Hence the
next best solution was to use __compiletime_error(), which
still gives a better diagnostic than what we have so far, but is
- see include/linux/compiler-gcc4.h - available only with gcc 4.3
and newer (simply using the construct even when expanding to
nothing - in include/linux/compile.h - wouldn't have the intended
effect of producing a compiler diagnostic at all, it would get
deferred to link time in _all_ cases, even if the compiler can tell
it's a constant).

Jan

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