From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
Alexey Dobriyan <adobriyan@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions
Date: Sun, 17 Aug 2008 09:56:35 -0700 (PDT) [thread overview]
Message-ID: <alpine.LFD.1.10.0808170936260.3324@nehalem.linux-foundation.org> (raw)
In-Reply-To: <20080817103241.GB21303@elte.hu>
On Sun, 17 Aug 2008, Ingo Molnar wrote:
> +/*
> + * Force a compilation error if condition is true [array index becomes
> + * negative], and a linker error if condition is not constant [non-defined
> + * variable is used as an array index]:
> + *
> + * ( The linker trick relies on gcc optimizing out a multiplication with
> + * constant zero - which should be reasonable enough. )
> + */
> +extern unsigned int __BUILD_BUG_ON_non_constant;
> +
> +#define BUILD_BUG_ON(condition) \
> +do { \
> + (void)sizeof(char[1 - 2*!!(condition)]); \
> + if (!__builtin_constant_p(condition)) \
> + __BUILD_BUG_ON_non_constant++; \
> +} while (0)
Gag me now.
Why not just do
#define __BBO(c) sizeof(const char[1 - 2*!!(c)])
#define __BBONC(c) __BBO(!__builtin_constant_p(c))
#define BUILD_BUG_ON_ZERO(c) (__BBO(c) - __BBONC(c))
#define BUILD_BUG_ON(c) (void)BUILD_BUG_ON_ZERO(c)
and be done with it?
The rules are trivial:
- __BBO() causes a warning if 'c' is a constant non-zero, returns a
(size_t) 1 otherwise
- __BBONC() causes a warning if 'c' is not a constant, returns a
(size_t) 1 otherwise
And then BUILD_BUG_ON[_ZERO] are totally _trivial_ on top of those.
Yeah, yeah, I didn't test this, but it looks a hell of a lot simpler, and
gives the warning about non-constant issues at compile time with line
numbers rather than at link-time.
Linus
next prev parent reply other threads:[~2008-08-17 16:57 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-08-16 10:09 [PATCH] BUILD_BUG_ON sucks Alexey Dobriyan
2008-08-16 10:55 ` Rusty Russell
2008-08-16 20:07 ` Linus Torvalds
2008-08-17 10:32 ` [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions Ingo Molnar
2008-08-17 16:56 ` Linus Torvalds [this message]
2008-08-17 17:33 ` Ingo Molnar
2008-08-17 17:53 ` Ingo Molnar
2008-08-17 18:39 ` Linus Torvalds
2008-08-17 18:45 ` Ingo Molnar
2008-08-18 1:09 ` Rusty Russell
2008-08-18 7:54 ` Ingo Molnar
2008-08-18 9:55 ` Boaz Harrosh
2008-08-18 12:32 ` Boaz Harrosh
2008-08-19 13:34 ` Ingo Molnar
2008-08-19 16:33 ` Boaz Harrosh
2008-08-20 10:59 ` Ingo Molnar
2008-08-20 12:31 ` Boaz Harrosh
2008-08-20 12:39 ` adobriyan
2008-08-20 13:07 ` Boaz Harrosh
2008-08-21 12:17 ` Ingo Molnar
2008-08-25 1:19 ` Rusty Russell
2008-08-20 13:21 ` Boaz Harrosh
2008-08-16 17:46 ` [PATCH] BUILD_BUG_ON sucks Andrew Morton
2008-08-17 12:19 ` Theodore Tso
2008-08-17 16:33 ` Andrew Morton
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