From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
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 20:45:10 +0200 [thread overview]
Message-ID: <20080817184510.GA16508@elte.hu> (raw)
In-Reply-To: <alpine.LFD.1.10.0808171129350.3324@nehalem.linux-foundation.org>
* Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>
> On Sun, 17 Aug 2008, Ingo Molnar wrote:
> >
> > try the patch below - it only gives this error during build:
>
> Well, you didn't do it right:
>
> > +#define __BBO(c) sizeof(const char[1 - 2*!!(c)])
> > +#define __BBONC(c) __BBO(!__builtin_constant_p(c))
> > +#define BUILD_BUG_ON_ZERO2(c) (__BBO(c) - __BBONC(c))
> > +#define BUILD_BUG_ON2(c) (void)BUILD_BUG_ON_ZERO(c)
>
> Look at the #define of BUILD_BUG_ON2 a bit more.
>
> Hint: you're using the _wrong_ BUILD_BUG_ON_ZERO. The old one, not the v2
> one!
yeah, i already tried various variants earlier today so i really didnt try
that hard with yours. (and i pointed out this mistake in the previous mail)
> That said, with that fixed, there's still something wrong. It does seem
> like gcc has some very odd interaction there with __builtin_constant_p.
> Odd.
yeah. I tried various integer arithmetic expressions (which the array trick
relies on) and it didnt work as expected - it's always zero. It only makes
a difference when used in comparisons. (and that's where the kernel uses
__builtin_constant_p quite heavily, and it works fine there.)
Odd indeed.
Ingo
next prev parent reply other threads:[~2008-08-17 18:45 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
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 [this message]
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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