linux-kernel - Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

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Message-ID: <alpine.LFD.1.10.0808171129350.3324@nehalem.linux-foundation.org>
Date:	Sun, 17 Aug 2008 11:39:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions



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!

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.

			Linus
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