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

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Date:	Tue, 19 Aug 2008 15:34:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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


* Boaz Harrosh <bharrosh@...asas.com> wrote:

> > Link time warnings are easy enough to miss.
> > 
> > So unless there's a better way of doing it all at compile time (i'd 
> > really prefer that!) i'd prefer the link time error about botched 
> > BUILD_BUG_ON() conditions - as my commits introduce.
> > 
> > 	Ingo
> > --
> 
> #define BUILD_BUG_ON(condition)						\
> do {
> 	enum { bad = !!(condition)}; 									\
> 	static struct { char arr[1 - 2*bad]; } x __maybe_unused;	\
> } while(0)
> 
> the enum definition will not let in anything not compile-time constant.

nice trick!

> But then I fail on: (include/linux/virtio_config.h:99)
> 
> 	if (__builtin_constant_p(fbit))
> 		BUILD_BUG_ON(fbit >= 32);
> 
> is that code broken?

hmm ... that's a bit sad, gcc ought to have been able to figure this 
out. Can this be fixed somehow, without losing the strength of the 
checking here? I think we should not change BUILD_BUG_ON() to make it 
less useful.

	Ingo
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