linux-kernel - Re: [PATCH 4/4] bug: mark disabled BUG() as unreachable() code

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Date:	Sat, 28 Apr 2012 09:10:18 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4] bug: mark disabled BUG() as unreachable() code

Konstantin Khlebnikov wrote:
> This patch suppress some compiler warnings (if CONFIG_BUG=n)
> "warning: control reaches end of non-void function"

With this patch gcc throw out loop at the end of do_exit():

	schedule();
	BUG();
	/* Avoid "noreturn function does return".  */
	for (;;)
		cpu_relax();	/* For when BUG is null */

Is this ok? Probably not, and we need here some BUG_NORETURN() which
really never returns even if CONFIG_BUG=n.

>
> Plus now gcc can throw out some dead code.
> bloat-o-meter: add/remove: 1/1 grow/shrink: 68/173 up/down: 1569/-5607 (-4038)
>
> Signed-off-by: Konstantin Khlebnikov<khlebnikov@...nvz.org>
> ---
>   include/asm-generic/bug.h |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> index aadb6fc..0b08199 100644
> --- a/include/asm-generic/bug.h
> +++ b/include/asm-generic/bug.h
> @@ -108,7 +108,7 @@ extern void warn_slowpath_null(const char *file, const int line);
>
>   #else /* !CONFIG_BUG */
>   #ifndef HAVE_ARCH_BUG
> -#define BUG() do {} while(0)
> +#define BUG() unreachable()
>   #endif
>
>   #ifndef HAVE_ARCH_BUG_ON
>

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