linux-kernel - Re: [PATCH][RFC] getting rid of stupid loop in BUG()

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Date:	Tue, 24 Jul 2007 19:31:36 -0700 (PDT)
From:	Trent Piepho <xyzzy@...akeasy.org>
To:	Al Viro <viro@....linux.org.uk>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH][RFC] getting rid of stupid loop in BUG()

On Tue, 24 Jul 2007, Al Viro wrote:
> 	AFAICS, the patch below should do it for i386; instead of
> using a dummy loop to tell gcc that this sucker never returns,
> we do
> static void __always_inline __noreturn __BUG(const char *file, int line);
> containing the actual asm we want to insert and define BUG() as
> __BUG(__FILE__, __LINE__).  It looks safe, but I don't claim enough
> experience with gcc __asm__ potential nastiness, so...

Sounds like it doesn't work:
http://gcc.gnu.org/ml/gcc/2007-02/msg00107.html

    [The] programmer won't get optimization he wants as after inlining this as
    after inlining this attribute information becomes completely lost.

    What about __builtin_trap?

    It results in int 6 that might not be applicable, but adding some control
    over it to i386 backend is definitly an option.

    Honza

It seems like if __BUG() is not inlined, you get the bogus noreturn does
return warning.  If it is inlined, then you lose the noreturn attribute and
un-reachable code paths aren't eliminated.  Adding __builtin_trap after the
asm might be an ok fix.  It will emit a spurious int 6, but that won't even be
reached since the asm doesn't return, and it probably be less extra code than
the loop.
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