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

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Message-Id: <20070725.123501.71119586.davem@davemloft.net>
Date:	Wed, 25 Jul 2007 12:35:01 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jeremy@...p.org
Cc:	viro@....linux.org.uk, torvalds@...ux-foundation.org,
	hpa@...or.com, kaos@....com.au, xyzzy@...akeasy.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH][RFC] getting rid of stupid loop in BUG()

From: Jeremy Fitzhardinge <jeremy@...p.org>
Date: Wed, 25 Jul 2007 10:36:49 -0700

> I tried that, but there's still no way of getting a pointer to the ud2a
> instruction in there.  gcc just generates garbage if you try to use use
> the &&label syntax on a label which isn't (potentially) the target of a
> goto (it just gets placed somewhere random).
> 
> But there's a bigger problem than that.  If the BUG is in code which can
> be replicated (ie inlined or unrolled), then it would also require
> replicating the static variable...

Another issue is that if you have a conditional trap instruction on
your cpu, and you try the __label__ trick, GCC no longer converts:

	BUG_ON(test)

into just a:

	set condition codes;
	conditional_trap;

sequence because the "stuff" inside the basic block is something
more than just the __builtin_trap().

The holy grail would be being able to get the perfect conditional
trap sequence, plus the annotations in a seperate section.
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