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Date: Wed, 25 Jul 2007 10:41:27 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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Subject: Re: [PATCH][RFC] getting rid of stupid loop in BUG()
References: <21803.1185331847@ocs10w.ocs.com.au> <46A6DB04.3000003@zytor.com> <46A6ECB7.7090208@goop.org> <20070724.232914.56812376.davem@davemloft.net> <alpine.LFD.0.999.0707250954400.3607@woody.linux-foundation.org> <20070725171956.GF27237@ftp.linux.org.uk>
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To: Al Viro <viro@ftp.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, David Miller <davem@davemloft.net>, hpa@zytor.com, kaos@ocs.com.au, xyzzy@speakeasy.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
List-ID: <linux-arch.vger.kernel.org>

Al Viro wrote:
> On Wed, Jul 25, 2007 at 09:56:12AM -0700, Linus Torvalds wrote:
>   
>> On Tue, 24 Jul 2007, David Miller wrote:
>>     
>>> From: Jeremy Fitzhardinge <jeremy@goop.org>
>>> Date: Tue, 24 Jul 2007 23:24:55 -0700
>>>
>>>       
>>>> H. Peter Anvin wrote:
>>>>         
>>>>> How far back was __builtin_trap() supported?
>>>>>           
>>>> I think its relatively recent, but it might be within our supported
>>>> compiler window.
>>>>         
>>> I'm pretty sure it is.
>>>       
>> .. and I'm pretty sure it's immaterial.
>>
>> We don't just do the "ud2" instruction - we also do the file and line 
>> number information after it. Which means that __builtin_trap() is useless.
>>
>> So we might as well keep the loop, since both are two-byte instructions 
>> that tell gcc that it will never continue.
>>     
>
> Umm...  Actually, we might be able to do something like
> {
> 	l: __builtin_trap();
> 	static struct ... v __attribute__((section(...))) = { &&l, n, file };
> }
>
> except that it would need block-local labels and those are ugly (so's
> &&<label>, while we are at it)...
>   

I couldn't get it to work.  It would be nice, because it would more or
less eliminate the need for asm in setting up BUGs - particularly the
nasty asm setting up the structure.

But it just doesn't work.  The label &&l ends up pointing to is not
anywhere near the __builtin_trap instruction; I found it tended to point
to the start of the function prologue.

I reported it as a gcc bug, but they refused to hear of it.  Details at
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29305

    J
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