linux-kernel - Re: [PATCH 02/10] x86: Convert BUG() to use unreachable()

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Date:	Fri, 11 Sep 2009 08:07:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	David Daney <ddaney@...iumnetworks.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Roland McGrath <roland@...hat.com>
Subject: Re: [PATCH 02/10] x86: Convert BUG() to use unreachable()


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, 10 Sep 2009, H. Peter Anvin wrote:
> 
> > On 09/10/2009 04:56 PM, David Daney wrote:
> > > Use the new unreachable() macro instead of for(;;);.  When
> > > allyesconfig is built with a GCC-4.5 snapshot on i686 the size of the
> > > text segment is reduced by 3987 bytes (from 6827019 to 6823032).
> > > 
> > > Signed-off-by: David Daney <ddaney@...iumnetworks.com>
> > > CC: Thomas Gleixner <tglx@...utronix.de>
> > > CC: Ingo Molnar <mingo@...hat.com>
> > > CC: "H. Peter Anvin" <hpa@...or.com>
> > > CC: x86@...nel.org
> > 
> > Acked-by: H. Peter Anvin <hpa@...or.com>
> > 
> > ... although of course this clashes with Roland McGrath's 
> > patchset for the same thing which I applied earlier.  I have to 
> > say I like unreachable() in lower case better though...
> 
> I like David's version a bit better, since it takes care of more 
> architectures, and also because it avoids that butt-ugly special 
> case for gcc-4.4.1-RH-relase-10 backporting this feature.
> 
> I realize that the RH backport thing is good for testing now, but 
> at the same time, it really does look nasty. I wonder if we could 
> add some config-time compiler feature testing - so that you'd not 
> have a version test at all, but a CONFIG_BUILTIN_UNREACHABLE.
> 
> There are other cases where that kind of config-time testing could 
> be useful, and we could avoid doing various gcc checks dynamically 
> from within 'make' (along with checking for known-buggy versions 
> etc).
> 
> And yeah, it looks better in lower case. That said, I don't care 
> _that_ much, and people can fight it out.

Another detail that would be nice to be fixed is to propagate the 
unreachable() call into the !CONFIG_BUG case as well in 
asm-generic/bug.h.

That would kill dozens of !CONFIG_BUG compiler warnings and would 
make a dont-allow-warnings policy a possibility for random builds.

	Ingo
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