linux-kernel - Re: [PATCH 8/10] bug.h: Make BUILD_BUG

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Message-ID: <20120929003239.GA14293@jtriplet-mobl1>
Date:	Fri, 28 Sep 2012 17:32:39 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Daniel Santos <daniel.santos@...ox.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christopher Li <sparse@...isli.org>,
	David Daney <david.daney@...ium.com>,
	David Howells <dhowells@...hat.com>,
	Joe Perches <joe@...ches.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-sparse@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Pavel Pisa <pisa@....felk.cvut.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 8/10] bug.h: Make BUILD_BUG_ON generate compile-time error

On Fri, Sep 28, 2012 at 06:20:09PM -0500, Daniel Santos wrote:
> Negative sized arrays wont create a compile-time error in some cases
> starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced
> the error function attribute that will.  This patch modifies
> BUILD_BUG_ON to behave like BUILD_BUG already does, using the error
> function attribute so that you don't have to build the entire kernel to
> discover that you have a problem, and then enjoy trying to track it down
> from a link-time error.

Rather than doing both, and potentially producing two errors for the
same issue, how about using __compiletime_error only, and only using the
negative-sized array when __compiletime_error has no useful definition?

For instance, in compiler.h, when defining __compiletime_error as an
empty macro in the fallback case, you could define a
__compiletime_error_fallback() macro that declares a negative-sized
array; you could then define __compiletime_error_fallback() as an empty
macro when it doesn't exist.

- Josh Triplett
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