linux-kernel - Re: linux-next: tree build failure

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Date:	Tue, 20 Oct 2009 11:42:32 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Hollis Blanchard <hollisb@...ibm.com>
Cc:	Jan Beulich <JBeulich@...ell.com>, sfr@...b.auug.org.au,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
	kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-next@...r.kernel.org
Subject: Re: linux-next: tree build failure

On Tue, 20 Oct 2009 04:49:29 am Hollis Blanchard wrote:
> On Thu, 2009-10-15 at 08:27 +0100, Jan Beulich wrote:
> > My perspective is that it just uncovered already existing brokenness.
> 
> Sorry, I thought it was clear, but to be more explicit: I propose the
> following patch, which replaces the current BUILD_BUG_ON implementation
> with Rusty's version.

OK, I switched my brain back on.  Yeah, I agree: we may still want
BUILD_OR_RUNTIME_BUG_ON one day, but I like this.

It's just missing the giant comment that it needs :)

/**
 * BUILD_BUG_ON - break compile if a condition is true.
 * @cond: the condition which the compiler should know is false.
 *
 * If you have some code which relies on certain constants being equal, or
 * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
 * detect if someone changes it.
 *
 * The implementation uses gcc's reluctance to create a negative array, but
 * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
 * to inline functions).  So as a fallback we use the optimizer; if it can't
 * prove the condition is false, it will cause a link error on the undefined
 * "__build_bug_on_failed".  This error is less neat, and can be harder to
 * track down.
 */

Thanks!
Rusty.
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