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

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Date:	Wed, 30 Sep 2009 07:29:38 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"roel kluin" <roel.kluin@...il.com>
Cc:	<sfr@...b.auug.org.au>, <akpm@...ux-foundation.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <hollisb@...ibm.com>,
	<kvm-ppc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-next@...r.kernel.org>
Subject: Re: linux-next: tree build failure

>>> roel kluin <roel.kluin@...il.com> 29.09.09 11:51 >>>
>On Tue, Sep 29, 2009 at 11:28 AM, Jan Beulich <jbeulich@...ell.com> wrote:
>>>>> Hollis Blanchard  09/29/09 2:00 AM >>>
>>>First, I think there is a real bug here, and the code should read like
>>>this (to match the comment):
>>>    /* type has to be known at build time for optimization */
>>>-    BUILD_BUG_ON(__builtin_constant_p(type));
>>>+    BUILD_BUG_ON(!__builtin_constant_p(type));
>>>
>>>However, I get the same build error *both* ways, i.e.
>>>__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
>>>the new BUILD_BUG_ON() macro isn't working...
>>
>> No, at this point of the compilation process it's neither zero nor one,
>> it's simply considered non-constant by the compiler at that stage
>> (this builtin is used for optimization, not during parsing, and the
>> error gets generated when the body of the function gets parsed,
>> not when code gets generated from it).
>>
>> Jan
>
>then maybe
>
>if(__builtin_constant_p(type))
>        BUILD_BUG_ON(1);
>
>would work?

Definitely not - this would result in the compiler *always* generating an
error.

Jan

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