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

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Date:	Fri, 02 Oct 2009 08:48:36 -0700
From:	Hollis Blanchard <hollisb@...ibm.com>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	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 Wed, 2009-09-30 at 07:35 +0100, Jan Beulich wrote:
> >>> Hollis Blanchard <hollisb@...ibm.com> 30.09.09 01:39 >>>
> >On Tue, 2009-09-29 at 10:28 +0100, Jan Beulich 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).
> >
> >I think I see what you're saying. Do you have a fix to suggest?
> 
> The one Rusty suggested the other day may help here. I don't like it
> as a drop-in replacement for BUILD_BUG_ON() though (due to it
> deferring the error generated to the linking stage), I'd rather view
> this as an improvement to MAYBE_BUILD_BUG_ON() (which should
> then be used here).

Can you be more specific?

I have no idea what Rusty suggested where. I can't even guess what
MAYBE_BUILD_BUG_ON() is supposed to do (sounds like a terrible name).

All I know is that this used to build...

-- 
Hollis Blanchard
IBM Linux Technology Center

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