linux-kernel - Re: [PATCH 10/10] bug.h: Add gcc 4.2+ versions of BUILD_BUG_ON

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Date:	Mon, 1 Oct 2012 17:55:26 -0700
From:	Michel Lespinasse <walken@...gle.com>
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,
	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 10/10] bug.h: Add gcc 4.2+ versions of BUILD_BUG_ON_* macros

On Fri, Sep 28, 2012 at 4:20 PM, Daniel Santos <daniel.santos@...ox.com> wrote:
> BUILD_BUG_ON42(arg)
> BUILD_BUG_ON_CONST42(arg)
>
> Prior to gcc 4.2, the optimizer was unable to determine that many
> constant values stored in structs were indeed compile-time constants and
> optimize them out.  Sometimes, it will find an intergral value to be a
> compile-time constant, but fail to perform a bit-wise AND at
> compile-time.  These two macros provide a mechanism to perform these
> build-time checks, but not break on older compilers where we already
> know they can't be checked at compile time.
>
> For specific details, consult the doc comments for BUILD_BUG_ON_CONST.
> These macros are used in the generic rbtree code.

I think the names are quite confusing. BUILD_BUG_ON_NON_CONST42 sounds
like it's checking if 42 is a constant.

The name probably shouldn't mention what compiler versions support
this check, but instead it should hint as to when you should use this
instead of BUILD_BUG_ON_CONST ? Maybe BUILD_BUG_ON_CONST_DEREF or
something (I'm pretty bad with names too :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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