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

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Message-ID: <50664E30.7050809@att.net>
Date:	Fri, 28 Sep 2012 20:26:08 -0500
From:	Daniel Santos <danielfsantos@....net>
To:	Josh Triplett <josh@...htriplett.org>
CC:	Daniel Santos <daniel.santos@...ox.com>,
	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 09/28/2012 07:32 PM, Josh Triplett wrote:
> 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.
It may also make sense to define
#define BUILD_BUG() BUILD_BUG_ON(1)

I haven't examined this really closely yet and my eyes are getting a
little bleary :)


Really, I would have liked to be able to set the error message as
well, but we would have to have the macro generate a unique function
name for each time its expanded to make that work.  Example:

#define BUILD_BUG_ON(cond) BUILD_BUG_ON_MSG(cond, #cond)
#define BUILD_BUG_ON_MSG(cond, msg)                                \
    do {                                                           \
        extern void __build_bug_on_failed ## something_unique(void)\
            __compiletime_error("BUILD_BUG_ON failed: " msg);      \
        __compiletime_error_fallback(cond);                        \
        if (cond)                                                  \
            __build_bug_on_failed();                               \
    } while(0)

I just don't know any tricks to generate unique pre-processor
value automatically.

Daniel
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