linux-kernel - Re: [PATCH 5/5 ver2] debug: BUILD_BUG_ON: error on non-const expressions

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Date:	Sun, 05 Oct 2008 11:34:13 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Ingo Molnar <mingo@...e.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Ivo van Doorn <IvDoorn@...il.com>,
	"John W. Linville" <linville@...driver.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Beulich <jbeulich@...ell.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5 ver2] debug: BUILD_BUG_ON: error on non-const expressions

Rusty Russell wrote:
> On Wednesday 03 September 2008 01:57:31 Boaz Harrosh wrote:
>> +#define BUILD_BUG_ON(e) \
>> +       do { struct {int:-!!(e); } x __maybe_unused;} while(0)
> 
> Why did you hate the void cast again?  Simplest should 
> be "(void)BUILD_BUG_ON_ZERO(e)".  But if not, it seems to me that it's 
> cleaner to do:
> 
> #define BUILD_BUG_ON(e) \
> 	do { } while(BUILD_BUG_ON_ZERO(e))
> 

I have not checked this exact variant, but the problems I had
is that wrong or non-const code was ignored by the compiler,
which is what I tried to solve.

> No chance of the compiler emitting unused vars.
> 

There is no unused vars because the result is a zero-sized
struct which will never emit any code.

> Cheers,
> Rusty.
> 

Is it that important? the code submitted does what is required
to the letter, should we spend more effort on this?

Boaz
 
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