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

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Date:	Tue, 02 Sep 2008 18:19:47 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...e.hu>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Ivo van Doorn <IvDoorn@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"John W. Linville" <linville@...driver.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] debug: BUILD_BUG_ON: error on non-const	 expressions

Jan Beulich wrote:
>>>> Boaz Harrosh <bharrosh@...asas.com> 01.09.08 18:41 >>>
>> Jan Beulich wrote:
>>>>>> Boaz Harrosh <bharrosh@...asas.com> 01.09.08 17:00 >>>
>>>> What is broken with my BUILD_BUG_ON_ZERO(). I tried all tests and
>>>> it works fine. Do you have a test with unwanted results?
>>>> (Actually it's the original one I have not touched it).
>>> That's the problem - it uses the same sizeof(char[]) approach, and hence
>>> has the same problems that you just try to fix for BUILD_BUG_ON().
>>>
>> No it does not have this problem. Have you tested it? 
>> I have! It works fine. (Complains on non-const expressions)
> 
> For static variables, yes. But not for automatic variables and the like:
> 
> #define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
> 
> int test(int i) {
> 	int x = BUILD_BUG_ON_ZERO(i);
> 
> 	return x;
> }
> 
> You could argue that I could place a simple BUILD_BUG_ON() later in
> the code, but that easily defeats the documentation purposes the
> construct also has (my general position on this is that the check should
> be in or immediately before the statement that depends on the
> enforced restriction).
> 
> Jan
> 

I was able to reproduce your problem now. 

I have made new patchset the final BUILD_BUG_ON_ZERO() I took from you.
But in BUILD_BUG_ON I use a slight variation that satisfies my wishes
and makes BUILD_BUG_ON more like BUG_ON in syntax.

I had to change virtio_has_feature() semantics, though, but I believe it's
not that bad.

Jan maybe you want to put your Signed-off-by: on the last patch?
Please review?

[ I'll post new version of patches that change as reply to the original
 patches ]

Boaz
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