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

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Date:	Mon, 01 Sep 2008 17:21:27 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	"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>,
	"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 15:28 >>>
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -195,7 +195,10 @@ extern void __chk_io_ptr(const volatile void __iomem *);
>> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
>>
>> /* Force a compilation error if condition is true */
>> -#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>> +#define BUILD_BUG_ON(condition)					  \
>> +do {									  \
>> +	static struct { char arr[1 - 2*!!(condition)]; } x __maybe_unused;\
>> +} while(0)
>>
>> /* Force a compilation error if condition is true, but also produce a
>>    result (of value 0 and type size_t), so the expression can be used
> 
> I have to admit that I'm surprise this compiles: You replace an expression
> with a statement, and hence you reduce the places where BUILD_BUG_ON()
> can validly be used. 

it is only an expression because of the (void)() cast, which is what
I'm trying to avoid.

> Of course you could wrap the whole thing in ({}),

"do{}while(0)" is effectively an "{}" plus the added bonus
of demanding an ";" ;-)

> but I can't see why not to use a bit-field to achieve the intended effect.

Sure, I can also use my suggested enum construct.
(http://www.spinics.net/lists/kernel/msg772904.html)

But the thing I'm avoiding most is the (void)() cast, which makes the
optimizer very lazy. See:
[PATCH 4/5] rt2x00: Compiler warning unmasked by fix of BUILD_BUG_ON
that has only popped out when I remove the (void)() cast.

> Also, are you sure the compiler will eliminate the dead variable in all
> cases?
> 
> Finally, using as common a variable as 'x' here seems dangerous, too:
> What if somewhere x is #define-d to something more complex than a
> simple identifier?

No it is scoped in a dead do{}while(0). What gets optimized out most
is the do nothing do{}while(0). The inside is just ignored.

> 
> Jan
> 

I will test for your approach, give me a sec ...

Boaz
 
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