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

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Date:	Mon, 01 Sep 2008 15:36:32 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Boaz Harrosh" <bharrosh@...asas.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

>>> Boaz Harrosh <bharrosh@...asas.com> 01.09.08 16:21 >>>
>> 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.

No, sizeof() alone is an expression, too. Also, by using a statement you'll
have more problems with fixing BUILD_BUG_ON_ZERO(), which must be
an expression.

>> Of course you could wrap the whole thing in ({}),
>
>"do{}while(0)" is effectively an "{}" plus the added bonus
>of demanding an ";" ;-)

An expression likewise demands a terminating ; (or a continuation of the
expression, i.e. by using an operator)

>> 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.

I don't think compilers in general and gcc in particular work this way
(i.e. automatically throwing away everything included in a dead block).

Jan

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