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Date: Mon, 01 Sep 2008 15:36:32 +0100
From: "Jan Beulich" <jbeulich@novell.com>
To: "Boaz Harrosh" <bharrosh@panasas.com>
Cc: "David S. Miller" <davem@davemloft.net>, "Ingo Molnar" <mingo@elte.hu>,
       "Alexey Dobriyan" <adobriyan@gmail.com>,
       "Ivo van Doorn" <IvDoorn@gmail.com>,
       "Andrew Morton" <akpm@linux-foundation.org>,
       "Linus Torvalds" <torvalds@linux-foundation.org>,
       "Theodore Tso" <tytso@mit.edu>, "Rusty Russell" <rusty@rustcorp.com.au>,
       "John W. Linville" <linville@tuxdriver.com>,
       "linux-kernel" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/5] debug: BUILD_BUG_ON: error on non-const	
	 expressions
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>>> Boaz Harrosh <bharrosh@panasas.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