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Date: Mon, 01 Sep 2008 14:55:17 +0100
From: "Jan Beulich" <jbeulich@novell.com>
To: "Boaz Harrosh" <bharrosh@panasas.com>,
       "Rusty Russell" <rusty@rustcorp.com.au>
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>,
       "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
References: <48BBE77D.7070007@panasas.com> <48BBEE04.30903@panasas.com>
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>>> Boaz Harrosh <bharrosh@panasas.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. Of course you could wrap the whole thing in ({}),
but I can't see why not to use a bit-field to achieve the intended effect.

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?

Jan