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

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Date:	Mon, 01 Sep 2008 14:55:17 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Boaz Harrosh" <bharrosh@...asas.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>
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>,
	"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 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

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