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Date:	Tue, 20 Oct 2009 15:43:13 +0100
From:	Alan Jenkins <>
To:	Américo Wang <>
Cc:	Rusty Russell <>,
	Hollis Blanchard <>,
	Jan Beulich <>,,,,,,
Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases

On 10/20/09, Américo Wang <> wrote:
> On Tue, Oct 20, 2009 at 02:15:33PM +1030, Rusty Russell wrote:
>>BUILD_BUG_ON used to use the optimizer to do code elimination or fail
>>at link time; it was changed to first the size of a negative array (a
>>nicer compile time error), then (in
>>8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>>bitfields: needs a literal constant at parse time, and can't be put under
>>	"if (__builtin_constant_p(x))" for example.
>>negative array: can handle anything, but if the compiler can't tell it's
>>	a constant, silently has no effect.
>>link time: breaks link if the compiler can't determine the value, but the
>>	linker output is not usually as informative as a compiler error.
>>If we use the negative-array-size method *and* the link time trick,
>>we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
>>branches, and maximal ability for the compiler to detect errors at
>>build time.
>>Signed-off-by: Rusty Russell <>
>>diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>>--- a/include/linux/kernel.h
>>+++ b/include/linux/kernel.h
>>@@ -683,12 +683,6 @@ struct sysinfo {
>> 	char _f[20-2*sizeof(long)-sizeof(int)];	/* Padding: libc5 uses this.. */
>> };
>>-/* Force a compilation error if condition is true */
>>-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
>>-/* Force a compilation error if condition is constant and true */
>>-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
>> /* 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
>>    e.g. in a structure initializer (or where-ever else comma expressions
>>@@ -696,6 +690,33 @@ struct sysinfo {
>> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
>>+ * BUILD_BUG_ON - break compile if a condition is true.
>>+ * @cond: the condition which the compiler should know is false.
>>+ *
>>+ * If you have some code which relies on certain constants being equal, or
>>+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
>>+ * detect if someone changes it.
>>+ *
>>+ * The implementation uses gcc's reluctance to create a negative array,
>> but
>>+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not
>> arguments
>>+ * to inline functions).  So as a fallback we use the optimizer; if it
>> can't
>>+ * prove the condition is false, it will cause a link error on the
>> undefined
>>+ * "__build_bug_on_failed".  This error message can be harder to track
>> down
>>+ * though, hence the two different methods.
>>+ */
>>+#ifndef __OPTIMIZE__
>>+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>>+extern int __build_bug_on_failed;
> Hmm, what exactly is __build_bug_on_failed?

Well, we haven't added a definition for it in this patch.  I'm sure
grep will tell you it wasn't defined before hand either.  So any
reference to it is an error - which will be reported at link time.

>>+#define BUILD_BUG_ON(condition)					\
>>+	do {							\
>>+		((void)sizeof(char[1 - 2*!!(condition)]));	\
>>+		if (condition) __build_bug_on_failed = 1;	\

If "condition" is known false at compile time, gcc -O will eliminate
the code which refers to __build_bug_on_failed.  If it's not proved to
be false - it will break the build, which is exactly what we want

>>+	} while(0)
>>+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
>> /* Trap pasters of __FUNCTION__ at compile-time */
>> #define __FUNCTION__ (__func__)
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