[PATCHv3 4/6] [RFC] bug.h: make BUILD_BUG_ON generate compile-time error

[PATCHv3 4/6] [RFC] bug.h: make BUILD_BUG_ON generate compile-time error

[Daniel Santos]

Negative sized arrays wont create a compile-time error in some cases
starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced
the error function attribute that will.  This patch modifies
BUILD_BUG_ON to behave like BUILD_BUG already does, using the error
function attribute so that you don't have to build the entire kernel to
discover that you have a problem, and then enjoy trying to track it down
from a link-time error.

This is not only nicer, but it makes it behave consistiently with
BUILD_BUG.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
---
 include/linux/bug.h |   23 +++++++++++++----------
 1 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/include/linux/bug.h b/include/linux/bug.h
index 298a916..7323287 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -42,24 +42,27 @@ struct pt_regs;
  * @condition: 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
+ * some 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.
+ * to inline functions).  Luckily, in 4.3 they added the "error" function
+ * attribute just for this type of case.  Thus, we use a negative sized array
+ * (should always create an error pre-gcc-4.4) and then call an undefined
+ * function with the error attribute (should always creates an error 4.3+).  If
+ * for some reason, neither creates a compile-time error, we'll still have a
+ * link-time error, which is harder to track down.
  */
 #ifndef __OPTIMIZE__
 #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
 #else
-extern int __build_bug_on_failed;
-#define BUILD_BUG_ON(condition)					\
-	do {							\
-		((void)sizeof(char[1 - 2*!!(condition)]));	\
-		if (condition) __build_bug_on_failed = 1;	\
+#define BUILD_BUG_ON(condition)						\
+	do {								\
+		extern void __build_bug_on_failed(void)			\
+			__compiletime_error("BUILD_BUG_ON failed");	\
+		((void)sizeof(char[1 - 2*!!(condition)]));		\
+		if (condition) __build_bug_on_failed();			\
 	} while(0)
 #endif
 -- 1.7.3.4