[PATCH v2 07/10] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

[danielfsantos]

Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3,
creating compile-time errors required a little trickery.
BUILD_BUG{,_ON} uses this attribute when available to generate
compile-time errors, but also uses the negative-sized array trick for
older compilers, resulting in two error messages in some cases.  The
reason it's "some" cases is that as of gcc 4.4, the negative-sized array
will not create an error in some situations, like inline functions.

This patch replaces the negative-sized array code with the new
__compiletime_error_fallback() macro which expands to the same thing
unless the the error attribute is available, in which case it expands to
(0), resulting in exactly one compile-time error on all versions of gcc.

Signed-off-by: Daniel Santos <daniel.santos@xxxxxxxxx>
---
 include/linux/bug.h      |    4 ++--
 include/linux/compiler.h |    5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/bug.h b/include/linux/bug.h
index 3d4b564..f8eae31 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -56,13 +56,13 @@ struct pt_regs;
  * link-time error, which is harder to track down.
  */
 #ifndef __OPTIMIZE__
-#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+#define BUILD_BUG_ON(condition) __compiletime_error_fallback(condition)
 #else
 #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)]));		\
+		__compiletime_error_fallback(condition);		\
 		if (condition)						\
 			__build_bug_on_failed();			\
 	} while(0)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index fd455aa..88ba201 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -296,6 +296,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 #endif
 #ifndef __compiletime_error
 # define __compiletime_error(message)
+# define __compiletime_error_fallback(condition) \
+	 ((void)sizeof(char[1 - 2*!!(condition)]))
+#endif
+#ifndef __compiletime_error_fallback
+# define __compiletime_error_fallback(condition) (void)(0)
 #endif
 /*
  * Prevent the compiler from merging or refetching accesses.  The compiler
-- 
1.7.3.4

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