[PATCH v3 07/10] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

danielfsantos
2012-10-24T16:40:25Z

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
do{}while(0), resulting in exactly one compile-time error on all
versions of gcc.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
---
include/linux/bug.h | 4 ++--
include/linux/compiler.h | 7 +++++++
2 files changed, 9 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 cbf6d9d..84926f2 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -298,6 +298,13 @@ 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) \
+ do { \
+ ((void)sizeof(char[1 - 2*!!(condition)])); \
+ } while (0)
+#endif
+#ifndef __compiletime_error_fallback
+# define __compiletime_error_fallback(condition) do { } while (0)
#endif
/*
* Prevent the compiler from merging or refetching accesses. The compiler
--
1.7.3.4