@@ -67,7 +67,7 @@ struct pt_regs;
__compiletime_error("BUILD_BUG_ON failed"); \
if (__cond) \
__build_bug_on_failed(); \
- ((void)sizeof(char[1 - 2 * __cond])); \
+ __compiletime_error_fallback(__cond); \
} while(0)
#endif
@@ -307,7 +307,12 @@ 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)
+#else
+# define __compiletime_error_fallback(condition) do { } while (0)
#endif
+
/*
* Prevent the compiler from merging or refetching accesses. The compiler
* is also forbidden from reordering successive instances of ACCESS_ONCE(),
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. Note that we are not changing the negative-sized array code for the unoptimized version of BUILD_BUG_ON, since it has the potential to catch problems that would be disabled in later versions of gcc were __compiletime_error_fallback used. The reason is that that an unoptimized build can't always remove calls to an error-attributed function call (like we are using) that should effectively become dead code if it were optimized. However, using a negative-sized array with a similar value will not result in an false-positive (error). The only caveat being that it will also fail to catch valid conditions, which we should be expecting in an unoptimized build anyway. Signed-off-by: Daniel Santos <daniel.santos@pobox.com> --- include/linux/bug.h | 2 +- include/linux/compiler.h | 5 +++++ 2 files changed, 6 insertions(+), 1 deletions(-)