[PATCH] compiler: Don't perform compiletime_assert with -O0.

[Joe Stringer <joe@ovn.org>]

Recent changes[0] to make use of __compiletime_assert() from
container_of() increased the usage of this macro, allowing developers to
notice type conflicts in usage of container_of() at compile time.
However, the implementation of __compiletime_assert relies on compiler
optimizations to report an error. This means that if a developer uses
"-O0" with any code that performs container_of(), the compiler will
always report an error regardless of whether there is an actual problem
in the code.

This patch disables compile_time_assert when optimizations are disabled
to allow such code to compile with CFLAGS="-O0".

Example compilation failure:

./include/linux/compiler.h:547:38: error: call to ‘__compiletime_assert_94’ declared with attribute error: pointer type mismatch in container_of()
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
                                      ^
./include/linux/compiler.h:530:4: note: in definition of macro ‘__compiletime_assert’
    prefix ## suffix();    \
    ^~~~~~
./include/linux/compiler.h:547:2: note: in expansion of macro ‘_compiletime_assert’
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:46:37: note: in expansion of macro ‘compiletime_assert’
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/kernel.h:860:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
  BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \
  ^~~~~~~~~~~~~~~~

[0] http://lkml.kernel.org/r/20170525120316.24473-7-abbotti@mev.co.uk

Signed-off-by: Joe Stringer <joe@ovn.org>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/compiler.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index eca8ad75e28b..b67e5ec9b810 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -517,7 +517,8 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 # define __compiletime_error_fallback(condition) do { } while (0)
 #endif
 
-#define __compiletime_assert(condition, msg, prefix, suffix)		\
+#ifdef __OPTIMIZE__
+# define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
 		bool __cond = !(condition);				\
 		extern void prefix ## suffix(void) __compiletime_error(msg); \
@@ -525,6 +526,9 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 			prefix ## suffix();				\
 		__compiletime_error_fallback(__cond);			\
 	} while (0)
+#else
+# define __compiletime_assert(condition, msg, prefix, suffix)
+#endif
 
 #define _compiletime_assert(condition, msg, prefix, suffix) \
 	__compiletime_assert(condition, msg, prefix, suffix)
-- 
2.14.1