[PATCH net-next] compiler: Document behavior compiling with -O0

[Joe Stringer]

Recent changes[0] to make use of __compiletime_assert() from container_of()
increased the scope of this macro, resulting in a wider set of
situations where developers cannot compile their code using "-O0". I
noticed this when making use of the macro in my own development, and
spent more time than I'd like to admit tracking the problem down. This
patch documents the behavior in lieu of a compile-time assertion
implementation that does not rely on optimizations.

Example compilation failure:

./include/linux/compiler.h:547:38: error: call to â__compiletime_assert_94â 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 â__compiletime_assertâ
    prefix ## suffix();    \
    ^~~~~~
./include/linux/compiler.h:547:2: note: in expansion of macro â_compiletime_assertâ
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:46:37: note: in expansion of macro âcompiletime_assertâ
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/kernel.h:860:2: note: in expansion of macro âBUILD_BUG_ON_MSGâ
  BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \
  ^~~~~~~~~~~~~~~~

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

Signed-off-by: Joe Stringer <joe@xxxxxxx>
---
 include/linux/compiler.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index eca8ad75e28b..bb640167fdac 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -517,6 +517,11 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 # define __compiletime_error_fallback(condition) do { } while (0)
 #endif
 
+/*
+ * __compiletime_assert() relies on compiler optimizations to remove the check
+ * against '__cond' if 'condition' is false. As a result, compiling with -O0
+ * will cause compilation errors here regardless of the value of 'condition'.
+ */
 #define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
 		bool __cond = !(condition);				\
-- 
2.14.1