[XEN PATCH] xen/include/xen/lib.h: avoid undefined behavior.

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

Redefine BUILD_BUG_ON_ZERO to fully comply with C99 avoiding
undefined behavior 58 ("A structure or union is defined as
containing no named members (6.7.2.1)."
This also avoids a dependency on the compiler and its version.

Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
 xen/include/xen/lib.h | 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)

diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 67fc7c1d7e..a266159b9f 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -40,22 +40,8 @@
     unlikely(ret_warn_on_);             \
 })
 
-/* All clang versions supported by Xen have _Static_assert. */
-#if defined(__clang__) || \
-    (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6))
-/* Force a compilation error if condition is true */
-#define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
-
-/* Force a compilation error if condition is true, but also produce a
-   result (of value 0 and type size_t), so the expression can be used
-   e.g. in a structure initializer (or where-ever else comma expressions
-   aren't permitted). */
-#define BUILD_BUG_ON_ZERO(cond) \
-    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
-#else
-#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON_ZERO(cond) (sizeof(char[(cond)? -1 : 1]) - 1U)
 #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
-#endif
 
 #ifndef NDEBUG
 #define ASSERT(p) \
-- 
2.34.1