[XEN PATCH v4] xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO.

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

Redefine BUILD_BUG_ON_ZERO to avoid using a compiler extension
that gives an acceptable semantics to C99 undefined behavior 58
("A structure or union is defined as containing no named members
(6.7.2.1)").

The chosen ill-formed construct is a negative bitwidth in a
bitfield within a struct containing at least one named member,
which prevents the UB while keeping the semantics of the construct.

The choice of the bitwise AND operation to bring the result to 0
when cond is false boils down to possibly better portability,
and the 'U' suffix to make it obvious that this operation results
in an unsigned value.

Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
Changes in V2:
- Avoid using a VLA as the compile-time assertion
- Do not drop _Static_assert
Changes in V3:
- Changed the operation to bring the result to 0 when the
construct does not lead to a compilation error
Changes in V4:
- Switched to a shorter construct for the second definition.
---
 xen/include/xen/lib.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 67fc7c1d7e..460cab6734 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -51,9 +51,10 @@
    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 ")"); })
+    (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) & 0U)
 #else
-#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON_ZERO(cond) \
+    (sizeof(struct { unsigned u : !(cond); }) & 0U)
 #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
 #endif
 
-- 
2.34.1