[xen master] xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO

[patchbot@xxxxxxx]

commit 5c02dbd0e0adbe6100128345328999876fc78d00
Author:     Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
AuthorDate: Wed Jul 5 08:32:14 2023 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jul 5 08:32:14 2023 +0200

    xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO
    
    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 first definition includes an additional named field of type
    char.
    
    The chosen ill-formed construct for the second definition is a struct
    with a named bitfield of width 0 when the condition is true,
    which prevents the UB without using the compiler extension while keeping
    the semantic 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.
    
    Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 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..a8958ed57b 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 ")"); }) & 0)
 #else
-#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON_ZERO(cond) \
+    (sizeof(struct { unsigned u : !(cond); }) & 0)
 #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
 #endif
 
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