compiler: rework BUG_ON using a struct

There are theoretical concerns that some compilers might not trigger
build failures on attempts to define an array of size (x ? -1 : 1) where
x is a variable and make it a variable sized array instead. Let rewrite
using a struct with a negative bit field size instead as there are no
dynamic bit field sizes.  This is similar to what Linux does.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 77b9ce3..057639a 100644 (file)
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -85,10 +85,14 @@
 #define typeof_field(type, field) typeof(((type *)0)->field)
 #define type_check(t1,t2) ((t1*)0 - (t2*)0)
 
+#define QEMU_BUILD_BUG_ON_STRUCT(x) \
+    struct { \
+        int:(x) ? -1 : 1; \
+    }
+
 #ifdef __COUNTER__
-#define QEMU_BUILD_BUG_ON(x) \
-    typedef char glue(qemu_build_bug_on__, __COUNTER__)[(x) ? -1 : 1] \
-        __attribute__((unused))
+#define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
+    glue(qemu_build_bug_on__, __COUNTER__) __attribute__((unused))
 #else
 #define QEMU_BUILD_BUG_ON(x)
 #endif