LinuxKPI: conditionally add __flex_counter()

__flex_counter() is used by overflow.h and needed for "flex allocations".
It is either a void * typed 0 (NULL) (like this for _Generic checks),
or uses __builtin_counted_by_ref.

The latter was added to gcc and llvm fairly recently and while for gcc
the __has_builtin() check suffices, clang had parts broken until recently
so needs an extra check for the next major version.  The fixed hash is
currently not part of any tag to use, so we play it save (and hope 23
will have it).  It will be a while until we will see the builting to be
used but at least we will be prepared for it.  See inline comments for
the commit hashes and versions which added the feature.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	dumbbell
Differential Revision: https://reviews.freebsd.org/D56393

1 files changed, 13 insertions, 0 deletions

diff --git a/sys/compat/linuxkpi/common/include/linux/compiler_types.h b/sys/compat/linuxkpi/common/include/linux/compiler_types.h
index 7151c03de690..25e69f5afd8e 100644
--- a/sys/compat/linuxkpi/common/include/linux/compiler_types.h
+++ b/sys/compat/linuxkpi/common/include/linux/compiler_types.h
@@ -42,4 +42,17 @@
 
 #define	__same_type(a, b)	__builtin_types_compatible_p(typeof(a), typeof(b))
 
+/*
+ * __builtin_counted_by_ref was introduced to
+ * - gcc in e7380688fa59 with a first release tag of gcc-15.1.0,
+ * - llvm in 7475156d49406 with a first release tag of llvmorg-20.1.0-rc1
+ *   but cannot be used before 23 (22.x.y possibly) (see 09a3d830a888).
+ */
+#if (__has_builtin(__builtin_counted_by_ref)) && \
+    (!defined(__clang__) || (__clang_major__ >= 23))
+#define	__flex_counter(_field)		__builtin_counted_by_ref(_field)
+#else
+#define	__flex_counter(_field)		((void *)NULL)
+#endif
+
 #endif