[ruby-core:111823] [PATCH] fiddle: Use C11 _Alignof to define ALIGN_OF when possible

WG14 N2350 made very clear that it is an UB having type definitions
within "offsetof" [1]. This patch enhances the implementation of macro
ALIGN_OF to use builtin "_Alignof" to avoid undefined behavior
when using std=c11 or newer

clang 16+ has started to flag this [2]

Fixes build when using -std >= gnu11 and using clang16+

Older compilers gcc < 4.9 or clang < 8 has buggy _Alignof even though it
may support C11, exclude those compiler versions

[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
[2] https://reviews.llvm.org/D133574

Signed-off-by: Khem Raj <[e-mail]>
---
 ext/fiddle/fiddle.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/ext/fiddle/fiddle.h b/ext/fiddle/fiddle.h
index 10eb9ce..ffb395e 100644
--- a/ext/fiddle/fiddle.h
+++ b/ext/fiddle/fiddle.h
@@ -196,7 +196,17 @@
 #endif
 #define TYPE_UINTPTR_T (-TYPE_INTPTR_T)
 
-#define ALIGN_OF(type) offsetof(struct {char align_c; type align_x;}, align_x)
+/* GCC releases before GCC 4.9 had a bug in _Alignof.  See GCC bug 52023
+   <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>.
+   clang versions < 8.0.0 have the same bug.  */
+#if (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
+     || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
+         && !defined __clang__) \
+     || (defined __clang__ && __clang_major__ < 8))
+# define ALIGN_OF(type) offsetof(struct {char align_c; type align_x;}, align_x)
+#else
+# define ALIGN_OF(type) _Alignof(type)
+#endif
 
 #define ALIGN_VOIDP  ALIGN_OF(void*)
 #define ALIGN_CHAR   ALIGN_OF(char)
-- 
2.39.0

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