RBIMPL_ALIGNOF: do not use __alignof__

It is reported that on a system of i386 System V ABI, GCC returns 8 for
__alignof__(double).  OTOH the ABI defines alignments of double to be 4,
and ISO/IEC 9899:2011 reads that _Alignof(double) shall return 4 on such
machine.  What we want in ruby is 4 instead of 8 there.  We cannot use
__alignof__.

Additionally, both old GCC / old clang return 8 for _Alignof(double) on
such platforms.  They are their bugs, and already fixed in recent
versions.  But we have to support older compilers for a while.  Shall
check sanity of _Alignof.

1 files changed, 49 insertions, 41 deletions

diff --git a/include/ruby/internal/stdalign.h b/include/ruby/internal/stdalign.h
index 8c56fbbd69..8d9a0d21e0 100644
--- a/include/ruby/internal/stdalign.h
+++ b/include/ruby/internal/stdalign.h
@@ -22,16 +22,19 @@
  */
 #include "ruby/internal/config.h"
 
-#ifdef HAVE_STDALIGN_H
-# include <stdalign.h>
+#ifdef STDC_HEADERS
+# include <stddef.h>
 #endif
 
+#include "ruby/internal/attr/artificial.h"
+#include "ruby/internal/attr/const.h"
+#include "ruby/internal/attr/constexpr.h"
+#include "ruby/internal/attr/forceinline.h"
 #include "ruby/internal/compiler_is.h"
-#include "ruby/internal/compiler_since.h"
-#include "ruby/internal/has/feature.h"
-#include "ruby/internal/has/extension.h"
 #include "ruby/internal/has/attribute.h"
 #include "ruby/internal/has/declspec_attribute.h"
+#include "ruby/internal/has/extension.h"
+#include "ruby/internal/has/feature.h"
 
 /**
  * Wraps (or simulates) `alignas`. This is C++11's `alignas` and is _different_
@@ -75,50 +78,55 @@
 #endif
 
 /**
- * Wraps (or simulates)  `alignof`.  Unlike #RBIMPL_ALIGNAS, we  can safely say
- * both C/C++ definitions are effective.
+ * Wraps (or simulates) `alignof`.
+ *
+ * We want C11's `_Alignof`.  However in spite of its clear language, compilers
+ * (including GCC  and clang) tend to  have buggy implementations.  We  have to
+ * avoid such things to resort to our own version.
+ *
+ * @see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023
+ * @see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69560
+ * @see https://bugs.llvm.org/show_bug.cgi?id=26547
  */
-#if defined(__cplusplus) && RBIMPL_HAS_EXTENSION(cxx_alignof)
-# define RBIMPL_ALIGNOF __extension__ alignof
-
-#elif defined(__cplusplus) && (__cplusplus >= 201103L)
-# define RBIMPL_ALIGNOF alignof
-
-#elif defined(__INTEL_CXX11_MODE__)
-# define RBIMPL_ALIGNOF alignof
-
-#elif defined(__GXX_EXPERIMENTAL_CXX0X__)
-# define RBIMPL_ALIGNOF alignof
-
-#elif defined(__STDC_VERSION__) && RBIMPL_HAS_EXTENSION(c_alignof)
-# define RBIMPL_ALIGNOF __extension__ _Alignof
-
-#elif defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
-# define RBIMPL_ALIGNOF _Alignof
+#if defined(__STDC_VERSION__) && defined(HAVE__ALIGNOF)
+# /* Autoconf detected availability of a sane `_Alignof()`. */
+# define RBIMPL_ALIGNOF(T) RB_GNUC_EXTENSION(_Alignof(T))
+
+#elif defined(__cplusplus)
+# /* C++11 `alignof()` can be buggy. */
+# /* see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69560 */
+# /* But don't worry, we can use templates and `constexpr`. */
+# define RBIMPL_ALIGNOF(T) ruby::rbimpl_alignof<T>()
+
+namespace ruby {
+template<typename T>
+RBIMPL_ATTR_CONSTEXPR(CXX11)
+RBIMPL_ATTR_ARTIFICIAL()
+RBIMPL_ATTR_FORCEINLINE()
+RBIMPL_ATTR_CONST()
+static size_t
+rbimpl_alignof()
+{
+    typedef struct {
+        char _;
+        T t;
+    } type;
+    return offsetof(type, t);
+}
+}
 
 #elif RBIMPL_COMPILER_IS(MSVC)
+# /* Windows have no alignment glitch.*/
 # define RBIMPL_ALIGNOF __alignof
 
-#elif defined(__GNUC__)
-# /* At least GCC 2.95 had this. */
-# define RBIMPL_ALIGNOF __extension__ __alignof__
-
-#elif defined(__alignof_is_defined) || defined(__DOXYGEN__)
-# /* OK, we can safely take <stdalign.h> definition. */
-# define RBIMPL_ALIGNOF alignof
-
-#elif RBIMPL_COMPILER_SINCE(SunPro, 5, 9, 0)
-# /* According to their  manual, Sun Studio 12 introduced  __alignof__ for both
-#  * C/C++. */
-# define RBIMPL_ALIGNOF __alignof__
-
-#elif 0
-# /* THIS IS NG, you cannot define a new type inside of offsetof. */
+#else
+# /* :BEWARE: It is  an undefined behaviour to define a  struct/union inside of
+#  * `offsetof()`!  This section is the  last resort.  If your compiler somehow
+#  * supports querying  alignment of a type  please add your own  definition of
+#  * `RBIMPL_ALIGNOF` instead. */
 # /* see: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm */
 # define RBIMPL_ALIGNOF(T) offsetof(struct { char _; T t; }, t)
 
-#else
-# error :FIXME: add your compiler here to obtain an alignment.
 #endif
 
 #endif /* RBIMPL_STDALIGN_H */