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.

2 files changed, 87 insertions, 61 deletions

diff --git a/configure.ac b/configure.ac
index 46fdecda23..f7bbc36366 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1460,26 +1460,44 @@ AS_IF([test "$rb_cv_va_args_macro" = yes], [
   AC_DEFINE(HAVE_VA_ARGS_MACRO)
 ])
 
-AC_CACHE_CHECK([for alignof() syntax], rb_cv_have_alignof,[
-rb_cv_have_alignof=no
-# Prefer alignof over _Alignof to allow C++ compiler to read ruby.h
-RUBY_WERROR_FLAG([
-for expr in \
-    "alignof" \
-    "_Alignof" \
-    "__alignof" \
-    "__alignof__" \
-;
-do
-    AC_TRY_COMPILE([
-    @%:@ifdef HAVE_STDALIGN_H
-    @%:@include <stdalign.h>
-    @%:@endif],[return (int)$expr(int);],
-        [rb_cv_have_alignof="$expr"; break], [])
-done
-])])
-AS_IF([test "$rb_cv_have_alignof" != no], [
-    AC_DEFINE_UNQUOTED(RUBY_ALIGNOF, $rb_cv_have_alignof)
+# We want C11's  `_Alignof`.  GCC (and alike) have  `__alignof__`, which behave
+# slightly differently  than the  C11's.  We cannot  use `__alignof__`  for our
+# purpose.   The problem  is, however,  that  old gcc  and old  clang had  both
+# implemented `_Alignof` as  a synonym of `__alignof__`.  They are  not what we
+# want.  We have to check sanity.
+#
+# See also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023
+# See also: https://bugs.llvm.org/show_bug.cgi?id=26547
+AC_CACHE_CHECK([if _Alignof() works], rb_cv_have__alignof,[
+    rb_cv_have__alignof=no
+    RUBY_WERROR_FLAG([
+        AC_TRY_COMPILE([
+            @%:@ifdef HAVE_STDALIGN_H
+            @%:@include <stdalign.h>
+            @%:@endif
+            @%:@ifdef STDC_HEADERS
+            @%:@include <stddef.h>
+            @%:@endif
+            @%:@ifndef __GNUC__
+            @%:@define __extension__
+            @%:@endif
+        ], [
+            typedef struct conftest_tag {
+                char _;
+                double d;
+            } T;
+            static int conftest_ary@<:@
+                offsetof(T, d) == __extension__ _Alignof(double)
+                ? 1 : -1
+            @:>@;
+            return conftest_ary@<:@0@:>@;
+        ], [
+            rb_cv_have__alignof=yes
+        ])
+    ])
+])
+AS_IF([test "$rb_cv_have__alignof" != no], [
+    AC_DEFINE(HAVE__ALIGNOF)
 ])
 
 RUBY_FUNC_ATTRIBUTE(__const__, CONSTFUNC)
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 */