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author卜部昌平 <shyouhei@ruby-lang.org>2020-09-23 01:57:35 (GMT)
committer卜部昌平 <shyouhei@ruby-lang.org>2020-09-25 02:38:33 (GMT)
commite9fb2bc873a55181ac7d581e7252db3754a3209d (patch)
tree7ea6012f6ed825b0f35d0ffbca3ea275266f2b8d
parentfe875be01a501a47a4bf75df9c6b42bb33d9842b (diff)
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.
Notes
Notes: Merged: https://github.com/ruby/ruby/pull/3570
-rw-r--r--configure.ac58
-rw-r--r--include/ruby/internal/stdalign.h90
2 files changed, 87 insertions, 61 deletions
diff --git a/configure.ac b/configure.ac
index 46fdecd..f7bbc36 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 8c56fbb..8d9a0d2 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 */