The previous code would never use __is_identifier and __is_builtin for a
compiler that defines __GNU_C__ >= 7. A hypothetical compiler that
supports __is_identifier and __is_builtin might define __GNUC__ to 8
and therefore the macros for BUILTIN_IS_CONSTANT_EVALUATED
and BUILTIN_IS_SAME would not get defined, and we would not check for
those intrinsics using __is_identifier or __is_builtin either.
This change means that after the conditions using __GNUC__ we then check
__is_identifier or __is_builtin for any features not yet defined.
This probably doesn't achieve anything in practice, because Clang
defines __GNUC__ to 4, and Intel defines it to match whatever GCC
defines, so it will claim to be GCC 11 if using the libstdc++ headers
from GCC 11. But it still seems more correct to do it this way.
libstdc++-v3/ChangeLog:
* include/bits/c++config: Do checks using __is_identifier and
__is_builtin independent of checks for __GNUC__.
Does this seem worth doing, despite the caveat above?
commit 358069de33f305d753c64c7e8e4819bf4638ed14
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Wed Nov 18 16:15:15 2020
libstdc++: Fix detection of intrinsics for __GNUC__ < 11
The previous code would never use __is_identifier and __is_builtin for a
compiler that defines __GNU_C__ >= 7. A hypothetical compiler that
supports __is_identifier and __is_builtin might define __GNUC__ to 8
and therefore the macros for BUILTIN_IS_CONSTANT_EVALUATED
and BUILTIN_IS_SAME would not get defined, and we would not check for
those intrinsics using __is_identifier or __is_builtin either.
This change means that after the conditions using __GNUC__ we then check
__is_identifier or __is_builtin for any features not yet defined.
This probably doesn't achieve anything in practice, because Clang
defines __GNUC__ to 4, and Intel defines it to match whatever GCC
defines, so it will claim to be GCC 11 if using the libstdc++ headers
from GCC 11. But it still seems more correct to do it this way.
libstdc++-v3/ChangeLog:
* include/bits/c++config: Do checks using __is_identifier and
__is_builtin independent of checks for __GNUC__.
diff --git a/libstdc++-v3/include/bits/c++config
b/libstdc++-v3/include/bits/c++config
index 2e6c880ad95a..abf6320082c0 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -664,21 +664,28 @@ namespace std
# if __GNUC__ >= 11
# define _GLIBCXX_HAVE_BUILTIN_IS_SAME 1
# endif
-#elif defined(__is_identifier) && defined(__has_builtin)
+#endif
+
+#if defined(__is_identifier) && defined(__has_builtin)
// For non-GNU compilers:
-# if ! __is_identifier(__has_unique_object_representations)
+# if ! defined _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP \
+ && ! __is_identifier(__has_unique_object_representations)
# define _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP 1
# endif
-# if ! __is_identifier(__is_aggregate)
+# if ! defined _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE \
+ && ! __is_identifier(__is_aggregate)
# define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
# endif
-# if __has_builtin(__builtin_launder)
+# if ! defined _GLIBCXX_HAVE_BUILTIN_LAUNDER \
+ && __has_builtin(__builtin_launder)
# define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
# endif
-# if __has_builtin(__builtin_is_constant_evaluated)
+# if ! defined _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED \
+ && __has_builtin(__builtin_is_constant_evaluated)
# define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
# endif
-# if ! __is_identifier(__is_same)
+# if ! defined _GLIBCXX_HAVE_BUILTIN_IS_SAME \
+ && ! __is_identifier(__is_same)
# define _GLIBCXX_HAVE_BUILTIN_IS_SAME 1
# endif
#endif // GCC
