[libcxx] Allow use of <atomic> in C++03. Try 3.
Summary:
After putting this question up on cfe-dev I have decided that it would be best to allow the use of `<atomic>` in C++03. Although static initialization is a concern the syntax required to get it is C++11 only. Meaning that C++11 constant static initialization cannot silently break in C++03, it will always cause a syntax error. Furthermore `ATOMIC_VAR_INIT` and `ATOMIC_FLAG_INIT` remain defined in C++03 even though they cannot be used because C++03 usages will cause better error messages.
The main change in this patch is to replace `__has_feature(cxx_atomic)`, which only returns true when C++ >= 11, to `__has_extension(c_atomic)` which returns true whenever clang supports the required atomic builtins.
This patch adds the following macros:
* `_LIBCPP_HAS_C_ATOMIC_IMP` - Defined on clang versions which provide the C `_Atomic` keyword.
* `_LIBCPP_HAS_GCC_ATOMIC_IMP` - Defined on GCC > 4.7. We must use the fallback atomic implementation.
* `_LIBCPP_HAS_NO_ATOMIC_HEADER` - Defined when it is not safe to include `<atomic>`.
`_LIBCPP_HAS_C_ATOMIC_IMP` and `_LIBCPP_HAS_GCC_ATOMIC_IMP` are mutually exclusive, only one should be defined. If neither is defined then `<atomic>` is not implemented and including `<atomic>` will issue an error.
Reviewers: chandlerc, jroelofs, mclow.lists
Subscribers: cfe-commits
Differential Revision: http://reviews.llvm.org/D11555
llvm-svn: 245463
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 5c15156..7667500 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -37,6 +37,9 @@
#ifndef __has_builtin
#define __has_builtin(__x) 0
#endif
+#ifndef __has_extension
+#define __has_extension(__x) 0
+#endif
#ifndef __has_feature
#define __has_feature(__x) 0
#endif
@@ -773,4 +776,15 @@
#define _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
#endif
-#endif // _LIBCPP_CONFIG
+#if __has_extension(c_atomic)
+#define _LIBCPP_HAS_C_ATOMIC_IMP
+#elif _GNUC_VER > 407
+#define _LIBCPP_HAS_GCC_ATOMIC_IMP
+#endif
+
+#if (!defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_GCC_ATOMIC_IMP)) \
+ || defined(_LIBCPP_HAS_NO_THREADS)
+#define _LIBCPP_HAS_NO_ATOMIC_HEADER
+#endif
+
+#endif // _LIBCPP_CONFIG