[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

1 files changed, 4 insertions, 2 deletions

diff --git a/libcxx/include/ios b/libcxx/include/ios
index ff79998b0bb..8843ed4ca85 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -216,7 +216,7 @@ storage-class-specifier const error_category& iostream_category() noexcept;
 #include <__locale>
 #include <system_error>
 
-#if __has_feature(cxx_atomic) && !defined(_LIBCPP_HAS_NO_THREADS)
+#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 #include <atomic>     // for __xindex_
 #endif
 
@@ -367,7 +367,9 @@ private:
     int*            __index_;
     size_t          __event_size_;
     size_t          __event_cap_;
-#if __has_feature(cxx_atomic) && !defined(_LIBCPP_HAS_NO_THREADS)
+// TODO(EricWF): Enable this for both Clang and GCC. Currently it is only
+// enabled with clang.
+#if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
     static atomic<int> __xindex_;
 #else
     static int      __xindex_;