Disable the use of std::call_once on OpenBSD with libstdc++.

It was noticed this caused performance regressions and deadlocks. PR30768.

Reorder the code to make it clearer what is tested.

PPC now disables the use of std::call_once only with libstdc++ with
the reordering of the code, as was the original intent.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285782 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Threading.h

@@ -20,11 +20,11 @@
 #include <ciso646> // So we can check the C++ standard lib macros.
 #include <functional>
 
-// We use std::call_once on all Unix platforms except for NetBSD with
-// libstdc++. That platform has a bug they are working to fix, and they'll
-// remove the NetBSD checks once fixed.
-#if defined(LLVM_ON_UNIX) &&                                                   \
-    !(defined(__NetBSD__) && !defined(_LIBCPP_VERSION)) && !defined(__ppc__)
+// std::call_once from libc++ is used on all Unix platforms. Other
+// implementations like libstdc++ are known to have problems on NetBSD,
+// OpenBSD and PowerPC.
+#if defined(LLVM_ON_UNIX) && (defined(_LIBCPP_VERSION) ||                      \
+    !(defined(__NetBSD__) || defined(__OpenBSD__) || defined(__ppc__)))
 #define LLVM_THREADING_USE_STD_CALL_ONCE 1
 #else
 #define LLVM_THREADING_USE_STD_CALL_ONCE 0