[Support] Replace Windows __declspec(thread) with thread_local for LLVM_THREAD_LOCAL

Windows minimum host tools version is now VS2017, which supports C++11
thread_local so use this for LLVM_THREAD_LOCAL instead of
declspec(thread). According to [1], thread_local is implemented with
declspec(thread) so this should be NFC.

[1] https://docs.microsoft.com/en-us/cpp/cpp/thread?view=vs-2017

Differential Revision: https://reviews.llvm.org/D72399

diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
index 913353c..6f6f65c 100644 (file)
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -512,19 +512,15 @@ void AnnotateIgnoreWritesEnd(const char *file, int line);
 /// extern globals, and static globals.
 ///
 /// This is essentially an extremely restricted analog to C++11's thread_local
-/// support, and uses that when available. However, it falls back on
-/// platform-specific or vendor-provided extensions when necessary. These
-/// extensions don't support many of the C++11 thread_local's features. You
-/// should only use this for PODs that you can statically initialize to
-/// some constant value. In almost all circumstances this is most appropriate
-/// for use with a pointer, integer, or small aggregation of pointers and
-/// integers.
+/// support. It uses thread_local if available, falling back on gcc __thread
+/// if not. __thread doesn't support many of the C++11 thread_local's
+/// features. You should only use this for PODs that you can statically
+/// initialize to some constant value. In almost all circumstances this is most
+/// appropriate for use with a pointer, integer, or small aggregation of
+/// pointers and integers.
 #if LLVM_ENABLE_THREADS
-#if __has_feature(cxx_thread_local)
+#if __has_feature(cxx_thread_local) || defined(_MSC_VER)
 #define LLVM_THREAD_LOCAL thread_local
-#elif defined(_MSC_VER)
-// MSVC supports this with a __declspec.
-#define LLVM_THREAD_LOCAL __declspec(thread)
 #else
 // Clang, GCC, and other compatible compilers used __thread prior to C++11 and
 // we only need the restricted functionality that provides.