[Support] Wrap extern TLS variable in getter function

This patch wraps an external thread local storage variable inside of a
getter function and makes it have internal linkage. This allows LLVM to
be built with BUILD_SHARED_LIBS on windows with MinGW. Additionally it
allows Clang versions prior to 10 to compile current trunk for MinGW.

Differential Revision: https://reviews.llvm.org/D73639

llvm/include/llvm/Support/TimeProfiler.h

@@ -14,7 +14,7 @@
 namespace llvm {
 
 struct TimeTraceProfiler;
-extern LLVM_THREAD_LOCAL TimeTraceProfiler *TimeTraceProfilerInstance;
+TimeTraceProfiler *getTimeTraceProfilerInstance();
 
 /// Initialize the time trace profiler.
 /// This sets up the global \p TimeTraceProfilerInstance
@@ -30,7 +30,7 @@ void timeTraceProfilerFinishThread();
 
 /// Is the time trace profiler enabled, i.e. initialized?
 inline bool timeTraceProfilerEnabled() {
-  return TimeTraceProfilerInstance != nullptr;
+  return getTimeTraceProfilerInstance() != nullptr;
 }
 
 /// Write profiling data to output file.
@@ -62,19 +62,19 @@ struct TimeTraceScope {
   TimeTraceScope &operator=(TimeTraceScope &&) = delete;
 
   TimeTraceScope(StringRef Name) {
-    if (TimeTraceProfilerInstance != nullptr)
+    if (getTimeTraceProfilerInstance() != nullptr)
       timeTraceProfilerBegin(Name, StringRef(""));
   }
   TimeTraceScope(StringRef Name, StringRef Detail) {
-    if (TimeTraceProfilerInstance != nullptr)
+    if (getTimeTraceProfilerInstance() != nullptr)
       timeTraceProfilerBegin(Name, Detail);
   }
   TimeTraceScope(StringRef Name, llvm::function_ref<std::string()> Detail) {
-    if (TimeTraceProfilerInstance != nullptr)
+    if (getTimeTraceProfilerInstance() != nullptr)
       timeTraceProfilerBegin(Name, Detail);
   }
   ~TimeTraceScope() {
-    if (TimeTraceProfilerInstance != nullptr)
+    if (getTimeTraceProfilerInstance() != nullptr)
       timeTraceProfilerEnd();
   }
 };

llvm/lib/Support/TimeProfiler.cpp

@@ -24,16 +24,22 @@
 #include <vector>
 
 using namespace std::chrono;
+using namespace llvm;
 
 namespace {
 std::mutex Mu;
-std::vector<llvm::TimeTraceProfiler *>
+// List of all instances
+std::vector<TimeTraceProfiler *>
     ThreadTimeTraceProfilerInstances; // guarded by Mu
+// Per Thread instance
+LLVM_THREAD_LOCAL TimeTraceProfiler *TimeTraceProfilerInstance = nullptr;
 } // namespace
 
 namespace llvm {
 
-LLVM_THREAD_LOCAL TimeTraceProfiler *TimeTraceProfilerInstance = nullptr;
+TimeTraceProfiler *getTimeTraceProfilerInstance() {
+  return TimeTraceProfilerInstance;
+}
 
 typedef duration<steady_clock::rep, steady_clock::period> DurationType;
 typedef time_point<steady_clock> TimePointType;