Index: include/llvm/Support/DebugCounter.h
===================================================================
--- include/llvm/Support/DebugCounter.h
+++ include/llvm/Support/DebugCounter.h
@@ -51,6 +51,10 @@
 #include "llvm/Support/raw_ostream.h"
 #include <string>
 
+#if LLVM_ENABLE_THREADS
+#include <atomic>
+#endif
+
 namespace llvm {
 
 class DebugCounter {
@@ -151,10 +155,30 @@
   }
   // Struct to store counter info.
   struct CounterInfo {
+    #if LLVM_ENABLE_THREADS
+    CounterInfo() = default;
+    CounterInfo(const CounterInfo &Other) { *this = Other; }
+    CounterInfo &operator =(const CounterInfo &Other) {
+      Count = Other.Count.load();
+      Skip = Other.Skip.load();
+      StopAfter = Other.StopAfter.load();
+      IsSet = Other.IsSet.load();
+      Desc = Other.Desc;
+      return *this;
+    }
+
+    // We use atomics merely to silence TSAN warnings. DebugCounter does not
+    // well-defined semantics for multi-threaded programs.
+    std::atomic<int64_t> Count{0};
+    std::atomic<int64_t> Skip{0};
+    std::atomic<int64_t> StopAfter{-1};
+    std::atomic<bool> IsSet{false};
+    #else
     int64_t Count = 0;
     int64_t Skip = 0;
     int64_t StopAfter = -1;
     bool IsSet = false;
+    #endif // LLVM_ENABLE_THREADS
     std::string Desc;
   };
   DenseMap<unsigned, CounterInfo> Counters;
Index: unittests/Support/DebugCounterTest.cpp
===================================================================
--- unittests/Support/DebugCounterTest.cpp
+++ unittests/Support/DebugCounterTest.cpp
@@ -11,6 +11,13 @@
 #include "gtest/gtest.h"
 
 #include <string>
+
+#if LLVM_ENABLE_THREADS
+#include <thread>
+#include <mutex>
+#include <condition_variable>
+#endif
+
 using namespace llvm;
 
 #ifndef NDEBUG
@@ -39,4 +46,46 @@
   DebugCounter::setCounterValue(TestCounter, 100);
   EXPECT_FALSE(DebugCounter::shouldExecute(TestCounter));
 }
-#endif
+
+#if LLVM_ENABLE_THREADS
+TEST(DebugCounterTest, MultipleThreads) {
+  constexpr int ThreadCount = 3;
+  constexpr int Iterations = 1000;
+
+  std::mutex Mut;
+  std::condition_variable Cond;
+  int ThreadsWaiting = 0;
+  bool StartRunning = false;
+
+  auto Run = [&]() {
+    {
+      std::unique_lock<std::mutex> Lock(Mut);
+      ++ThreadsWaiting;
+      Cond.notify_all();
+      Cond.wait(Lock, [&]() { return StartRunning; });
+    }
+    for (int I = 0; I < Iterations; ++I)
+      DebugCounter::shouldExecute(TestCounter);
+  };
+
+  // This test is here to make sure TSAN does not show warnings about data races.
+  // DebugCounter should not be used in multi-threaded environments.
+  std::vector<std::thread> Threads;
+  for (int I = 0; I < ThreadCount; ++I) {
+    Threads.emplace_back(Run);
+  }
+
+  {
+    std::unique_lock<std::mutex> Lock(Mut);
+    // Wait for the threads to start.
+    Cond.wait(Lock, [&]() { return ThreadsWaiting == ThreadCount; });
+    StartRunning = true;
+    // Indicate they can start running.
+    Cond.notify_all();
+  }
+
+  for (auto & T : Threads)
+    T.join();
+}
+#endif // LLVM_ENABLE_THREADS
+#endif // NDEBUG