Index: flang/runtime/lock.h
===================================================================
--- flang/runtime/lock.h
+++ flang/runtime/lock.h
@@ -15,23 +15,19 @@
 
 // Avoid <mutex> if possible to avoid introduction of C++ runtime
 // library dependence.
-#ifndef _WIN32
-#define USE_PTHREADS 1
+#ifdef _WIN32
+// Do not define macros for "min" and "max"
+#define NOMINMAX
+#include <windows.h>
 #else
-#undef USE_PTHREADS
-#endif
-
-#if USE_PTHREADS
 #include <pthread.h>
-#else
-#include <mutex>
 #endif
 
 namespace Fortran::runtime {
 
 class Lock {
 public:
-#if USE_PTHREADS
+#ifndef _WIN32
   Lock() { pthread_mutex_init(&mutex_, nullptr); }
   ~Lock() { pthread_mutex_destroy(&mutex_); }
   void Take() {
@@ -41,9 +37,11 @@
   bool Try() { return pthread_mutex_trylock(&mutex_) == 0; }
   void Drop() { pthread_mutex_unlock(&mutex_); }
 #else
-  void Take() { mutex_.lock(); }
-  bool Try() { return mutex_.try_lock(); }
-  void Drop() { mutex_.unlock(); }
+  Lock() { InitializeCriticalSection(&cs_); }
+  ~Lock() { DeleteCriticalSection(&cs_); }
+  void Take() { EnterCriticalSection(&cs_); }
+  bool Try() { return TryEnterCriticalSection(&cs_); }
+  void Drop() { LeaveCriticalSection(&cs_); }
 #endif
 
   void CheckLocked(const Terminator &terminator) {
@@ -54,10 +52,10 @@
   }
 
 private:
-#if USE_PTHREADS
+#ifndef _WIN32
   pthread_mutex_t mutex_{};
 #else
-  std::mutex mutex_;
+  CRITICAL_SECTION cs_;
 #endif
 };