diff --git a/llvm/lib/Support/Host.cpp b/llvm/lib/Support/Host.cpp
--- a/llvm/lib/Support/Host.cpp
+++ b/llvm/lib/Support/Host.cpp
@@ -1326,7 +1326,7 @@
   }
   return count;
 }
-#elif defined(_WIN32)
+#elif defined(_WIN32) && LLVM_THREADS_ENABLED
 // Defined in llvm/lib/Support/Windows/Threading.inc
 int computeHostNumPhysicalCores();
 #else