diff --git a/llvm/include/llvm/Support/Threading.h b/llvm/include/llvm/Support/Threading.h
--- a/llvm/include/llvm/Support/Threading.h
+++ b/llvm/include/llvm/Support/Threading.h
@@ -130,7 +130,7 @@
   /// Get the amount of currency to use for tasks requiring significant
   /// memory or other resources. Currently based on physical cores, if
   /// available for the host system, otherwise falls back to
-  /// thread::hardware_concurrency().
+  /// llvm::hardware_concurrency().
   /// Returns 1 when LLVM is configured with LLVM_ENABLE_THREADS=OFF
   unsigned heavyweight_hardware_concurrency();
 
diff --git a/llvm/lib/Support/Threading.cpp b/llvm/lib/Support/Threading.cpp
--- a/llvm/lib/Support/Threading.cpp
+++ b/llvm/lib/Support/Threading.cpp
@@ -68,7 +68,7 @@
   // ADL.
   int NumPhysical = sys::getHostNumPhysicalCores();
   if (NumPhysical == -1)
-    return std::thread::hardware_concurrency();
+    return llvm::hardware_concurrency();
   return NumPhysical;
 }