Index: llvm/cmake/modules/LLVMExternalProjectUtils.cmake
===================================================================
--- llvm/cmake/modules/LLVMExternalProjectUtils.cmake
+++ llvm/cmake/modules/LLVMExternalProjectUtils.cmake
@@ -56,6 +56,8 @@
     set(ARG_TOOLCHAIN_TOOLS clang lld llvm-ar llvm-ranlib llvm-nm llvm-objdump)
     if(_cmake_system_name STREQUAL Darwin)
       list(APPEND ARG_TOOLCHAIN_TOOLS llvm-libtool-darwin llvm-lipo)
+    elseif(_cmake_system_name STREQUAL Windows)
+      list(APPEND ARG_TOOLCHAIN_TOOLS llvm-lib)
     else()
       # TODO: These tools don't fully support Mach-O format yet.
       list(APPEND ARG_TOOLCHAIN_TOOLS llvm-objcopy llvm-strip)
@@ -144,7 +146,11 @@
       endif()
     endif()
     if(llvm-ar IN_LIST TOOLCHAIN_TOOLS)
-      list(APPEND compiler_args -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar${CMAKE_EXECUTABLE_SUFFIX})
+      if(_cmake_system_name STREQUAL Windows)
+        list(APPEND compiler_args -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-lib${CMAKE_EXECUTABLE_SUFFIX})
+      else()
+        list(APPEND compiler_args -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar${CMAKE_EXECUTABLE_SUFFIX})
+      endif()
     endif()
     if(llvm-libtool-darwin IN_LIST TOOLCHAIN_TOOLS)
       list(APPEND compiler_args -DCMAKE_LIBTOOL=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-libtool-darwin${CMAKE_EXECUTABLE_SUFFIX})