diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
index f0ef353a2c66..d466c69d1ca8 100644
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -5,15 +5,18 @@
 
 set(COMMON_CMAKE_ARGS "-DHAVE_LLVM_LIT=ON;-DCLANG_RESOURCE_DIR=${CLANG_RESOURCE_DIR}")
 foreach(proj ${LLVM_ENABLE_RUNTIMES})
-  set(proj_dir "${CMAKE_CURRENT_SOURCE_DIR}/../../${proj}")
-  if(IS_DIRECTORY ${proj_dir} AND EXISTS ${proj_dir}/CMakeLists.txt)
-    list(APPEND runtimes ${proj_dir})
-  else()
-    message(FATAL_ERROR "LLVM_ENABLE_RUNTIMES requests ${proj} but directory not found: ${proj_dir}")
-  endif()
   string(TOUPPER "${proj}" canon_name)
   STRING(REGEX REPLACE "-" "_" canon_name ${canon_name})
-  set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../../${proj}")
+  if(NOT LLVM_EXTERNAL_${canon_name}_SOURCE_DIR)
+    set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../../${proj}")
+  endif()
+  set(proj_dir "${LLVM_EXTERNAL_${canon_name}_SOURCE_DIR}")
+  if(NOT IS_DIRECTORY ${proj_dir} OR NOT EXISTS ${proj_dir}/CMakeLists.txt)
+    message(FATAL_ERROR "LLVM_ENABLE_RUNTIMES requests ${proj} but directory not found: ${proj_dir}")
+  endif()
+  list(APPEND runtimes ${proj_dir})
+  list(APPEND RUNTIMES_CMAKE_ARGS
+    "-DLLVM_EXTERNAL_${canon_name}_SOURCE_DIR=${LLVM_EXTERNAL_${canon_name}_SOURCE_DIR}")
 endforeach()
 
 function(get_compiler_rt_path path)
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index 157e87750e19..c12e2406b5d0 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -46,15 +46,17 @@ endif()
 include(SortSubset)
 sort_subset("${LLVM_SUPPORTED_RUNTIMES}" "${LLVM_ENABLE_RUNTIMES}" LLVM_ENABLE_RUNTIMES)
 foreach(proj ${LLVM_ENABLE_RUNTIMES})
-  set(proj_dir "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  string(TOUPPER "${proj}" canon_name)
+  STRING(REGEX REPLACE "-" "_" canon_name ${canon_name})
+  if(NOT LLVM_EXTERNAL_${canon_name}_SOURCE_DIR)
+    set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  endif()
+  set(proj_dir "${LLVM_EXTERNAL_${canon_name}_SOURCE_DIR}")
   if(IS_DIRECTORY ${proj_dir} AND EXISTS ${proj_dir}/CMakeLists.txt)
     list(APPEND runtimes ${proj_dir})
   else()
     message(FATAL_ERROR "LLVM_ENABLE_RUNTIMES requests ${proj} but directory not found: ${proj_dir}")
   endif()
-  string(TOUPPER "${proj}" canon_name)
-  STRING(REGEX REPLACE "-" "_" canon_name ${canon_name})
-  set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
 endforeach()
 
 function(runtime_register_component name)