diff --git a/libcxx/TODO.TXT b/libcxx/TODO.TXT
--- a/libcxx/TODO.TXT
+++ b/libcxx/TODO.TXT
@@ -73,3 +73,4 @@
 * Document the "conditionally-supported" bits of libc++
 * Look at basic_string's move assignment operator, re LWG 2063 and POCMA
 * Put a static_assert in std::allocator to deny const/volatile types (LWG 2447)
+
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -671,7 +671,12 @@
 set(LLVM_TARGET_TRIPLE_ENV CACHE STRING "The name of environment variable to override default target. Disabled by blank.")
 mark_as_advanced(LLVM_TARGET_TRIPLE_ENV)
 
-set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR OFF CACHE BOOL
+if(CMAKE_SYSTEM_NAME MATCHES "Linux")
+  set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default ON)
+else()
+  set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default OFF)
+endif()
+set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ${LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default} CACHE BOOL
   "Enable per-target runtimes directory")
 
 set(LLVM_PROFDATA_FILE "" CACHE FILEPATH