Index: cmake/modules/LLVMExternalProjectUtils.cmake
===================================================================
--- cmake/modules/LLVMExternalProjectUtils.cmake
+++ cmake/modules/LLVMExternalProjectUtils.cmake
@@ -28,7 +28,7 @@
 #   CMAKE_ARGS arguments...
 #     Optional cmake arguments to pass when configuring the project
 #   TOOLCHAIN_TOOLS targets...
-#     Targets for toolchain tools (defaults to clang;lld)
+#     Targets for toolchain tools (defaults to clang;lld;llvm-ar)
 #   DEPENDS targets...
 #     Targets that this project depends on
 #   EXTRA_TARGETS targets...
@@ -44,7 +44,7 @@
     ${ARGN})
   canonicalize_tool_name(${name} nameCanon)
   if(NOT ARG_TOOLCHAIN_TOOLS)
-    set(ARG_TOOLCHAIN_TOOLS clang lld)
+    set(ARG_TOOLCHAIN_TOOLS clang lld llvm-ar)
   endif()
   foreach(tool ${ARG_TOOLCHAIN_TOOLS})
     if(TARGET ${tool})
@@ -71,6 +71,11 @@
     set(CLANG_IN_TOOLCHAIN On)
   endif()
 
+  list(FIND TOOLCHAIN_TOOLS llvm-ar FOUND_ARCHIVER)
+  if(FOUND_ARCHIVER GREATER -1)
+    set(ARCHIVER_IN_TOOLCHAIN On)
+  endif()
+
   if(RUNTIME_LIBRARIES AND CLANG_IN_TOOLCHAIN)
     list(APPEND TOOLCHAIN_BINS ${RUNTIME_LIBRARIES})
   endif()
@@ -104,6 +109,9 @@
       set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang
                         -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++)
     endif()
+    if(ARCHIVER_IN_TOOLCHAIN)
+      list(APPEND compiler_args -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar)
+    endif()
     list(APPEND ARG_DEPENDS ${TOOLCHAIN_TOOLS})
   endif()
 
Index: runtimes/CMakeLists.txt
===================================================================
--- runtimes/CMakeLists.txt
+++ runtimes/CMakeLists.txt
@@ -139,12 +139,40 @@
   # is required because the other runtimes need the builtin libraries present
   # before the just-built compiler can pass the configuration tests.
   if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt)
-    llvm_ExternalProject_Add(builtins
-                             ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt/lib/builtins
-                             CMAKE_ARGS -DLLVM_LIBRARY_OUTPUT_INTDIR=${LLVM_LIBRARY_DIR}
-                                        -DLLVM_RUNTIME_OUTPUT_INTDIR=${LLVM_TOOLS_BINARY_DIR}
-                             PASSTHROUGH_PREFIXES COMPILER_RT
-                             USE_TOOLCHAIN)
+    if(APPLE OR NOT LLVM_BUILTIN_TARGETS)
+      llvm_ExternalProject_Add(builtins
+                               ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt/lib/builtins
+                               CMAKE_ARGS -DLLVM_LIBRARY_OUTPUT_INTDIR=${LLVM_LIBRARY_DIR}
+                                          -DLLVM_RUNTIME_OUTPUT_INTDIR=${LLVM_TOOLS_BINARY_DIR}
+                                          -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
+                                          -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
+                               PASSTHROUGH_PREFIXES COMPILER_RT
+                               USE_TOOLCHAIN)
+    else()
+      get_cmake_property(variableNames VARIABLES)
+      add_custom_target(builtins)
+      foreach(target ${LLVM_BUILTIN_TARGETS})
+        foreach(variableName ${variableNames})
+          if(variableName MATCHES "^BUILTINS_${target}")
+            string(REPLACE "BUILTINS_${target}_" "" new_name ${variableName})
+            list(APPEND ${target}_extra_args "-D${new_name}=${${variableName}}")
+          endif()
+        endforeach()
+        llvm_ExternalProject_Add(builtins-${target}
+                               ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt/lib/builtins
+                               CMAKE_ARGS -DLLVM_LIBRARY_OUTPUT_INTDIR=${LLVM_LIBRARY_DIR}
+                                          -DLLVM_RUNTIME_OUTPUT_INTDIR=${LLVM_TOOLS_BINARY_DIR}
+                                          -DCMAKE_C_COMPILER_TARGET=${target}
+                                          -DCMAKE_ASM_COMPILER_TARGET=${target}
+                                          -DCMAKE_C_COMPILER_WORKS=On
+                                          -DCMAKE_ASM_COMPILER_WORKS=On
+                                          -DCOMPILER_RT_DEFAULT_TARGET_ONLY=On
+                                          ${${target}_extra_args}
+                               PASSTHROUGH_PREFIXES COMPILER_RT
+                               USE_TOOLCHAIN)
+        add_dependencies(builtins builtins-${target})
+      endforeach()
+    endif()
     set(deps builtins)
     # We don't need to depend on the builtins if we're building instrumented
     # because the next stage will use the same compiler used to build this stage.