diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index 515b43bc9d2d47..cdafdfc7ecc3b5 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -621,6 +621,32 @@ add_subdirectory(include)
 
 option(COMPILER_RT_USE_LIBCXX
   "Enable compiler-rt to use libc++ from the source tree" ON)
+if(COMPILER_RT_USE_LIBCXX)
+  if(LLVM_ENABLE_PROJECTS_USED)
+    # Don't use libcxx if LLVM_ENABLE_PROJECTS does not enable it.
+    set(COMPILER_RT_LIBCXX_PATH ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR})
+    set(COMPILER_RT_LIBCXXABI_PATH ${LLVM_EXTERNAL_LIBCXXABI_SOURCE_DIR})
+  else()
+    foreach(path IN ITEMS ${LLVM_MAIN_SRC_DIR}/projects/libcxx
+                          ${LLVM_MAIN_SRC_DIR}/runtimes/libcxx
+                          ${LLVM_MAIN_SRC_DIR}/../libcxx
+                          ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR})
+      if(IS_DIRECTORY ${path})
+        set(COMPILER_RT_LIBCXX_PATH ${path})
+        break()
+      endif()
+    endforeach()
+    foreach(path IN ITEMS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi
+                          ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi
+                          ${LLVM_MAIN_SRC_DIR}/../libcxxabi
+                          ${LLVM_EXTERNAL_LIBCXXABI_SOURCE_DIR})
+      if(IS_DIRECTORY ${path})
+        set(COMPILER_RT_LIBCXXABI_PATH ${path})
+        break()
+      endif()
+    endforeach()
+  endif()
+endif()
 
 set(COMPILER_RT_LLD_PATH ${LLVM_MAIN_SRC_DIR}/tools/lld)
 if(EXISTS ${COMPILER_RT_LLD_PATH}/ AND LLVM_TOOL_LLD_BUILD)
diff --git a/compiler-rt/cmake/Modules/AddCompilerRT.cmake b/compiler-rt/cmake/Modules/AddCompilerRT.cmake
index f3c26c8a8b9946..b7eb04327bb1da 100644
--- a/compiler-rt/cmake/Modules/AddCompilerRT.cmake
+++ b/compiler-rt/cmake/Modules/AddCompilerRT.cmake
@@ -585,6 +585,13 @@ endmacro(add_compiler_rt_script src name)
 #                   CFLAGS <list of compile flags>
 #                   USE_TOOLCHAIN)
 macro(add_custom_libcxx name prefix)
+  if(NOT COMPILER_RT_LIBCXX_PATH)
+    message(FATAL_ERROR "libcxx not found!")
+  endif()
+  if(NOT COMPILER_RT_LIBCXXABI_PATH)
+    message(FATAL_ERROR "libcxxabi not found!")
+  endif()
+
   cmake_parse_arguments(LIBCXX "USE_TOOLCHAIN" "" "DEPS;CFLAGS;CMAKE_ARGS" ${ARGN})
 
   if(LIBCXX_USE_TOOLCHAIN)
diff --git a/compiler-rt/lib/fuzzer/CMakeLists.txt b/compiler-rt/lib/fuzzer/CMakeLists.txt
index 18c156d739d8c3..4500c4f1340933 100644
--- a/compiler-rt/lib/fuzzer/CMakeLists.txt
+++ b/compiler-rt/lib/fuzzer/CMakeLists.txt
@@ -59,7 +59,9 @@ CHECK_CXX_SOURCE_COMPILES("
 
 set(LIBFUZZER_CFLAGS ${COMPILER_RT_COMMON_CFLAGS})
 
-if(OS_NAME MATCHES "Linux|Fuchsia" AND COMPILER_RT_USE_LIBCXX)
+if(OS_NAME MATCHES "Linux|Fuchsia" AND
+   COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH)
   list(APPEND LIBFUZZER_CFLAGS -D_LIBCPP_ABI_VERSION=Fuzzer)
   append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ LIBFUZZER_CFLAGS)
 elseif(TARGET cxx-headers OR HAVE_LIBCXX)
@@ -132,7 +134,9 @@ add_compiler_rt_runtime(clang_rt.fuzzer_interceptors
   CFLAGS ${LIBFUZZER_CFLAGS}
   PARENT_TARGET fuzzer)
 
-if(OS_NAME MATCHES "Linux|Fuchsia" AND COMPILER_RT_USE_LIBCXX)
+if(OS_NAME MATCHES "Linux|Fuchsia" AND
+   COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH)
   macro(partially_link_libcxx name dir arch)
     get_target_flags_for_arch(${arch} target_cflags)
     if(CMAKE_CXX_COMPILER_ID MATCHES Clang)
diff --git a/compiler-rt/lib/fuzzer/tests/CMakeLists.txt b/compiler-rt/lib/fuzzer/tests/CMakeLists.txt
index 6ed5b86c1e21e1..10fcfbaa083e5c 100644
--- a/compiler-rt/lib/fuzzer/tests/CMakeLists.txt
+++ b/compiler-rt/lib/fuzzer/tests/CMakeLists.txt
@@ -30,7 +30,9 @@ else()
   list(APPEND LIBFUZZER_UNITTEST_LINK_FLAGS -lpthread)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND COMPILER_RT_USE_LIBCXX)
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND
+   COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH)
   list(APPEND LIBFUZZER_UNITTEST_CFLAGS -nostdinc++ -fno-exceptions)
   list(APPEND LIBFUZZER_UNITTEST_LINK_FLAGS -nostdlib++ -fno-exceptions)
 endif()
@@ -58,7 +60,9 @@ if(COMPILER_RT_DEFAULT_TARGET_ARCH IN_LIST FUZZER_SUPPORTED_ARCH)
     ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
     FOLDER "Compiler-RT Runtime tests")
 
-  if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND COMPILER_RT_USE_LIBCXX)
+  if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND
+     COMPILER_RT_LIBCXX_PATH AND
+     COMPILER_RT_LIBCXXABI_PATH)
     file(GLOB libfuzzer_headers ../*.h)
     set(LIBFUZZER_TEST_RUNTIME_DEPS libcxx_fuzzer_${arch}-build ${libfuzzer_headers})
     set(LIBFUZZER_TEST_RUNTIME_CFLAGS -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
diff --git a/compiler-rt/lib/msan/tests/CMakeLists.txt b/compiler-rt/lib/msan/tests/CMakeLists.txt
index 3dfc16bcc30496..3ed888b81c6e58 100644
--- a/compiler-rt/lib/msan/tests/CMakeLists.txt
+++ b/compiler-rt/lib/msan/tests/CMakeLists.txt
@@ -130,7 +130,9 @@ macro(add_msan_tests_for_arch arch kind cflags)
 endmacro()
 
 # We should only build MSan unit tests if we can build instrumented libcxx.
-if(COMPILER_RT_CAN_EXECUTE_TESTS AND COMPILER_RT_USE_LIBCXX)
+if(COMPILER_RT_CAN_EXECUTE_TESTS AND
+   COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH)
   foreach(arch ${MSAN_SUPPORTED_ARCH})
     get_target_flags_for_arch(${arch} TARGET_CFLAGS)
     set(LIBCXX_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/../libcxx_msan_${arch})
diff --git a/compiler-rt/lib/tsan/CMakeLists.txt b/compiler-rt/lib/tsan/CMakeLists.txt
index 454f435341cff4..a327fb14de9a5d 100644
--- a/compiler-rt/lib/tsan/CMakeLists.txt
+++ b/compiler-rt/lib/tsan/CMakeLists.txt
@@ -24,7 +24,8 @@ else()
 endif()
 
 # Build libcxx instrumented with TSan.
-if(COMPILER_RT_USE_LIBCXX AND
+if(COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH AND
    COMPILER_RT_TEST_COMPILER_ID STREQUAL "Clang" AND
    NOT ANDROID)
   set(libcxx_tsan_deps)
diff --git a/compiler-rt/test/msan/CMakeLists.txt b/compiler-rt/test/msan/CMakeLists.txt
index bd7e8934ad8887..01b832b5ae3f9d 100644
--- a/compiler-rt/test/msan/CMakeLists.txt
+++ b/compiler-rt/test/msan/CMakeLists.txt
@@ -45,7 +45,9 @@ if(NOT COMPILER_RT_STANDALONE_BUILD)
   list(APPEND MSAN_TEST_DEPS msan)
 endif()
 
-if(COMPILER_RT_INCLUDE_TESTS AND COMPILER_RT_USE_LIBCXX)
+if(COMPILER_RT_INCLUDE_TESTS AND
+   COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH)
   configure_lit_site_cfg(
     ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.py.in
     ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg.py)
diff --git a/compiler-rt/test/tsan/CMakeLists.txt b/compiler-rt/test/tsan/CMakeLists.txt
index c0570801a271bd..25e95aa98cf950 100644
--- a/compiler-rt/test/tsan/CMakeLists.txt
+++ b/compiler-rt/test/tsan/CMakeLists.txt
@@ -7,7 +7,8 @@ endif()
 if(NOT COMPILER_RT_STANDALONE_BUILD)
   list(APPEND TSAN_TEST_DEPS tsan)
 endif()
-if(COMPILER_RT_USE_LIBCXX AND
+if(COMPILER_RT_LIBCXX_PATH AND
+   COMPILER_RT_LIBCXXABI_PATH AND
    COMPILER_RT_TEST_COMPILER_ID STREQUAL "Clang"
    AND NOT APPLE AND NOT ANDROID)
   list(APPEND TSAN_TEST_DEPS libcxx_tsan)