diff --git a/clang/test/CMakeLists.txt b/clang/test/CMakeLists.txt
--- a/clang/test/CMakeLists.txt
+++ b/clang/test/CMakeLists.txt
@@ -35,6 +35,16 @@
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py
+  PATHS
+  "LLVM_SOURCE_DIR"
+  "LLVM_BINARY_DIR"
+  "LLVM_TOOLS_DIR"
+  "LLVM_LIBS_DIR"
+  "SHLIBDIR"
+  "LLVM_LIT_TOOLS_DIR"
+  "CLANG_BINARY_DIR"
+  "CLANG_SOURCE_DIR"
+  "CLANG_TOOLS_DIR"
   )
 
 configure_lit_site_cfg(
@@ -42,6 +52,13 @@
   ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.cfg.py
+  PATHS
+  "LLVM_SOURCE_DIR"
+  "LLVM_BINARY_DIR"
+  "LLVM_TOOLS_DIR"
+  "LLVM_LIBS_DIR"
+  "CLANG_BINARY_DIR"
+  "SHLIBDIR"
   )
 
 option(CLANG_TEST_USE_VG "Run Clang tests under Valgrind" OFF)
diff --git a/clang/test/Unit/lit.site.cfg.py.in b/clang/test/Unit/lit.site.cfg.py.in
--- a/clang/test/Unit/lit.site.cfg.py.in
+++ b/clang/test/Unit/lit.site.cfg.py.in
@@ -2,14 +2,14 @@
 
 import sys
 
-config.llvm_src_root = "@LLVM_SOURCE_DIR@"
-config.llvm_obj_root = "@LLVM_BINARY_DIR@"
-config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
-config.llvm_libs_dir = "@LLVM_LIBS_DIR@"
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
+config.llvm_libs_dir = path("@LLVM_LIBS_DIR@")
 config.llvm_build_mode = "@LLVM_BUILD_MODE@"
-config.clang_obj_root = "@CLANG_BINARY_DIR@"
+config.clang_obj_root = path("@CLANG_BINARY_DIR@")
 config.enable_shared = @ENABLE_SHARED@
-config.shlibdir = "@SHLIBDIR@"
+config.shlibdir = path("@SHLIBDIR@")
 config.target_triple = "@TARGET_TRIPLE@"
 
 # Support substitution of the tools_dir, libs_dirs, and build_mode with user
@@ -25,4 +25,5 @@
     lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
 
 # Let the main config do the real work.
-lit_config.load_config(config, "@CLANG_SOURCE_DIR@/test/Unit/lit.cfg.py")
+lit_config.load_config(
+    config, os.path.join(path("@CLANG_SOURCE_DIR@"), "test/Unit/lit.cfg.py"))
diff --git a/clang/test/lit.site.cfg.py.in b/clang/test/lit.site.cfg.py.in
--- a/clang/test/lit.site.cfg.py.in
+++ b/clang/test/lit.site.cfg.py.in
@@ -2,16 +2,16 @@
 
 import sys
 
-config.llvm_src_root = "@LLVM_SOURCE_DIR@"
-config.llvm_obj_root = "@LLVM_BINARY_DIR@"
-config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
-config.llvm_libs_dir = "@LLVM_LIBS_DIR@"
-config.llvm_shlib_dir = "@SHLIBDIR@"
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
+config.llvm_libs_dir = path("@LLVM_LIBS_DIR@")
+config.llvm_shlib_dir = path("@SHLIBDIR@")
 config.llvm_plugin_ext = "@LLVM_PLUGIN_EXT@"
-config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
-config.clang_obj_root = "@CLANG_BINARY_DIR@"
-config.clang_src_dir = "@CLANG_SOURCE_DIR@"
-config.clang_tools_dir = "@CLANG_TOOLS_DIR@"
+config.lit_tools_dir = path("@LLVM_LIT_TOOLS_DIR@")
+config.clang_obj_root = path("@CLANG_BINARY_DIR@")
+config.clang_src_dir = path("@CLANG_SOURCE_DIR@")
+config.clang_tools_dir = path("@CLANG_TOOLS_DIR@")
 config.host_triple = "@LLVM_HOST_TRIPLE@"
 config.target_triple = "@TARGET_TRIPLE@"
 config.host_cxx = "@CMAKE_CXX_COMPILER@"
@@ -50,4 +50,5 @@
     config.available_features.add('platform-linker')
 
 # Let the main config do the real work.
-lit_config.load_config(config, "@CLANG_SOURCE_DIR@/test/lit.cfg.py")
+lit_config.load_config(
+    config, os.path.join(config.clang_src_dir, "test/lit.cfg.py"))
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1396,8 +1396,14 @@
 # variables needed for the 'lit.site.cfg' files. This function bundles the
 # common variables that any Lit instance is likely to need, and custom
 # variables can be passed in.
+# The keyword PATHS is followed by a list of cmake variable names that are
+# mentioned as `path("@varname@")` in the lit.cfg.py.in file. Variables in that
+# list are treated as paths that are relative to the directory the generated
+# lit.cfg.py file is in, and the `path()` function converts the relative
+# path back to absolute form. This makes it possible to move a build directory
+# containing lit.cfg.py files from one machine to another.
 function(configure_lit_site_cfg site_in site_out)
-  cmake_parse_arguments(ARG "" "" "MAIN_CONFIG;OUTPUT_MAPPING" ${ARGN})
+  cmake_parse_arguments(ARG "" "" "MAIN_CONFIG;OUTPUT_MAPPING;PATHS" ${ARGN})
 
   if ("${ARG_MAIN_CONFIG}" STREQUAL "")
     get_filename_component(INPUT_DIR ${site_in} DIRECTORY)
@@ -1447,12 +1453,17 @@
   set(HOST_CXX "${CMAKE_CXX_COMPILER} ${CMAKE_CXX_COMPILER_ARG1}")
   set(HOST_LDFLAGS "${CMAKE_EXE_LINKER_FLAGS}")
 
-  set(LIT_SITE_CFG_IN_HEADER  "## Autogenerated from ${site_in}\n## Do not edit!")
+  set(LIT_SITE_CFG_IN_HEADER  "# Autogenerated from ${site_in}\n# Do not edit!")
+
+  string(CONCAT LIT_SITE_CFG_IN_HEADER "${LIT_SITE_CFG_IN_HEADER}\n\n"
+    "# Allow generated lit.site.cfg.py to be relocatable.\n"
+    "def path(p): return os.path.join(os.path.dirname(__file__), p) if p else ''\n"
+    )
 
   # Override config_target_triple (and the env)
   if(LLVM_TARGET_TRIPLE_ENV)
     # This is expanded into the heading.
-    string(CONCAT LIT_SITE_CFG_IN_HEADER "${LIT_SITE_CFG_IN_HEADER}\n\n"
+    string(CONCAT LIT_SITE_CFG_IN_HEADER "${LIT_SITE_CFG_IN_HEADER}"
       "import os\n"
       "target_env = \"${LLVM_TARGET_TRIPLE_ENV}\"\n"
       "config.target_triple = config.environment[target_env] = os.environ.get(target_env, \"${TARGET_TRIPLE}\")\n"
@@ -1462,7 +1473,45 @@
     set(TARGET_TRIPLE "\"+config.target_triple+\"")
   endif()
 
+  if (ARG_PATH_VALUES)
+    # Walk ARG_PATHS and collect the current value of the variables in there.
+    foreach(path ${ARG_PATHS})
+      list(APPEND ARG_PATH_VALUES "${${path}}")
+    endforeach()
+
+    # Compute paths relative to the directory containing output lit.site.cfg.py.
+    # Passing ARG_PATH_VALUES as-is to execute_process() makes cmake strip
+    # empty list entries. So escape the ;s in the list and do the splitting
+    # outselves. cmake has no relpath function, so use Python for that.
+    string(REPLACE ";" "\\;" ARG_PATH_VALUES_ESCAPED "${ARG_PATH_VALUES}")
+    get_filename_component(OUTPUT_DIR ${site_out} DIRECTORY)
+    execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c"
+      "import os, sys; sys.stdout.write(';'.join(os.path.relpath(p, sys.argv[1]) if p else '' for p in sys.argv[2].split(';')))"
+      ${OUTPUT_DIR}
+      ${ARG_PATH_VALUES_ESCAPED}
+      OUTPUT_VARIABLE ARG_PATH_VALUES_RELATIVE)
+
+    list(LENGTH ARG_PATHS len_paths)
+    list(LENGTH ARG_PATH_VALUES len_path_values)
+    list(LENGTH ARG_PATH_VALUES_RELATIVE len_path_value_rels)
+    if ((NOT ${len_paths} EQUAL ${len_path_values}) OR
+        (NOT ${len_paths} EQUAL ${path_value_len_rels}))
+      message(SEND_ERROR "PATHS lengths got confused")
+    endif()
+
+    # Transform variables mentioned in ARG_PATHS to relative paths for
+    # the configure_file() call. Variables are copied to subscopeds by cmake,
+    # so this only modifies the local copy of the variables.
+    math(EXPR arg_path_limit "${len_paths} - 1")
+    foreach(i RANGE ${arg_path_limit})
+      list(GET ARG_PATHS ${i} val1)
+      list(GET ARG_PATH_VALUES_RELATIVE ${i} val2)
+      set(${val1} ${val2})
+    endforeach()
+  endif()
+
   configure_file(${site_in} ${site_out} @ONLY)
+
   if (EXISTS "${ARG_MAIN_CONFIG}")
     set(PYTHON_STATEMENT "map_config('${ARG_MAIN_CONFIG}', '${site_out}')")
     get_property(LLVM_LIT_CONFIG_MAP GLOBAL PROPERTY LLVM_LIT_CONFIG_MAP)
diff --git a/llvm/test/CMakeLists.txt b/llvm/test/CMakeLists.txt
--- a/llvm/test/CMakeLists.txt
+++ b/llvm/test/CMakeLists.txt
@@ -22,12 +22,23 @@
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py
+  PATHS
+  "LLVM_SOURCE_DIR"
+  "LLVM_BINARY_DIR"
+  "LLVM_TOOLS_DIR"
+  "LLVM_LIBRARY_DIR"
+  "SHLIBDIR"
   )
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.py.in
   ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.cfg.py
+  PATHS
+  "LLVM_SOURCE_DIR"
+  "LLVM_BINARY_DIR"
+  "LLVM_TOOLS_DIR"
+  "SHLIBDIR"
   )
 
 # Set the depends list as a variable so that it can grow conditionally.
diff --git a/llvm/test/Unit/lit.site.cfg.py.in b/llvm/test/Unit/lit.site.cfg.py.in
--- a/llvm/test/Unit/lit.site.cfg.py.in
+++ b/llvm/test/Unit/lit.site.cfg.py.in
@@ -2,12 +2,12 @@
 
 import sys
 
-config.llvm_src_root = "@LLVM_SOURCE_DIR@"
-config.llvm_obj_root = "@LLVM_BINARY_DIR@"
-config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
 config.llvm_build_mode = "@LLVM_BUILD_MODE@"
 config.enable_shared = @ENABLE_SHARED@
-config.shlibdir = "@SHLIBDIR@"
+config.shlibdir = path("@SHLIBDIR@")
 
 # Support substitution of the tools_dir and build_mode with user parameters.
 # This is used when we can't determine the tool dir at configuration time.
@@ -20,4 +20,5 @@
     lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
 
 # Let the main config do the real work.
-lit_config.load_config(config, "@LLVM_SOURCE_DIR@/test/Unit/lit.cfg.py")
+lit_config.load_config(
+    config, os.path.join(config.llvm_src_root, "test/Unit/lit.cfg.py"))
diff --git a/llvm/test/lit.site.cfg.py.in b/llvm/test/lit.site.cfg.py.in
--- a/llvm/test/lit.site.cfg.py.in
+++ b/llvm/test/lit.site.cfg.py.in
@@ -4,14 +4,14 @@
 
 config.host_triple = "@LLVM_HOST_TRIPLE@"
 config.target_triple = "@TARGET_TRIPLE@"
-config.llvm_src_root = "@LLVM_SOURCE_DIR@"
-config.llvm_obj_root = "@LLVM_BINARY_DIR@"
-config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
-config.llvm_lib_dir = "@LLVM_LIBRARY_DIR@"
-config.llvm_shlib_dir = "@SHLIBDIR@"
+config.llvm_src_root = path("@LLVM_SOURCE_DIR@")
+config.llvm_obj_root = path("@LLVM_BINARY_DIR@")
+config.llvm_tools_dir = path("@LLVM_TOOLS_DIR@")
+config.llvm_lib_dir = path("@LLVM_LIBRARY_DIR@")
+config.llvm_shlib_dir = path("@SHLIBDIR@")
 config.llvm_shlib_ext = "@SHLIBEXT@"
 config.llvm_exe_ext = "@EXEEXT@"
-config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
+config.lit_tools_dir = path("@LLVM_LIT_TOOLS_DIR@")
 config.python_executable = "@PYTHON_EXECUTABLE@"
 config.gold_executable = "@GOLD_EXECUTABLE@"
 config.ld64_executable = "@LD64_EXECUTABLE@"
@@ -63,4 +63,5 @@
 lit.llvm.initialize(lit_config, config)
 
 # Let the main config do the real work.
-lit_config.load_config(config, "@LLVM_SOURCE_DIR@/test/lit.cfg.py")
+lit_config.load_config(
+    config, os.path.join(config.llvm_src_root, "test/lit.cfg.py"))
diff --git a/llvm/utils/gn/secondary/clang/test/BUILD.gn b/llvm/utils/gn/secondary/clang/test/BUILD.gn
--- a/llvm/utils/gn/secondary/clang/test/BUILD.gn
+++ b/llvm/utils/gn/secondary/clang/test/BUILD.gn
@@ -12,7 +12,10 @@
     input = invoker.input
     output = invoker.output
     values = [
-      "LIT_SITE_CFG_IN_HEADER=## Autogenerated from $input, do not edit",
+      # FIXME: Write relative paths for path()s.
+      "LIT_SITE_CFG_IN_HEADER=" +
+          "## Autogenerated from $input, do not edit\n\n" +
+          "def path(p): return p if p else \"\"",
       "CLANG_BINARY_DIR=" +
           rebase_path(get_label_info("//clang", "target_out_dir")),
       "CLANG_SOURCE_DIR=" + rebase_path("//clang"),
diff --git a/llvm/utils/gn/secondary/llvm/test/BUILD.gn b/llvm/utils/gn/secondary/llvm/test/BUILD.gn
--- a/llvm/utils/gn/secondary/llvm/test/BUILD.gn
+++ b/llvm/utils/gn/secondary/llvm/test/BUILD.gn
@@ -14,7 +14,10 @@
     input = invoker.input
     output = invoker.output
     values = [
-      "LIT_SITE_CFG_IN_HEADER=## Autogenerated from $input, do not edit",
+      # FIXME: Write relative paths for path()s.
+      "LIT_SITE_CFG_IN_HEADER=" +
+          "## Autogenerated from $input, do not edit\n\n" +
+          "def path(p): return p if p else \"\"",
       "ENABLE_SHARED=0",
       "LLVM_BINARY_DIR=" +
           rebase_path(get_label_info("//llvm", "target_out_dir")),