diff --git a/clang/cmake/modules/CMakeLists.txt b/clang/cmake/modules/CMakeLists.txt
--- a/clang/cmake/modules/CMakeLists.txt
+++ b/clang/cmake/modules/CMakeLists.txt
@@ -1,3 +1,4 @@
+include(GNUInstallPackageDir)
 include(ExtendPath)
 include(LLVMDistributionSupport)
 include(FindPrefixFromConfig)
@@ -5,12 +6,16 @@
 # Generate a list of CMake library targets so that other CMake projects can
 # link against them. LLVM calls its version of this file LLVMExports.cmake, but
 # the usual CMake convention seems to be ${Project}Targets.cmake.
-set(CLANG_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/clang)
-set(clang_cmake_builddir "${CMAKE_BINARY_DIR}/${CLANG_INSTALL_PACKAGE_DIR}")
+set(CLANG_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/clang" CACHE STRING
+  "Path for CMake subdirectory for Clang (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/clang')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(clang_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/clang")
 
 # Keep this in sync with llvm/cmake/CMakeLists.txt!
-set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
-set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
+  "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 
 get_property(CLANG_EXPORTS GLOBAL PROPERTY CLANG_EXPORTS)
 export(TARGETS ${CLANG_EXPORTS} FILE ${clang_cmake_builddir}/ClangTargets.cmake)
@@ -40,8 +45,8 @@
 
 # Generate ClangConfig.cmake for the install tree.
 find_prefix_from_config(CLANG_CONFIG_CODE CLANG_INSTALL_PREFIX "${CLANG_INSTALL_PACKAGE_DIR}")
-set(CLANG_CONFIG_CMAKE_DIR "\${CLANG_INSTALL_PREFIX}/${CLANG_INSTALL_PACKAGE_DIR}")
-set(CLANG_CONFIG_LLVM_CMAKE_DIR "\${CLANG_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")
+extend_path(CLANG_CONFIG_CMAKE_DIR "\${CLANG_INSTALL_PREFIX}" "${CLANG_INSTALL_PACKAGE_DIR}")
+extend_path(CLANG_CONFIG_LLVM_CMAKE_DIR "\${CLANG_INSTALL_PREFIX}" "${LLVM_INSTALL_PACKAGE_DIR}")
 get_config_exports_includes(Clang CLANG_CONFIG_INCLUDE_EXPORTS)
 extend_path(base_includedir "\${CLANG_INSTALL_PREFIX}" "${CMAKE_INSTALL_INCLUDEDIR}")
 set(CLANG_CONFIG_INCLUDE_DIRS
diff --git a/cmake/Modules/FindPrefixFromConfig.cmake b/cmake/Modules/FindPrefixFromConfig.cmake
--- a/cmake/Modules/FindPrefixFromConfig.cmake
+++ b/cmake/Modules/FindPrefixFromConfig.cmake
@@ -26,16 +26,22 @@
 #   Path from the prefix to the config file, a relative path which we wish to
 #   go up and out from to find the prefix directory.
 function(find_prefix_from_config out_var prefix_var path_to_leave)
-  set(config_code
-    "# Compute the installation prefix from this LLVMConfig.cmake file location."
-    "get_filename_component(${prefix_var} \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
-  # Construct the proper number of get_filename_component(... PATH)
-  # calls to compute the installation prefix.
-  string(REGEX REPLACE "/" ";" _count "${path_to_leave}")
-  foreach(p ${_count})
-    list(APPEND config_code
-      "get_filename_component(${prefix_var} \"\${${prefix_var}}\" PATH)")
-  endforeach(p)
+  if(IS_ABSOLUTE "${path_to_leave}")
+    set(prefix_var
+      "# Installation prefix is fixed absolute path"
+      "set(${prefix_var} \"${CMAKE_INSTALL_PREFIX}\"")
+  else()
+    set(config_code
+      "# Compute the installation prefix from this LLVMConfig.cmake file location."
+      "get_filename_component(${prefix_var} \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
+    # Construct the proper number of get_filename_component(... PATH)
+    # calls to compute the installation prefix.
+    string(REGEX REPLACE "/" ";" _count "${path_to_leave}")
+    foreach(p ${_count})
+      list(APPEND config_code
+        "get_filename_component(${prefix_var} \"\${${prefix_var}}\" PATH)")
+    endforeach(p)
+  endif()
   string(REPLACE ";" "\n" config_code "${config_code}")
   set("${out_var}" "${config_code}" PARENT_SCOPE)
 endfunction()
diff --git a/cmake/Modules/GNUInstallPackageDir.cmake b/cmake/Modules/GNUInstallPackageDir.cmake
new file mode 100644
--- /dev/null
+++ b/cmake/Modules/GNUInstallPackageDir.cmake
@@ -0,0 +1,32 @@
+# Mimmick GNUInstallDirs for one more install directory, the one where
+# project's installed cmake subdirs gos.
+
+# These functions are internal functions vendored in from GNUInstallDirs (with
+# new names) lest it changes.
+#
+# The purpose would appear to be making `CACHE PATH` vars in a way that
+# bypasses the legacy oddity that `-D<PATH>` gets canonicalized, despite
+# non-canonical `CACHE PATH`s being perfectly valid.
+
+macro(_GNUInstallPackageDir_cache_convert_to_path var description)
+  get_property(_GNUInstallPackageDir_cache_type CACHE ${var} PROPERTY TYPE)
+  if(_GNUInstallPackageDir_cache_type STREQUAL "UNINITIALIZED")
+    file(TO_CMAKE_PATH "${${var}}" _GNUInstallPackageDir_cmakepath)
+    set_property(CACHE ${var} PROPERTY TYPE PATH)
+    set_property(CACHE ${var} PROPERTY VALUE "${_GNUInstallPackageDir_cmakepath}")
+    set_property(CACHE ${var} PROPERTY HELPSTRING "${description}")
+    unset(_GNUInstallPackageDir_cmakepath)
+  endif()
+  unset(_GNUInstallPackageDir_cache_type)
+endmacro()
+
+# Create a cache variable with default for a path.
+macro(_GNUInstallPackageDir_cache_path var default description)
+  if(NOT DEFINED ${var})
+    set(${var} "${default}" CACHE PATH "${description}")
+  endif()
+  _GNUInstallPackageDir_cache_convert_to_path("${var}" "${description}")
+endmacro()
+
+_GNUInstallPackageDir_cache_path(CMAKE_INSTALL_PACKAGEDIR "lib${LLVM_LIBDIR_SUFFIX}/cmake"
+  "Directories containing installed CMake modules (lib/cmake)")
diff --git a/flang/cmake/modules/CMakeLists.txt b/flang/cmake/modules/CMakeLists.txt
--- a/flang/cmake/modules/CMakeLists.txt
+++ b/flang/cmake/modules/CMakeLists.txt
@@ -1,15 +1,20 @@
+include(GNUInstallPackageDir)
 include(ExtendPath)
 include(FindPrefixFromConfig)
 
 # Generate a list of CMake library targets so that other CMake projects can
 # link against them. LLVM calls its version of this file LLVMExports.cmake, but
 # the usual CMake convention seems to be ${Project}Targets.cmake.
-set(FLANG_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/flang)
-set(flang_cmake_builddir "${CMAKE_BINARY_DIR}/${FLANG_INSTALL_PACKAGE_DIR}")
+set(FLANG_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/flang" CACHE STRING
+  "Path for CMake subdirectory for Flang (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/flang')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(flang_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/flang")
 
 # Keep this in sync with llvm/cmake/CMakeLists.txt!
-set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
-set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
+  "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(llvm_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 
 get_property(FLANG_EXPORTS GLOBAL PROPERTY FLANG_EXPORTS)
 export(TARGETS ${FLANG_EXPORTS} FILE ${flang_cmake_builddir}/FlangTargets.cmake)
diff --git a/lld/cmake/modules/CMakeLists.txt b/lld/cmake/modules/CMakeLists.txt
--- a/lld/cmake/modules/CMakeLists.txt
+++ b/lld/cmake/modules/CMakeLists.txt
@@ -1,15 +1,20 @@
+include(GNUInstallPackageDir)
 include(ExtendPath)
 include(FindPrefixFromConfig)
 
 # Generate a list of CMake library targets so that other CMake projects can
 # link against them. LLVM calls its version of this file LLVMExports.cmake, but
 # the usual CMake convention seems to be ${Project}Targets.cmake.
-set(LLD_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/lld)
-set(lld_cmake_builddir "${CMAKE_BINARY_DIR}/${LLD_INSTALL_PACKAGE_DIR}")
+set(LLD_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/lld" CACHE STRING
+  "Path for CMake subdirectory for LLD (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/lld')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(lld_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/lld")
 
 # Keep this in sync with llvm/cmake/CMakeLists.txt!
-set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
-set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
+  "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(llvm_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 
 get_property(LLD_EXPORTS GLOBAL PROPERTY LLD_EXPORTS)
 export(TARGETS ${LLD_EXPORTS} FILE ${lld_cmake_builddir}/LLDTargets.cmake)
@@ -31,8 +36,8 @@
 
 # Generate LLDConfig.cmake for the install tree.
 find_prefix_from_config(LLD_CONFIG_CODE LLD_INSTALL_PREFIX "${LLD_INSTALL_PACKAGE_DIR}")
-set(LLD_CONFIG_CMAKE_DIR "\${LLD_INSTALL_PREFIX}/${LLD_INSTALL_PACKAGE_DIR}")
-set(LLD_CONFIG_LLVM_CMAKE_DIR "\${LLD_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")
+extend_path(LLD_CONFIG_CMAKE_DIR "\${LLD_INSTALL_PREFIX}" "${LLD_INSTALL_PACKAGE_DIR}")
+extend_path(LLD_CONFIG_LLVM_CMAKE_DIR "\${LLD_INSTALL_PREFIX}" "${LLVM_INSTALL_PACKAGE_DIR}")
 get_config_exports_includes(LLD LLD_CONFIG_INCLUDE_EXPORTS)
 extend_path(LLD_CONFIG_INCLUDE_DIRS "\${LLD_INSTALL_PREFIX}" "${CMAKE_INSTALL_INCLUDEDIR}")
 configure_file(
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
@@ -1049,8 +1049,11 @@
 
       ## Part 1: Extension header to be included whenever we need extension
       #  processing.
-      set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
-      set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+      if(NOT DEFINED LLVM_INSTALL_PACKAGE_DIR)
+          message(FATAL_ERROR "LLVM_INSTALL_PACKAGE_DIR must be defined and writable. GEN_CONFIG should only be passe when building LLVM proper.")
+      endif()
+      # LLVM_INSTALL_PACKAGE_DIR might be absolute, so don't reuse below.
+      set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
       file(WRITE
           "${llvm_cmake_builddir}/LLVMConfigExtensions.cmake"
           "set(LLVM_STATIC_EXTENSIONS ${LLVM_STATIC_EXTENSIONS})")
diff --git a/llvm/cmake/modules/CMakeLists.txt b/llvm/cmake/modules/CMakeLists.txt
--- a/llvm/cmake/modules/CMakeLists.txt
+++ b/llvm/cmake/modules/CMakeLists.txt
@@ -1,9 +1,12 @@
+include(GNUInstallPackageDir)
 include(ExtendPath)
 include(LLVMDistributionSupport)
 include(FindPrefixFromConfig)
 
-set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
-set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
+  "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(llvm_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 
 # First for users who use an installed LLVM, create the LLVMExports.cmake file.
 set(LLVM_EXPORTS_FILE ${llvm_cmake_builddir}/LLVMExports.cmake)
diff --git a/mlir/cmake/modules/CMakeLists.txt b/mlir/cmake/modules/CMakeLists.txt
--- a/mlir/cmake/modules/CMakeLists.txt
+++ b/mlir/cmake/modules/CMakeLists.txt
@@ -1,3 +1,4 @@
+include(GNUInstallPackageDir)
 include(ExtendPath)
 include(LLVMDistributionSupport)
 include(FindPrefixFromConfig)
@@ -5,12 +6,16 @@
 # Generate a list of CMake library targets so that other CMake projects can
 # link against them. LLVM calls its version of this file LLVMExports.cmake, but
 # the usual CMake convention seems to be ${Project}Targets.cmake.
-set(MLIR_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/mlir)
-set(mlir_cmake_builddir "${CMAKE_BINARY_DIR}/${MLIR_INSTALL_PACKAGE_DIR}")
+set(MLIR_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/mlir" CACHE STRING
+  "Path for CMake subdirectory for Polly (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/polly')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(mlir_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/mlir")
 
 # Keep this in sync with llvm/cmake/CMakeLists.txt!
-set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)
-set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
+set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
+  "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(llvm_cmake_builddir "${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 
 get_property(MLIR_EXPORTS GLOBAL PROPERTY MLIR_EXPORTS)
 export(TARGETS ${MLIR_EXPORTS} FILE ${mlir_cmake_builddir}/MLIRTargets.cmake)
@@ -50,8 +55,8 @@
 
 # Generate MLIRConfig.cmake for the install tree.
 find_prefix_from_config(MLIR_CONFIG_CODE MLIR_INSTALL_PREFIX "${MLIR_INSTALL_PACKAGE_DIR}")
-set(MLIR_CONFIG_CMAKE_DIR "\${MLIR_INSTALL_PREFIX}/${MLIR_INSTALL_PACKAGE_DIR}")
-set(MLIR_CONFIG_LLVM_CMAKE_DIR "\${MLIR_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")
+extend_path(MLIR_CONFIG_CMAKE_DIR "\${MLIR_INSTALL_PREFIX}" "${MLIR_INSTALL_PACKAGE_DIR}")
+extend_path(MLIR_CONFIG_LLVM_CMAKE_DIR "\${MLIR_INSTALL_PREFIX}" "${LLVM_INSTALL_PACKAGE_DIR}")
 get_config_exports_includes(MLIR MLIR_CONFIG_INCLUDE_EXPORTS)
 extend_path(base_includedir "\${MLIR_INSTALL_PREFIX}" "${CMAKE_INSTALL_INCLUDEDIR}")
 set(MLIR_CONFIG_INCLUDE_DIRS
diff --git a/polly/cmake/CMakeLists.txt b/polly/cmake/CMakeLists.txt
--- a/polly/cmake/CMakeLists.txt
+++ b/polly/cmake/CMakeLists.txt
@@ -1,10 +1,19 @@
 # Keep this in sync with llvm/cmake/CMakeLists.txt!
 
+include(GNUInstallPackageDir)
 include(ExtendPath)
 include(FindPrefixFromConfig)
 
-set(LLVM_INSTALL_PACKAGE_DIR "lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
-set(POLLY_INSTALL_PACKAGE_DIR "lib${LLVM_LIBDIR_SUFFIX}/cmake/polly")
+set(POLLY_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/polly" CACHE STRING
+  "Path for CMake subdirectory for Polly (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/polly')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(polly_cmake_builddir "${POLLY_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/polly")
+
+set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
+  "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
+# CMAKE_INSTALL_PACKAGEDIR might be absolute, so don't reuse below.
+set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
+
 if (CMAKE_CONFIGURATION_TYPES)
   set(POLLY_EXPORTS_FILE_NAME "PollyExports-$<LOWER_CASE:$<CONFIG>>.cmake")
 else()
@@ -28,9 +37,6 @@
   set(POLLY_CONFIG_TARGET_${tgt}_TYPE ${tgt_type})
 endforeach()
 
-set(llvm_cmake_builddir "${LLVM_BINARY_DIR}/${LLVM_INSTALL_PACKAGE_DIR}")
-set(POLLY_CONFIG_LLVM_CMAKE_DIR "${llvm_cmake_builddir}")
-
 # generate the import code for bundled/undbundled libisl versions
 if (NOT POLLY_BUNDLED_ISL)
   get_property(incl TARGET ISL PROPERTY INTERFACE_INCLUDE_DIRECTORIES)
@@ -50,7 +56,8 @@
 
 # Generate PollyConfig.cmake for the build tree.
 set(POLLY_CONFIG_CODE "")
-set(POLLY_CONFIG_CMAKE_DIR "${CMAKE_BINARY_DIR}/${POLLY_INSTALL_PACKAGE_DIR}")
+set(POLLY_CONFIG_LLVM_CMAKE_DIR "${llvm_cmake_builddir}")
+set(POLLY_CONFIG_CMAKE_DIR "${polly_cmake_builddir}")
 set(POLLY_CONFIG_INCLUDE_DIRS
   ${POLLY_SOURCE_DIR}/include
   ${ISL_INCLUDE_DIRS}
@@ -73,11 +80,11 @@
 # the imported locations
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/PollyConfig.cmake.in
-  ${POLLY_CONFIG_CMAKE_DIR}/PollyConfig.cmake
+  ${polly_cmake_builddir}/PollyConfig.cmake
   @ONLY)
 
 file(GENERATE
-  OUTPUT ${POLLY_CONFIG_CMAKE_DIR}/${POLLY_EXPORTS_FILE_NAME}
+  OUTPUT ${polly_cmake_builddir}/${POLLY_EXPORTS_FILE_NAME}
   CONTENT "${POLLY_EXPORTS}")