[cmake] Support custom package install paths

Firstly, we we make an additional GNUInstallDirs-style variable. With
NixOS, for example, this is crucial as we want those to go in
`${dev}/lib/cmake` not `${out}/lib/cmake` as that would a cmake subdir
of the "regular" libdir, which is installed even when no one needs to do
any development.

Secondly, we make *Config.cmake robust to absolute package install
paths. We for NixOS will in fact be passing them absolute paths to make
the `${dev}` vs `${out}` distinction mentioned above, and the
GNUInstallDirs-style variables are suposed to support absolute paths in
general so it's good practice besides the NixOS use-case.

Thirdly, we make `${project}_INSTALL_PACKAGE_DIR` CACHE PATHs like other
install dirs are.

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D117973

2 files changed, 55 insertions, 10 deletions

diff --git a/cmake/Modules/FindPrefixFromConfig.cmake b/cmake/Modules/FindPrefixFromConfig.cmake
index aa9fb0d03413..c583a65e6738 100644
--- a/cmake/Modules/FindPrefixFromConfig.cmake
+++ b/cmake/Modules/FindPrefixFromConfig.cmake
@@ -26,16 +26,28 @@
 #   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}")
+    # Because the path is absolute, we don't care about `path_to_leave`
+    # because we can just "jump" to the absolute path rather than work
+    # our way there relatively.
+    set(config_code
+      "# Installation prefix is fixed absolute path"
+      "set(${prefix_var} \"${CMAKE_INSTALL_PREFIX}\"")
+  else()
+    # `path_to_leave` is relative. Relative to what? The install prefix.
+    # We therefore go up enough parent directories to get back to the
+    # install prefix, and avoid hard-coding any absolute paths.
+    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
index 000000000000..e4a058e68f4f
--- /dev/null
+++ b/cmake/Modules/GNUInstallPackageDir.cmake
@@ -0,0 +1,33 @@
+# Mimick `GNUInstallDirs` for one more install directory, the one where
+# project's installed cmake subdirs go.
+
+# These functions are internal functions vendored in from GNUInstallDirs (with
+# new names), so we don't depend on unstable implementation details. They are
+# also simplified to only handle the cases we need.
+#
+# 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)")