[CMake] Factor out config prefix finding logic

See the docs in the new function for details.

 I think I found every instance of this copy pasted code. Polly could
 also use it, but currently does something different, so I will save the
 behavior change for a future revision.

We get the shared, non-installed CMake modules following the pattern
established in D116472.

It might be good to have LLD and Flang also use this, but that would be
a functional change and so I leave it as future work.

Reviewed By: beanz, lebedev.ri

Differential Revision: https://reviews.llvm.org/D116521

1 files changed, 41 insertions, 0 deletions

diff --git a/cmake/Modules/FindPrefixFromConfig.cmake b/cmake/Modules/FindPrefixFromConfig.cmake
new file mode 100644
index 000000000000..aa9fb0d03413
--- /dev/null
+++ b/cmake/Modules/FindPrefixFromConfig.cmake
@@ -0,0 +1,41 @@
+# Find the prefix from the `*Config.cmake` file being generated.
+#
+# When generating an installed `*Config.cmake` file, we often want to be able
+# to refer to the ancestor directory which contains all the installed files.
+#
+# We want to do this without baking in an absolute path when the config file is
+# generated, in order to allow for a "relocatable" binary distribution that
+# doesn't need to know what path it ends up being installed at when it is
+# built.
+#
+# The solution that we know the relative path that the config file will be at
+# within that prefix, like `"${prefix_var}/lib/cmake/${project}"`, so we count
+# the number of components in that path to figure out how many parent dirs we
+# need to traverse from the location of the config file to get to the prefix
+# dir.
+#
+# out_var:
+#   variable to set the "return value" of the function, which is the code to
+#   include in the config file under construction.
+#
+# prefix_var:
+#   Name of the variable to define in the returned code (not directory for the
+#   faller!) that will contain the prefix path.
+#
+# path_to_leave:
+#   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} \"\${${prefix_var}}\" PATH)")
+  endforeach(p)
+  string(REPLACE ";" "\n" config_code "${config_code}")
+  set("${out_var}" "${config_code}" PARENT_SCOPE)
+endfunction()