[llvm] [cmake] Resolve symlink when finding install prefix (PR #124743)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/124743

When determining the install prefix in LLVMConfig.cmake etc resolve symlinks in CMAKE_CURRENT_LIST_FILE first. The motivation for this is to support symlinks like `/usr/lib64/cmake/llvm` to `/usr/lib64/llvm19/lib/cmake/llvm`. This only works correctly if the paths are relative to the resolved symlink.

It's worth noting that this *mostly* already works out of the box, because cmake automatically does the symlink resolution when the library is found via CMAKE_PREFIX_PATH. It just doesn't happen when it's found via the default prefix path.

>From 06774eb8a7dc0bc36b59e53310c7f5b5d89f6c29 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Tue, 28 Jan 2025 12:31:49 +0100
Subject: [PATCH] [cmake] Resolve symlink when finding install prefix

When determining the install prefix in LLVMConfig.cmake etc resolve
symlinks in CMAKE_CURRENT_LIST_FILE first. The motivation for this
is to support symlinks like `/usr/lib64/cmake/llvm` to
`/usr/lib64/llvm19/lib/cmake/llvm`. This only works correctly if
the paths are relative to the resolved symlink.

It's worth noting that this *mostly* already works out of the box,
because cmake automatically does the symlink resolution when the
library is found via CMAKE_PREFIX_PATH. It just doesn't happen
when it's found via the default prefix path.
---
 cmake/Modules/FindPrefixFromConfig.cmake | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/cmake/Modules/FindPrefixFromConfig.cmake b/cmake/Modules/FindPrefixFromConfig.cmake
index 22211e4b72f283..3daff607ff847e 100644
--- a/cmake/Modules/FindPrefixFromConfig.cmake
+++ b/cmake/Modules/FindPrefixFromConfig.cmake
@@ -39,10 +39,10 @@ function(find_prefix_from_config out_var prefix_var path_to_leave)
     # 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(${prefix_var} \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
+      "get_filename_component(${prefix_var} \"\${CMAKE_CURRENT_LIST_FILE}\" REALPATH)")
     # Construct the proper number of get_filename_component(... PATH)
     # calls to compute the installation prefix.
-    string(REGEX REPLACE "/" ";" _count "${path_to_leave}")
+    string(REGEX REPLACE "/" ";" _count "${path_to_leave}/plus_one")
     foreach(p ${_count})
       list(APPEND config_code
         "get_filename_component(${prefix_var} \"\${${prefix_var}}\" PATH)")