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macOS: Speed up rpath install name dir lookup with a cache

Lukas Böger requested to merge lubgr/cmake:cache-macosx-rpath-install-namedir into master

This MR provides a speedup for larger MacOS projects by introducing a new cache in cmGeneratorTarget. The issue has been described in #20602 (closed).

In particular, the lookup table is a nested hash map with the configuration (std::string) as the primary and the cmTarget* pointer the secondary key. While it would be nicer to have a single std::pair<std::string, cmTarget*> key, as far as I can see there is no project-wide utility to properly combine hashes (similar to boost::hash_combine), so I chose the nested approach. It has the desired affect, and while I couldn't reproduce the same speedup as described in the issue, my test runs showed a worthwile 1.1x speedup for one large project under consideration.

Here's a screenshot of the profile of the cmake invocation in an already configured build directory (using Apple's "Instruments" frontend to dtrace, i.e. this is from the usual stacktrace samples). The rectangle highlights the function in question with 8.5% of the total time spent (this is without the patch this MR provides. With this patch, the function drops below 0.5%.

cmake-macosx-rpath-install-name-dir

Fixes: #20602 (closed)
Topic-rename: macos-cache-rpath-install-namedir

Edited by Brad King

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