[llvm-dev] llvm-lit and cmake and out-of-tree builds.

[Stephen Neuendorffer via llvm-dev]

 (Abandon all hope, ye who enter here?)

I've been looking at better support for out-of-tree dialects in mlir.  One
of the natural things that we'd like to do is have llvm-lit tests for the
dialects.  Unfortunately,  attempting to support seems non-trivial because
there's a huge amount of configuration code duplicated between
clang/libcxx/lld/libunwind to retrieve information in out-of-tree builds.
This code is often slightly different and large amounts of the code is
deprecated (1) related to using llvm-config. It's also very complicated
since it has to deal with many cases (including using build areas, install
areas, windows, pulling lit.py from some random source tree, and packaged
lit installs to name a few).

It seems to me that the right thing to do is to pass this information from
LLVMConfig.cmake so that it is not rebuilt in every subproject (and in fact
a simple patch to do this solves *my* problem (2)), however I fear adding
yet another mechanism to this large amount of legacy.   Are there thoughts
on how much this is ready for general refactoring?  I fear that making any
progress in this is going to take significant input from a number of people
across different sub-projects which certainly have different use cases.  Is
it time to start simplifying this code by removing use of llvm-config and
relying solely on LLVMConfig.cmake?

1) https://reviews.llvm.org/D51714
2) https://reviews.llvm.org/D77110

Steve
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