llvm-config --ldflags incorrect when llvm is installed inside build directory

[DavidSpickett]

Originally reported in https://discourse.llvm.org/t/compiled-llm-config-executable-programs-do-not-correctly-point-to-the-installation-directory/87766.

Build llvm as follows:

$ mkdir build-llvm-config && cd llvm-config
$ make ../llvm-project/llvm -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/home/david.spickett/build-llvm-config/llvm.install -G Ninja -DLLVM_CCACHE_BUILD=ON && ninja && ninja install

Note that the install directory is inside the build directory.

This causes the llvm-config in the installed directory to report an incorrect library path:

~/build-llvm-config$ ./llvm.install/bin/llvm-config --ldflags
-L/home/david.spickett/build-llvm-config/Release/lib

The one in the build directory's bin points to the build's library dir as expected:

~/build-llvm-config$ ./bin/llvm-config --ldflags
-L/home/david.spickett/build-llvm-config/lib 

I think llvm-config has autodetected as InDevelopmentTree true (which it is, sort of) and DevelopmentTreeLayout of CMakeBuildModeStyle. This causes it to put build_mode in the path which is read from LLVM_BUILDMODE which would be Release.

[DavidSpickett]

Found what CMakeBuildModeStyle is looking for and why it expects to find the build type in the path. It's for multi-config generators like https://cmake.org/cmake/help/latest/generator/Ninja%20Multi-Config.html.

build-llvm-config$ rm -rf CMakeCache.txt CMakeFiles/ && cmake ../llvm-project/llvm -DCMAKE_BUILD_TYPE=Release -G "Ninja Multi-Config" -DLLVM_CCACHE_BUILD=ON
build-llvm-config$ ninja -f build-Release.ninja
build-llvm-config$ ./Release/bin/llvm-config --ldflags
-L/home/david.spickett/build-llvm-config/Release/lib 
build-llvm-config$ ./Release/bin/llvm-config --obj-root
/home/david.spickett/build-llvm-config

So when you have installed into the build directory, object root is the build directory and is one level up from where the installed llvm-config is, so it looks like this multi-config layout.

Could we look only for known build types? No. CMake lets you define anything as a build type and though llvm by default complains, you can set LLVM_ADDITIONAL_BUILD_TYPES to bypass that.

So in theory you could have a build type "llvm_install" that you use for building llvm in a way suitable to be installed.

Will think about other clues we might be able to look at, but trending towards won't fix on this.