[CMake] Correctly handle LLVM_ENABLE_RUNTIMES in targets

[petrhosek]

When a target sets LLVM_ENABLE_RUNTIMES, we should only generate proxy targets for those runtimes rather than using the global list which may contain runtimes that are not supported by that particular target.

[smeenai]

LGTM

[JDevlieghere]

Looks like this broke the cxx target: https://green.lab.llvm.org/green/view/LLDB/job/as-lldb-cmake/8459/

$ cmake /Users/jonas/llvm/llvm-project/llvm \
                                       -G Ninja \
                                       -DCMAKE_BUILD_TYPE='RelWithDebInfo' \
                                       -DLLVM_ENABLE_ASSERTIONS:BOOL=ON \
                                       -DLLVM_ENABLE_PROJECTS='llvm;clang' \
                                       -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi'
...
$ ninja cxx
ninja: error: unknown target 'cxx', did you mean 'lli'?

[ldionne]

@petrhosek I kind of missed that change, but can you give an example of what this change is doing? What is an example of a "proxy" target?

[JDevlieghere]

@ldionne pointed me at the runtimes target (which seems like the right thing to do regardless of this change) so I've changed the lldb build to depend on that (dfa3570).

[petrhosek]

@ldionne I ran into this issue when trying to build LLVM libc in the bootstrapping build. I need to include libc in LLVM_ENABLE_RUNTIMES to ensure that the host tools used by libc—that is libc-hdrgen—are built, but I only want build libc for theriscv32-unknown-elf target so I exclude it from all RUNTIMES_<target>_LLVM_ENABLE_RUNTIMES (with the exception of RUNTIMES_riscv32-unknown-elf_LLVM_ENABLE_RUNTIMES).

The issue is that the logic in runtimes prior to this patch would always iterate over the global LLVM_ENABLE_RUNTIMES and set up the proxy targets for the child builds. Concretely, in my example libc would depend on libc-x86_64-unknown-linux-gnu which is a proxy for the libc target in the runtimes-x86_64-unknown-linux-gnu-bins build, except that there's no libc target in that build because RUNTIMES_x86_64-unknown-linux-gnu_LLVM_ENABLE_RUNTIMES doesn't include libc and that leads to a build failure.

When we first implemented the bootstrapping (aka runtimes build), we only supported the global LLVM_ENABLE_RUNTIMES as the assumption was that every target would build the same set of runtimes, which is not the case in practice so we introduced support for RUNTIMES_<target>_LLVM_ENABLE_RUNTIMES but we never really considered interactions between the two which is what this issue is about.

@JDevlieghere sorry about the breakage, that was unintentional. I'll try to reproduce your failure locally and see if it's a problem with this change.

[estewart08]

In response to the issue seen by @JDevlieghere where the cxx and cxxabi targets are not present:

I believe entry needs changed to name for the SUBSTRING on line 177.

When using -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi', ${name} is being returned from the function instead of the SUBSTRING ${entry}.

With your original patch the targets remain libcxx and libcxxabi.

[petrhosek]

Thanks for catching that, I fixed it in fb619b3.