llvm (all versions): add missing versioned libLLVM symlink; fix Mojave sysroot

[Bo98]

llvm-config expects libLLVM-9.dylib (or the equivalent for LLVM 6-8) in order to properly function. It is however missing and I believe this to be the cause of #47142.

Since I was here revision bumping to include this fix, I've also taken the opportunity to fix the default sysroot for Mojave. We didn't get any reported issues about it AFAIK (only for GCC) - but now the behavior matches that of Xcode's LLVM build.

[fxcoudert]

If llvm-config expects this, why isn't llvm installing the symlink itself? Something seems not right here… we shouldn't have to symlink manually, the build process should install all necessary files.

[Bo98]

That question has been asked before but unanswered. It apparently does work on Linux.

The issue was first picked up in a comment of this code review however it was only noticed a few months later. A bug was eventually reported but however did not get a response.

It is documented as supposed to still work - though those docs haven't been updated in years and still mention LLVM 2.7.

[fxcoudert]

The relevant cmake file is there: https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-shlib/CMakeLists.txt
I'm not sure what's making it work on Linux but not on Darwin. But I don't like to hardcode symlinks that upstream doesn't provide.

[Bo98]

There's an unexplained NOT APPLE in the symlink part of their CMake script (introduced in the above code review - but I think the prior code didn't do it either for CMake) but llvm-config explicitly declares in code for Darwin that it should be there. One or the other is wrong.

I believe this did not affect their old, and long gone, autoconf system.

[Bo98]

I can see if it's third time lucky and reraise the question upstream if you want.

[fxcoudert]

Please do. If we add hack after hack in our formulas, they will only get unmaintainable. And the root issue is never fixed.

To some extent, if LLVM ships a compiler with breakage in some corner cases, it's their problem and we don't want to patch every little issue.

[fxcoudert]

I'm gonna close this: it is for LLVM to fix, it's clearly a bug in their build system.

[Bo98]

If you want an update: nobody responded to my query on the mailing list. I think I’ve exhausted all options bar submitting a patch to them which I’ll do when the time allows.