Can LLVM be built so that its 'lib' directory has higher priority by default?

[Ken Cunningham]

Hi,

WIth the way MacPorts builds llvm/clang, libc++ is not actually built or installed in the directory you mention. It could be -- the source is there -- but it isn't. We just use the source for the llvm build, and install the headers, but don't build or install libc++ that way.

The MacPorts-sanctioned way to do what you want to do is to use the libcxx port, and if you look at the variants, there is a "replacement" libcxx that forces a new libcxx to install in "/usr/lib/". This is presently set to be the libc++ that comes with llvm-5.0. I have personally built the libc++ that corresponds to llvm-7.01, but haven't yet pushed that through as I've been working with the libcxx port quite a bit lately and didn't want to do too many changes at once.

Certainly if I was going to do that on a current system I'd back up my libc++ first, I'd bump the libcxx port to the 7.0.1 release, and then cross my fingers :> It will almost certainly work fine, but you need a method to get the original libc++ and libc++abi back in place in case of gremlins, because if there is a gremlin, your system won't boot.

It is possible to adjust clang's library search paths, but it's not quite as easy as we thought. Ideally you should be able to pass -syslibroot, but that didn't really work out <https://trac.macports.org/ticket/57612>. Even if you did alter and SDK and use isysroot to build against a different libc++, you'd wind up using the one in /usr/lib at runtime. 

If you dig into the clang source code, you can quite easily find where the behaviour to look for libraries is handled;  there is a custom modification to clang on MacPorts where we allow it link against a different libstdc++ by passing in an added stdlib Marcus made up. So it would be possible, I believe, to set up a clang modification to use another custom stdlib (ie -stdlib=myspeciallibc++) that uses directories and headers you choose. It would be a slightly "advanced" project to do that, but do-able, I believe, in a day or three (or less, if you eat this stuff for breakfast). 

See <https://github.com/macports/macports-ports/blob/master/lang/llvm-7.0/Portfile> and <https://github.com/macports/macports-ports/blob/master/lang/llvm-7.0/files/9001-macports-libstdcxx.diff> for a clue as to how someone might proceed if they wanted to try that.

Maybe there are easier ways I haven't thought of here -- this is just what came to mind between events at work.

Hope this at least gets you started !

Ken



On 2019-01-21, at 1:54 PM, Ces VLC wrote:

> Hi,
> 
> I'm building LLVM 7.0.1 manually (well, following the MacPorts port file, but doing it manually), and I'm aiming for getting a clang/clang++ that by default looks for libs in the LLVMHOME/lib directory (where LLVMHOME is the installation prefix used when building LLVM) before looking at /usr/lib and at /usr/local/lib
> 
> The main purpose is that I want that the LLVM libc++ is always used by default, instead of the MacOS one.
> 
> Of course I know I can add LLVMHOME/lib as a -L option passed to clang++, but I want it to be a default rather than a -L option, because some packages come with broken Makefiles that overwrite any tuning that you wish to perform through environment variables such as LDFLAGS or CXXFLAGS.
> 
> How can I build LLVM with this behavior?
> 
> Thanks a lot!
> 
> César