clang effectively ignores the LIBRARY_PATH env. variable

[RJVB (René Bertin)]

Clang has a subtly different way than GCC of handling the LIBRARY_PATH env. variable which can have significant implications for libraries that are present on the default (system) search path as well as in $prefix.

With GCC, LIBRARY_PATH=${prefix}/lib will translate into a link editor command (collect2) that has -L${prefix}/lib *BEFORE* all -lfoo libspecs. Clang translates the variable to the same -L expression but puts this *AFTER* all -lfoo libspecs it was invoked with. In other words, clang applies LIBRARY_PATH only to libraries that are added automatically:

> env LIBRARY_PATH=/opt/local/lib gcc -v test.o -lA -lB
...
collect2 [...] test.o -L/opt/local/lib -lA -lB [standard libraries] [...]

vs.

> env LIBRARY_PATH=/opt/local/lib clang-mp-4.0 -v test.o -lA -lB
...
dyld [...] test.o -lA -lB -L/opt/local/lib [standard libraries] [...]

I'll admit that I discovered this (= it bit me) on Linux, and even there only with cmake-based projects (recent CMake versions will rewrite absolute libspecs /opt/local/lib/libfoo.dylib into the shorthand libspecs -lfoo if it thinks the library ought to be found on the search path).

Still, I think it's a bug, or at least something that would be good to set straight in a MacPorts context. My query about it on the clang ML remains unanswered after 3 weeks so I'm signalling the issue here. I'll leave it up to port maintainers to decide if and how they'll address it here or upstream.

[jeremyhu (Jeremy Huddleston Sequoia)]

This isn't something that I would want to change in MacPorts. We should discuss this with the larger community. Can you file a bug at llvm.org and reference it from here?

[RJVB (René Bertin)]

I was hoping a port maintainer would already have an LLVM bugzilla account and something of a track record on it. The (even) larger community on the ML hasn't been showing any interest in discussing this at all. I did look into filing a ticket before I filed one here but wasn't myself particularly keen on following the account creation procedure (ask nicely and then wait until someone creates the account).

[RJVB (René Bertin)]

Actually, it turns out this is a longstanding bug and someone even wrote a patch:

​https://bugs.llvm.org/show_bug.cgi?id=16786

[kencu (Ken)]

<​https://reviews.llvm.org/D65880>

[kencu (Ken)]

rjvb -- thanks