Starting with LLVM 8, clang does no longer use llvm-config to detect the
LLVM installation: https://github.com/llvm/llvm-project/commit/e4faa5c7986b7
Consequently, there is no point passing LLVM_CONFIG_PATH (in fact the
variable is unused currently).
When using GNU binutils, clang passes the LLVMgold.so plugin to the
linker for certain operations that require special support in the linker
like doing link time optimization (LTO). When passing the plugin to the
linker's command line, clang assumes that llvm and itself are installed
in the same prefix and thus `/path/to/clang/bin/../lib/LLVMgold.so` is
the plugin.
Since we install clang and llvm to separate store paths, this assumption
does not hold. When clang-unwrapped only had a single output, we worked
around this issue by symlinking `$out/lib/LLVMgold.so` to
`${llvm}/lib/LLVMgold.so`. However since we split all llvm packages into
multiple outputs clang's `$out` no longer has a lib directory and clang
can't discover clangs lib output on its own. As a result LTO was broken.
Instead of introducing yet another hack and having a symlink to
LLVMgold.so in `$out/lib` (despite having `$lib/lib` as well), we patch
clang to use a hard coded path to `${libllvm.lib}/lib` for discovering
`LLVMgold.so`.
Resolves#123361.
Also begin to start work on cross compilation, though that will have to
be finished later.
The patches are based on the first version of
https://reviews.llvm.org/D99484. It's very annoying to do the
back-porting but the review has uncovered nothing super major so I'm
fine sticking with what I've got.
Beyond making the outputs work, I also strove to re-sync the packages,
as they have been drifting pointlessly apart for some time.
----
Other misc notes, highly incomplete
- lvm-config-native and llvm-config are put in `dev` because they are
tools just for build time.
- Clang no longer has an lld dep. That was introduced in
db29857eb3, but if clang needs help
finding lld when it is used we should just pass it flags / put in the
resource dir. Providing it at build time increases critical path
length for no good reason.
----
A note on `nativeCC`:
`stdenv` takes tools from the previous stage, so:
1. `pkgsBuildBuild`: `(?1, x, x)`
2. `pkgsBuildBuild.stdenv.cc`: `(?0, ?1, x)`
while:
1. `pkgsBuildBuild`: `(?1, x, x)`
2. `pkgsBuildBuild.targetPackages`: `(x, x, ?2)`
3. `pkgsBuildBuild.targetPackages.stdenv.cc`: `(?1, x, x)`
