Out-of-tree clang and flang can't find compiler-rt

[DavidTruby]

If either clang or flang are built out-of-tree against an already compiled LLVM, they are unable to find compiler-rt when linking. This is due to the path to compiler-rt being hard-coded to the same install tree as the clang driver, meaning that if compiler-rt is installed elsewhere it doesn't get picked up.

[llvmbot]

@llvm/issue-subscribers-flang-driver

Author: David Truby (DavidTruby)

If either clang or flang are built out-of-tree against an already compiled LLVM, they are unable to find compiler-rt when linking. This is due to the path to compiler-rt being hard-coded to the same install tree as the clang driver, meaning that if compiler-rt is installed elsewhere it doesn't get picked up.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: David Truby (DavidTruby)

If either clang or flang are built out-of-tree against an already compiled LLVM, they are unable to find compiler-rt when linking. This is due to the path to compiler-rt being hard-coded to the same install tree as the clang driver, meaning that if compiler-rt is installed elsewhere it doesn't get picked up.

[DavidTruby]

This is causing the flang-aarch64-libcxx and flang-aarch64-out-of-tree bots to fail as flang on AArch64 always links to compiler-rt for a couple of symbols missing in libgcc, however this is not a flang specific issue and an out-of-tree clang configured with -DCLANG_DEFAULT_RTLIB=compiler-rt or using -rtlib=compiler-rt will also fail. We had some discussion in llvm/llvm-zorg#482 but I wanted to open a ticket here for more visibility in case anyone has any ideas what we can do about this.