[llvm-dev] Failed to configure LLVM for use with Musl

[Dmitry Golovin via llvm-dev]

I want to build LLVM-based toolchain with Musl, I have LLVM sources with clang and lld (under `tools` directory) and libunwind, compiler-rt, libcxx and libcxxabi (under `projects` directory). All are the latest versions cloned from GitHub mirror, branch `master`.

I'm trying to configure with the following options:
    CLANG_DEFAULT_CXX_STDLIB = libc++
    CLANG_DEFAULT_RTLIB = compiler-rt
    LIBCXX_CXX_ABI = libcxxabi
    LLVM_DEFAULT_TARGET_TRIPLE = x86_64-pc-linux-musl
    LIBCXXABI_TARGET_TRIPLE = x86_64-pc-linux-musl
    LIBUNWIND_TARGET_TRIPLE = x86_64-pc-linux-musl
    DEFAULT_SYSROOT = /path/to/musl
    GCC_INSTALL_PREFIX = /path/to/gcc-stub
    LLVM_TARGETS_TO_BUILD = X86
    LIBCXXABI_USE_COMPILER_RT = ON
    LIBCXXABI_USE_LLVM_UNWINDER = ON
    LIBCXX_HAS_MUSL_LIBC = ON
    LIBCXX_USE_COMPILER_RT = ON
    LLVM_ENABLE_LIBCXX = ON
    LLVM_ENABLE_LLD = ON

When I try to configure, I get the following error messages:

 CMake Warning at cmake/modules/HandleLLVMStdlib.cmake:24 (message):
   Can't specify libc++ with '-stdlib='
 Call Stack (most recent call first):
   cmake/config-ix.cmake:15 (include)
   CMakeLists.txt:566 (include)

 CMake Error at projects/libunwind/src/CMakeLists.txt:70 (message):
   Compiler doesn't support generation of unwind tables if exception support
   is disabled.  Building libunwind DSO with runtime dependency on C++ ABI
   library is not supported.

I'm probably doing something wrong, but I can't figure out what exactly. If I omit some of the flags, I get different errors, but I never get successful configuration.

I'm on Ubuntu 16.04 (if it is important) and have not yet tried on other machines, but I'm pretty sure it is possible to reproduce it.

Regards,
Dmitry Golovin