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Unable to specify libc++ include directory for Musl libc headers #66459

Open solidDoWant opened 1 year ago

solidDoWant commented 1 year ago

Specifying the LIBCXX_HAS_MUSL_LIBC flag requires Musl libc headers to be installed and resolvable by the compiler when building libc++. When building libc++ as a runtime there is not currently a way to configure CMake to include a specific directory in the header search path. As a result, it does not appear to be possible to build a toolchain with libc++ support for Musl libc when the headers for the specific version of Musl libc needed are not installed onto the build host. For example it does not seem to be possible to use a build system with triplet x86_64-pc-linux-gnu to build toolchain that targets x86_64-pc-linux-musl without first installing the Musl headers onto the build system.

I have attempted to get around this issue by setting both CMAKE_C_FLAGS and CMAKE_CXX_FLAGS to -isystem/path/to/musl/headers, but this value does not get passed down to the runtimes because the runtimes are loaded in CMake via an add_subdirectory directive here which does not preserve variable scope. The COMPILER_RT_CMAKE_CXX_FLAGS flag is passed down to the runtime cmake files, but the generated build files do not include the flag when invoking the compiler.

ldionne commented 6 days ago

The proper way to do this IMO is to provide a CMake interface target representing the C library that we can link against, and to allow replacing that. Similarly to how we allow the C++ ABI library to be customized in https://github.com/llvm/llvm-project/blob/d617371375cba53e44eccedbac976f2b74df4f23/libcxx/cmake/Modules/HandleLibCXXABI.cmake