[llvm-dev] LLVM cross-compilation cmake issues

[Shoaib Meenai via llvm-dev]

I'm an idiot and sent to llvm-commits instead of llvm-dev. Fixing.

On 10/17/17, 5:09 PM, "llvm-commits on behalf of Shoaib Meenai via llvm-commits" <llvm-commits-bounces at lists.llvm.org on behalf of llvm-commits at lists.llvm.org> wrote:

    Hi all (CC beanz for cmake advice),
    
    
    
    I'm running into a cmake problem when I try to cross-compile a Windows-hosted
    
    toolchain (building on Linux). I'm using clang-cl as my cross-compiler, and I
    
    set up the compiler paths in a cmake toolchain file. Unfortunately, as part of
    
    setting up the compiler, cmake sets the environment variables ASM, CC, CXX,
    
    etc. to the paths it found. It's easy enough to reproduce this; just configure
    
    the following CMakeLists.txt:
    
    
    
    message(STATUS "CC is $ENV{CC}")
    
    project(null C)
    
    message(STATUS "CC is $ENV{CC}")
    
    
    
    The first printout should be empty (assuming you don't have CC set in your
    
    environment already, of course), and the second value should be the full path
    
    the compiler found by cmake. Note that this will only occur on the initial
    
    configure; subsequent configures should have the compiler setup cached, and
    
    will therefore not set the environment variable. I've confirmed this behavior
    
    on cmake 3.5, 3.6, and 3.9, so I'm fairly sure it's universal.
    
    
    
    This causes problems for me because when I'm running the initial cmake
    
    configure for my cross-compile, ASM, CC, and CXX get set to the values for my
    
    cross-compiler (clang-cl). LLVM's build then launches the configure for the
    
    native portions of the build (TableGen, etc.), but this configure is launched
    
    with the aforementioned environment variables, and cmake's compiler detection
    
    picks up on those environment variables, so it then tries to compile the
    
    native portions with my cross-compiler, which goes about as well as you'd
    
    expect.
    
    
    
    My current workaround is to simply unset these environment variables before
    
    running the native configure step:
    
    
    
    --- a/cmake/modules/CrossCompile.cmake
    
    +++ b/cmake/modules/CrossCompile.cmake
    
    @@ -45,6 +45,9 @@ function(llvm_create_cross_target_internal target_name toolchain buildtype)
    
              # Propagate LLVM_EXTERNAL_CLANG_SOURCE_DIR so that clang-tblgen can be built
    
              set(external_clang_dir "-DLLVM_EXTERNAL_CLANG_SOURCE_DIR=${LLVM_EXTERNAL_CLANG_SOURCE_DIR}")
    
            endif()
    
    +    unset(ENV{ASM})
    
    +    unset(ENV{CC})
    
    +    unset(ENV{CXX})
    
         execute_process(COMMAND ${CMAKE_COMMAND} ${build_type_flags}
    
             -G "${CMAKE_GENERATOR}" -DLLVM_TARGETS_TO_BUILD=${LLVM_TARGETS_TO_BUILD}
    
             ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
    
    
    
    It would be more proper to save the values of these environment variables at
    
    the start of the configure (before any project command is run) and restore
    
    those values before starting the native configure, of course. This still feels
    
    pretty ugly though, and I'm sure I'm not the first person to run into a
    
    similar issue. What would be the best way to fix this?
    
    
    
    Thanks,
    
    Shoaib
    
    
    
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