[llvm-dev] LLVM cross-compilation cmake issues

[Pirama Arumuga Nainar via llvm-dev]

CROSS_TOOLCHAIN_FLAGS_NATIVE can be used to pass CMake variables when
building the native tools.  What I do is set up a toolchain file:

set(CMAKE_C_COMPILER {host-cc})
> set(CMAKE_CXX_COMPILER {host-cxx})
>
...
>

And pass this to the top level CMake:

-DCROSS_TOOLCHAIN_FLAGS_NATIVE="-DCMAKE_TOOLCHAIN_FILE=<path-to-above-file>"
>

CMAKE_C_COMPILER etc can instead be directly set in this flag (instead of a
toolchain file):

-DCROSS_TOOLCHAIN_FLAGS_NATIVE="-DCMAKE_C_COMPILER=<host-cc> ..."
>


On Tue, Oct 17, 2017 at 5:11 PM, Shoaib Meenai via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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