[Buildroot] [PATCH 1/1] rust: make sure the cxx compiler is also set for the target

[Eric Le Bihan <eric.le.bihan.dev@free.fr>]

Hi!

On 2018-04-02 19:54, Charles Hardin wrote:
> so? hopefully the following makes sense...
>
> When you fix the CXX linking you will see this in the stage0 - notice the target is unknown - not buildroot, and
> so what happens is the tools use the ?same? compiler as the target for the host side, which means that I ended
> up getting ?target? compiled with avx2 on a host that doesn?t support it
>
> Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
> Building stage0 test artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
>
> ? snip snip ?
>
> Building LLVM for x86_64-unknown-linux-gnu
> running: "cmake" "/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/build/host-rust-1.23.0/src/llvm" "-DLLVM_ENABLE_ASSERTIONS=OFF" "-DLLVM_TARGETS_TO_BUILD=X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon" "-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=" "-DLLVM_INCLUDE_EXAMPLES=OFF" "-DLLVM_INCLUDE_TESTS=OFF" "-DLLVM_INCLUDE_DOCS=OFF" "-DLLVM_ENABLE_ZLIB=OFF" "-DWITH_POLLY=OFF" "-DLLVM_ENABLE_TERMINFO=OFF" "-DLLVM_ENABLE_LIBEDIT=OFF" "-DLLVM_PARALLEL_COMPILE_JOBS=2" "-DLLVM_TARGET_ARCH=x86_64" "-DLLVM_DEFAULT_TARGET_TRIPLE=x86_64-unknown-linux-gnu" "-DLLVM_LINK_LLVM_DYLIB=ON" "-DCMAKE_C_COMPILER=/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-gcc" "-DCMAKE_CXX_COMPILER=/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-g++" "-DCMAKE_C_FLAGS=-ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_CXX_FLAGS=-ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_AR=/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-ar" "-DCMAKE_INSTALL_PREFIX=/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/build/host-rust-1.23.0/build/x86_64-unknown-linux-gnu/llvm" "-DCMAKE_BUILD_TYPE=Release"
> -- The C compiler identification is GNU 7.3.0
> -- The CXX compiler identification is GNU 7.3.0
> -- The ASM compiler identification is GNU
> -- Found assembler: /home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-gcc
> -- Check for working C compiler: /home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-gcc
> -- Check for working C compiler: /home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-gcc -- works
> -- Detecting C compiler ABI info
> -- Detecting C compiler ABI info - done
> -- Detecting C compile features
> -- Detecting C compile features - done
> -- Check for working CXX compiler: /home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-g++
> -- Check for working CXX compiler: /home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/host/bin/x86_64-buildroot-linux-gnu-g++ -- works
> -- Detecting CXX compiler ABI info
> -- Detecting CXX compiler ABI info - done
> -- Detecting CXX compile features
> -- Detecting CXX compile features - done
> CMake Warning at CMakeLists.txt:140 (message):
>   Job pooling is only available with Ninja generators.
>
> leads to building this ?host? tool
>
> [  2%] Built target LLVMTableGen
>
> which cann?t run on that host because the compiler it used was the ?buildroot? compiler
>
> [ 19%] Linking CXX static library ../../libLLVMDebugInfoPDB.a
> [ 19%] Built target LLVMDebugInfoPDB
> Makefile:151: recipe for target 'all' failed
> make[3]: *** [all] Error 2
> thread 'main' panicked at '
> command did not execute successfully, got: exit code: 2
>
> build script failed, must exit now', src/vendor/cmake/src/lib.rs:631<http://lib.rs:631>:4
> note: Run with `RUST_BACKTRACE=1` for a backtrace.
> finished in 267.033
> failed to run: /home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/build/host-rust-1.23.0/build/bootstrap/debug/bootstrap build
> Build completed unsuccessfully in 0:06:22
> package/pkg-generic.mk:247: recipe for target '/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/build/host-rust-1.23.0/.stamp_built' failed
> make[2]: *** [/home/vagrant/oneblox-buildroot/toolchains/br-x86_64-core_avx2-glibc_toolchains.build/build/host-rust-1.23.0/.stamp_built] Error 1
> Makefile:79: recipe for target '_all' failed
> make[1]: *** [_all] Error 2
> make[1]: Leaving directory '/home/vagrant/oneblox-buildroot/repos/buildroot'
> Makefile:93: recipe for target 'br-x86_64-core_avx2-glibc-tmp.tar.bz2' failed
> make: *** [br-x86_64-core_avx2-glibc-tmp.tar.bz2] Error 2

Building Rust for x86_64 on a x86_64 is indeed tricky!

As seen in the build section of src/bootstrap/config.toml.example, Rust
manages "build", "host" and "target" parameters, which all default to
"x86_64-unknown-linux-gnu".

When building, the compiler is built for the host and the standard
library for the host and the target.

So when the target is the same as the host, only the compiler and the
standard library for the host should be built.

As the compiler is built on LLVM, coded in C++, a C++ compiler for the
host is indeed required. It happens that in the build of LLVM, a host
tool named FileCheck is built and run for test.

IIUC, the C compiler is only used for building the standard library
(this is what I've seen when cross-compiling for ARM).

So when the target is the same as the host, it might be sensible to
avoid defining c and cxx in the target configuration file, as suggested
by your patch.

I'll poke upstream for clarifications on this special case, though.

The first versions of the patch series did try to work with new targets
using triples with "buildroot" instead of "unknown", but defining new
targets instead of using the available ones turned out to be
difficult/cumbersome.

But maybe it would have avoided the issue raised here where the target
is the same as the host.

Out of curiosity, does using the pre-built toolchain may be of use in
your situation?

Regards,

--
ELB