[llvm-dev] Cross compiling for Baremetal ARM without using GCC
Peter Smith via llvm-dev
llvm-dev at lists.llvm.org
Wed Nov 1 05:02:29 PDT 2017
Hello Daniel,
I don't have full answers but I may be able to move you slightly
forward. I'm assuming that you want an answer that involves only open
source tools.
I've managed to successfully build compiler-rt using Jonathan Roelofs
BaremetalARM.cmake in clang/cmake/caches and an arm-none-eabi gcc
installation as sysroot. One thing that I needed to do for it to work
was to move compiler-rt from llvm/projects/compiler-rt to
llvm/runtimes/compiler-rt. I haven't got any experience at building
newlib or Musl with clang instead of gcc.
I've been working on Arm support in LLD, in theory most of the parts
are there [*] and the linker script support has come a long way
recently. However I've not actively tested on embedded systems yet as
my focus has been on Linux/BSD like targets. At least a few people
have had problems with LLD always allocating the ELF header and
Program Header in the first PT_LOAD Program Header, which is
definitely not what you want for an embedded system. If you do run
into problems please let us know and raise bug reports if possible.
Peter
[*] There aren't any range extension thunks that work for Cortex-M0,
but in the vast majority of cases you won't need these.
On 31 October 2017 at 23:55, Daniel Kang via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Dear LLVM developers,
>
>
>
> Hello,
>
> I’m trying to find a way of cross-compiling my c code against Baremetal
> Cortex-M device (so target triple will be arm-none-eabi) only using
> LLVM/Clang, and not using anything from GNU (ld or libc).
>
> I’m doing this to know which one of LLVM/clang and GCC produces smaller
> flash image size because saving flash is a big deal in our projects.
>
>
>
> 1) When I just follow this -
> https://clang.llvm.org/docs/CrossCompilation.html using LLVM/Clang Prebuilt
> binary 5.0.0, I got this error,
>
> daniel at daniel-VirtualBox:~/clang/crosscompile$ make clean all
>
> rm -f main *.o *.map *.a
>
> "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -static
> --target=arm-none-eabi -mcpu=cortex-m0 -c -o main.o main.c
>
> "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -static
> --target=arm-none-eabi -mcpu=cortex-m0 -c -o a.o a.c
>
> "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -static
> --target=arm-none-eabi -mcpu=cortex-m0 -c -o reset.o reset.c
>
> "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -v -static
> --target=armv6m-none-eabi -o main main.o a.o reset.o
>
> clang version 5.0.0 (tags/RELEASE_500/final)
>
> Target: armv6m-none--eabi
>
> Thread model: single
>
> InstalledDir: clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin
>
> "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld" main.o a.o reset.o
> -Bstatic
> -Lclang+llvm-5.0.0-linux-x86_64-ubuntu16.04/lib/clang/5.0.0/lib/baremetal
> -lc -lm -lclang_rt.builtins-armv6m.a -o main
>
> clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld: error: unable to find
> library -lc
>
> clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld: error: unable to find
> library -lm
>
> clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld: error: unable to find
> library -lclang_rt.builtins-armv6m.a
>
> clang-5.0: error: ld.lld command failed with exit code 1 (use -v to see
> invocation)
>
> makefile:33: recipe for target 'main' failed
>
> make: *** [main] Error 1
>
>
>
> 2) To get clang_rt.builtins-armv6m.a or libc, libm, I tried to build
> LLVM/Clang (compiler_rt) from LLVM/Clang source following this -
> http://llvm.org/docs/HowToCrossCompileLLVM.html but I got this error,
>
> TARGET_TRIPLE=arm-none-eabi
>
> MYHOSTBIN=${HOME}/clang/source/build_x64/bin
>
> MYGNUARM_ROOT=${HOME}/opt/gcc-arm-none-eabi-6-2017-q2-update
>
> MYCFLAGS="--specs=nosys.specs -mcpu=cortex-m4 -mthumb -mfloat-abi=softfp
> -mfpu=fpv4-sp-d16"
>
> cmake -G "Ninja" \
>
> -DCMAKE_CROSSCOMPILING=True \
>
> -DCMAKE_BUILD_TYPE=Release \
>
> -DCMAKE_INSTALL_PREFIX=${HOME}/clang/toInstall \
>
> -DLLVM_TABLEGEN=${MYHOSTBIN}/llvm-tblgen \
>
> -DCLANG_TABLEGEN=${MYHOSTBIN}/clang-tblgen \
>
> -DLLVM_DEFAULT_TARGET_TRIPLE=arm-none-eabi \
>
> -DLLVM_TARGET_ARCH=ARM \
>
> -DLLVM_TARGETS_TO_BUILD=ARM \
>
> -DLLVM_ENABLE_LTO=Full \
>
> -DCMAKE_C_FLAGS="${MYCFLAGS}" \
>
> -DCMAKE_CXX_FLAGS="${MYCFLAGS}" \
>
> -DCMAKE_SYSROOT=${MYGNUARM_ROOT} \
>
> -DCMAKE_TOOLCHAIN_FILE=../toolchain.txt \
>
> -DCMAKE_C_COMPILER=${MYGNUARM_ROOT}/bin/arm-none-eabi-gcc \
>
> -DCMAKE_CXX_COMPILER=${MYGNUARM_ROOT}/bin/arm-none-eabi-g++ \
>
> -DCMAKE_ASM_COMPILER=${MYGNUARM_ROOT}/bin/arm-none-eabi-as \
>
> ../llvm
>
> …
>
> -- Looking for __atomic_load_8 in atomic
>
> -- Looking for __atomic_load_8 in atomic - not found
>
> CMake Error at cmake/modules/CheckAtomic.cmake:74 (message):
>
> Host compiler appears to require libatomic, but cannot find it.
>
> Call Stack (most recent call first):
>
> cmake/config-ix.cmake:350 (include)
>
> CMakeLists.txt:585 (include)
>
>
>
> arm-none-eabi toolchain doesn’t contain libatomic, but I need to build
> compiler_rt using this tool to get the runtime for ARM, so it’s confusing.
> Or building compiler_rt using arm-none-eabi toolchain is completely wrong
> idea in the first place.
>
>
>
> 3) Then I found this mail thread
> http://lists.llvm.org/pipermail/llvm-dev/2017-August/116134.html which seems
> very close to the thing I’m trying, but the mail thread is unfinished.
>
>
>
> So, my question is,
>
> Is there a way of cross-compiling c code against Baremetal Cortex-M device
> only using LLVM/Clang, and not using anything from GNU (ld or libc)?
>
> If so, do I have to build LLVM/Clang source code to get
> clang_rt.builtins-armv6m.a or libc, libm for ARM?
>
> If so, the method 2) or 3) above is the right approach? Or are there any
> other instruction?
>
>
>
> Any advice would be a great help for me.
>
>
>
> Thank you
>
> Daniel
>
>
>
>
>
>
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