[llvm-dev] Cross compiling for Baremetal ARM without using GCC

[Daniel Kang via llvm-dev]

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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