[llvm-dev] Compiling libc++ using GNU Arm Embedded Toolchain for arm-cortex-m4

[Peter Smith via llvm-dev]

On Wed, 13 Nov 2019 at 20:34, Kacper Kowalski via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello,
>
> lately, I have been thinking about how to resolve the problem with that the program size increases enormously when including <iostream> when compiling with libstdc++. In this library, in <iostream> there is a static object __ioinit initialized like so:
>
>   ...
>   // For construction of filebuffers for cout, cin, cerr, clog et. al.
>   static ios_base::Init __ioinit;
>   ...
>
> This makes the program size increase enormously. The thing is that I often compile programs for small platforms, like arm-cortex-m3 or arm-cortex-m4 (stm32, nrf52, ...). Including this header leads to the program size exceed the FLASH memory size.
>
> This is not a problem when you have full control over what you include. But if you use external libraries and you don't want to mess with their sources, or you are not able to, then it gets complicated.
>
> To resolve this issue I found out that libc++ can be compiled with the flags:
>
>     -D_LIBCPP_HAS_NO_STDOUT:BOOL=ON
>     -D_LIBCPP_HAS_NO_STDIN:BOOL=ON
>
> so I gave it a try: I started a struggle to compile libc++ and libc++ ABI with ARMToolchain-9-2019-q4-major. Unfortunately, I failed.
>
> This is how I build libc++:
>
> TOOLCHAIN_ROOT_DIR="${HOME}/ARMToolchain-9-2019-q4-major"
>
> export CC="${TOOLCHAIN_ROOT_DIR}/bin/arm-none-eabi-gcc"
> export CXX="${TOOLCHAIN_ROOT_DIR}/bin/arm-none-eabi-g++"
>
> C_AND_CXX_COMMON_FLAGS_BASE="-mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard --specs=nosys.specs\
>     --sysroot=${TOOLCHAIN_ROOT_DIR} -I${TOOLCHAIN_ROOT_DIR}/arm-none-eabi/include\
>     -L${TOOLCHAIN_ROOT_DIR}/lib -L${TOOLCHAIN_ROOT_DIR}/lib/gcc/arm-none-eabi/9.2.1"
> C_AND_CXX_COMMON_FLAGS_MORE="-mthumb -mabi=aapcs"
> C_AND_CXX_COMMON_FLAGS="$C_AND_CXX_COMMON_FLAGS_BASE $C_AND_CXX_COMMON_FLAGS_MORE"
>
> cmake \
>     -DCMAKE_CROSS_COMPILING:BOOL=ON \
>     -DLLVM_ENABLE_PROJECTS='libcxxabi;libcxx' \
>     -DCMAKE_INSTALL_PREFIX='${HOME}/ClangLibcxx' \
>     -DCMAKE_BUILD_TYPE='Release' \
>     -DLLVM_ENABLE_ASSERTIONS:BOOL=ON \
>     -DLLVM_TARGET_ARCH='ARM' \
>     -DLLVM_TARGETS_TO_BUILD='ARM' \
>     -DLIBCXX_BUILD_32_BITS:BOOL=ON \
>     -DLLVM_BUILD_32_BITS:BOOL=ON \
>     -DLLVM_ENABLE_STATIC:BOOL=ON \
>     -DLLVM_ENABLE_SHARED:BOOL=OFF \
>     -DLLVM_DISABLE_PIC:BOOL=OFF \
>     -D_LIBCPP_HAS_NO_STDOUT:BOOL=ON \
>     -D_LIBCPP_HAS_NO_STDIN:BOOL=ON \
>     -DCMAKE_C_FLAGS="$C_AND_CXX_COMMON_FLAGS" \
>     -DCMAKE_CXX_FLAGS="$C_AND_CXX_COMMON_FLAGS" \
>     ../llvm
>
> Firstly, I encounter the known problem with __atomic_load_8. arm-cortex-m{3,4} most probably doesn't support 64bit atomic read/write, but as long as I don't use it, changing the line in CheckAtomic.cmake:
>
>       message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
>
> to:
>
>       message(WARNING "Host compiler appears to require libatomic, but cannot find it.")
>
> could push the things further.
> But then I encounter that error:
>
>     -- Targeting ARM
>     CMake Error at /home/kacper/Workspace/LLVMProject/libcxxabi/CMakeLists.txt:216 (message):
>         LIBCXXABI_BUILD_32_BITS=ON is not supported on this platform.
>
> which is confusing.
>
> Can't the libc++abi be compiled for 32B ARM targets?
>

libc++abi can be compiled for 32-bit ARM targets. I think the intent
behind that particular option is for 64-bit X86 building for 32-bit
i386. The ARM target is only 32-bit so there isn't any need for an
option, I suggest leaving out LIBCXXABI_BUILD_32_BITS=ON,
-DLIBCXX_BUILD_32_BITS:BOOL=ON and  -DLLVM_BUILD_32_BITS:BOOL=ON

It may be worth disabling threads for libc++. From memory libc++
relies on a pthread API by default which the GNU embedded toolchain
won't supply. I think that there are ways to supply an alternative
threading implementation but I don't know the details.

Hope this helps and good luck

Peter

> Best regards,
> Kacper Kowalski
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