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

[Kacper Kowalski via llvm-dev]

Thank you for your answer. So I continue my struggle.

How can I disable threads for libc++? I can't find any switch in the
documentation.

Kacper

czw., 14 lis 2019 o 11:18 Peter Smith <peter.smith at linaro.org> napisał(a):

> 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
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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