[llvm-dev] Cross compiling for ARMv7-m

Peter Smith via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 14 10:14:04 PDT 2020 

> ________________________________________
> From: Christophe Lyon <christophe.lyon at linaro.org>
> Sent: 14 September 2020 15:42
> To: Peter Smith
> Cc: llvm-dev at lists.llvm.org; Arslan Khan; nd
> Subject: Re: [llvm-dev] Cross compiling for ARMv7-m
> 
> Hi Peter,
> 
> 
> On Wed, 26 Feb 2020 at 18:37, Peter Smith via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hello Arslan,
> >
> > > I am guessing I can borrow libraries such as lc and lm from GCC
> > > arm-none-eabi*, but for clang_rt.builtins-arm.a (which I thought
> > > should've been built with LLVM build but I couldn't find that library
> > > in my build directory) I tried building it from LLVM source.
> >
> > You can take these from an arm-none-eabi toolchain. The GCC equivalent of clang_rt.builtins-arm.a is libgcc.a,
> > one way to use these with clang is use one of the options that suppress -lc -lm and -lclang_rt.builtins-arm.a
> > I think either --nostdlib or --nodefaultlibs will do that. You'll need to tell clang where to find these in your arm-none-eabi toolchain.
> >
> > > To generate the library, I tried generating builtins by following the guide at:
> > > https://llvm.org/docs/HowToCrossCompileBuiltinsOnArm.html
> > > I tried with different configurations, but each one ends up in some
> > > error which left me wondering that this document might be obsolete.
> >
> > I'd hope the information in there isn't completely obsolete, but I haven't tried the build for over a year so it may well need an update.
> > I'll try to reproduce my steps and send a patch to update the guide if there is anything broken. I can't promise when I'll get that done by though.
> >
> 
> I recently tried again to build compiler-rt for cortex-m using the
> info from the guide mentioned above, but it still fails for me.
> 
> Something that I don't understand is that when I call
> clang --target=armv7m-none-eabi
> --gcc-toolchain=/XX/gcc-linaro-7.3.1-2018.05-x86_64_arm-eabi
> --sysroot=/XX/gcc-linaro-7.3.1-2018.05-x86_64_arm-eabi/arm-eabi/
> -L$PWD/llvm-project/cross-install/lib hello.o -o hello.exe.arm.clang
> 
> I get:
> clang-12: warning: argument unused during compilation:
> '--gcc-toolchain=/XX/gcc-linaro-7.3.1-2018.05-x86_64_arm-eabi'
> [-Wunused-command-line-argument]
> ld.lld: error: unable to find library -lc
> ld.lld: error: unable to find library -lm
> clang-12: error: ld.lld command failed with exit code 1 (use -v to see
> invocation)
> 
> Not sure why --gcc-toolchain isn't used, anyway libc.a and libm.a are
> under $SYSROOT/libc/usr/lib
> (and I did try --sysroot=...../libc --sysroot=.../libc/usr and
> --sysroot=../libc/usr/lib, none of which worked)
> 
> Any idea?
> 
> Thanks,
> 
> Christophe

Hello Christophe,

When the bare metal driver is selected with --target=arm-none-eabi the --gcc-toolchain option is not checked.
>From the code in clang/lib/Driver/ToolChains/BareMetal.cpp it looks like sysroot is used in the include paths but not for the library paths.

If libc.a and libm.a are under $SYSROOT/libc/usr/lib then with the bare-metal driver I think you would need to use -L $SYSROOT/libc/usr/lib

If it helps, this is what a colleague had been using recently to compile compiler-rt for bare-metal. I've not tested it myself though. The interesting parts that may be
new are -DCOMPILER_RT_BAREMETAL_BUILD=ON and -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY I expect that a lot of that can be trimmed as well.

cmake -G Ninja ../llvm-project/compiler-rt/
        -DCMAKE_BUILD_TYPE=Release \
        -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
        -DCOMPILER_RT_BUILD_XRAY=OFF \
        -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
        -DCOMPILER_RT_BUILD_PROFILE=OFF \
        -DCOMPILER_RT_BAREMETAL_BUILD=ON \
        -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
        -DCMAKE_C_COMPILER_TARGET="$TARGET" \
        -DCMAKE_ASM_COMPILER_TARGET="$TARGET" \
        -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
        -DLLVM_CONFIG_PATH="$BUILDTOOLS/llvm-config" \
        -DCMAKE_C_COMPILER="$TOOLS/clang" \
        -DCMAKE_C_FLAGS="--sysroot $INSTALL/targets/$TARGET" \
        -DCMAKE_AR="$TOOLS/llvm-ar" \
        -DCMAKE_NM="$TOOLS/llvm-nm" \
        -DCMAKE_RANLIB="$TOOLS/llvm-ranlib" \
        -DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld" \
        -DCMAKE_INSTALL_PREFIX="$INSTALL/targets/$TARGET"

Hope this gets you a bit further. Apologies if the webmail client has mangled the reply. Let me know if you are still having problems and I'll try and take a look.

Peter

> > > cmake -G "Unix Makefiles"
> > > -DBAREMETAL_ARMV6M_SYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/
> > > -DBAREMETAL_ARMV7M_SYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/
> > > -DBAREMETAL_ARMV7EM_SYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/
> > > -DCMAKE_BUILD_TYPE=Release -C ../clang/cmake/caches/BaremetalARM.cmake
> > > ../llvm
> >
> > That particular recipe predates the monorepo, and relied upon compiler-rt being checked out in the runtimes subdirectory. I suspect that this will likely not work with the monorepo.
>
> Peter
>
> ________________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Arslan Khan via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: 26 February 2020 16:57
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] Cross compiling for ARMv7-m
>
> Hi,
> I am trying to use LLVM/clang to build a binary for ARM Cortex M platforms.
> When I build my binary the linker fails to find these libraries.
>
> ./bin/clang++ -target arm-none-eabi -mcpu=cortex-m4 ./temp.cpp -L
> ./lib/ -L ./libexec/ -L ../build3_v7m/lib/
> ld.lld: error: unable to find library -lunwind
> ld.lld: error: unable to find library -lc
> ld.lld: error: unable to find library -lm
> ld.lld: error: unable to find library -lclang_rt.builtins-arm.a
>
> I am guessing I can borrow libraries such as lc and lm from GCC
> arm-none-eabi*, but for clang_rt.builtins-arm.a (which I thought
> should've been built with LLVM build but I couldn't find that library
> in my build directory) I tried building it from LLVM source.
>
>
> To generate the library, I tried generating builtins by following the guide at:
> https://llvm.org/docs/HowToCrossCompileBuiltinsOnArm.html
> I tried with different configurations, but each one ends up in some
> error which left me wondering that this document might be obsolete.
>
> Anyhow googling things led me to this thread.
>
> http://llvm.1065342.n5.nabble.com/llvm-dev-Compiling-for-baremetal-ARMv4-on-Ubuntu-Linux-td124226.html
>
> After going through this I figured my best bet would be using GCC
> sysroot. After which my cmake command looks like this.
>
> cmake -G "Unix Makefiles"
> -DBAREMETAL_ARMV6M_SYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/
> -DBAREMETAL_ARMV7M_SYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/
> -DBAREMETAL_ARMV7EM_SYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/
> -DCMAKE_BUILD_TYPE=Release -C ../clang/cmake/caches/BaremetalARM.cmake
> ../llvm
>
> But with this, I am getting this error attached to the email. Is there
> something I am missing here? Is there any update to the
> cross-compilation guide? My end goal is cross-compiling C++ programs
> for ARM on x86. Any pointers for this would be helpful.
>
> Thanks,
> Arslan
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