[llvm-dev] Cross compiling for ARMv7-m

Tue Sep 15 03:24:47 PDT 2020

On Mon, 14 Sep 2020 at 19:14, Peter Smith <Peter.Smith at arm.com> wrote:
>
> > ________________________________________
> > 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.

Hi Peter,

Thanks, it did help. Looks like I was missing
-DCMAKE_C_FLAGS="--sysroot XXX"
so now build_and_install_compilerrt() completes.

I guess my next step is now to understand how to build a toolchain for
eg. rtems to see what I need to update to build the sanitizers for a
new "OS" (baremetal for me).

Thanks,

Christophe

>
> 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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