[llvm-dev] Cross compiling for ARMv7-m

Mon Sep 14 07:42:16 PDT 2020

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

> > 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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