[llvm-dev] Cross compiling for ARMv7-m (original) (raw)

Peter Smith via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 26 09:36:55 PST 2020

Hello Arslan,

I am guessing I can borrow libraries such as lc and lm from GCC arm-none-eabi*, but for clangrt.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.

cmake -G "Unix Makefiles" -DBAREMETALARMV6MSYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/ -DBAREMETALARMV7MSYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/ -DBAREMETALARMV7EMSYSROOT=../../gcc-arm-none-eabi-9-2019-q4-major/arm-none-eabi/ -DCMAKEBUILDTYPE=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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