[cfe-dev] Consult how to cross compile clang for arm on x86 linux ubuntu box

Thu Aug 20 18:35:32 PDT 2020

Dear David,

I appreciate your experience. I also successfully built the clang for
target ARM platform on x86 Linux by directly modifying
"./llvm/CMakeLists.txt" as below in green color. It is ugly workaround. I
prefer your approach and will try later.

# Override the default target with an environment variable named by
LLVM_TARGET_TRIPLE_ENV.
#set(LLVM_TARGET_TRIPLE_ENV CACHE STRING "The name of environment variable
to override default target. Disabled by blank.")
set(LLVM_TARGET_TRIPLE_ENV "armv7-linux-gnueabihf")
set(LLVM_TARGET_TRIPLE_ENV "armv7-linux-gnueabihf"  CACHE STRING "ydd
override default target to armv7-linux-gnueabihf.")
mark_as_advanced(LLVM_TARGET_TRIPLE_ENV)

# By default, we target the host, but this can be overridden at CMake
# invocation time.
set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_HOST_TRIPLE}" CACHE STRING
  "Default target for which LLVM will generate code." )
set(TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")
message(STATUS "LLVM host triple: ${LLVM_HOST_TRIPLE}")
message(STATUS "LLVM default target triple: ${LLVM_DEFAULT_TARGET_TRIPLE}")
set(LLVM_TARGET_TRIPLE_ENV "armv7-linux-gnueabihf")
message(STATUS "ydd LLVM_TARGET_TRIPLE_ENV: ${LLVM_TARGET_TRIPLE_ENV}")
message(STATUS "ydd LLVM_TARGETS_TO_BUILD: ${LLVM_TARGETS_TO_BUILD}")

Many Thanks,
Andy

On Thu, Aug 20, 2020 at 5:38 PM David Spickett <david.spickett at linaro.org>
wrote:

> Hi Andy,
>
> Assuming you are compiling with clang, I think -target and --sysroot
> need to be added to CMAKE_CXX_FLAGS instead of being cmake arguments
> themselves.
>
> "However, if you’re using Clang, the driver might not be up-to-date
> with your specific Linux distribution, version or GCC layout, so
> you’ll need to fudge.
>
> In addition to the ones above, you’ll also need:"
>
> Means you'll need extra CMAKE_CXX_FLAGS. (I also made the same mistake
> on first reading) So try:
> -DCMAKE_CXX_FLAGS="-target arm-linux-gnueabihf
> --sysroot=/usr/arm-linux-gnueabihf"
>
> Also I think (not tried your exact config) you need to tell cmake you
> want to use clang by doing:
> -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++
>
> If you're using GCC, skip the target and sysroot CFLAGS. Also I found
> that I had to explicitly give cmake the gcc name (similar to above):
> -DCMAKE_C_COMPILER=arm-linux-gnueabihf-gcc
> -DCMAKE_CXX_COMPILER=arm-linux-gnueabihf-g++-5
> I have a feeling CMake should find this on its own but I don't know
> enough to say.
>
> If the default triple is still x86 by that point you can set that
> with: -DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf
> (probably won't be an issue)
>
> Thanks,
> David Spickett.
>
> On Thu, 20 Aug 2020 at 03:15, Dongdong Yang via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > I can get llvm source code and the information “cross-compiling from
> x86_64 to ARM” from https://llvm.org/docs/HowToCrossCompileLLVM.html.
> However, I still fail to build clang for ARM. It is strange, I have already
> set a target with "   -target arm-linux-gnueabihf  "  .  I am describing
> the failure progress as below. Expecting any idea.
> >
> >
> >
> > The build steps:
> > cd llvm-project
> > mkdir build
> > cd build
> > cmake -DLLVM_ENABLE_PROJECTS=clang -G "Unix Makefiles"
> > make
> > // After I built host llvm-tblgen  and clang-tblgen, I begin to return
> to build arm target clang as below steps:
> > cd ..
> > mkdir build_arm_onx86
> > cd build_arm_onx86
> > mkdir stall_prefix
> > cmake -G "Unix Makefiles" -DCMAKE_CROSSCOMPILING=True
> -DCMAKE_INSTALL_PREFIX=./stall_prefix
> -DLLVM_TABLEGEN=/home_2t/workspace/llvm_clang/llvm-project/build/bin/llvm-tblgen
> -DCLANG_TABLEGEN=/home_2t/workspace/llvm_clang/llvm-project/build/bin/clang-tblgen
> -DLLVM_TARGET_ARCH=ARM -DLLVM_TARGETS_TO_BUILD=ARM  -target
> arm-linux-gnueabihf  -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi"
> ../llvm    2>&1 | tee out_cofiguration.txt
> > make  2>&1 | tee out_make.txt
> >
> > The result:
> >
> > build_arm_onx86$ file bin/clang-12
> > bin/clang-12: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux),
> dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux
> 2.6.32, BuildID[sha1]=3b669c10e300bb5d9bb96cfb77f927ce95728ca6, not stripped
> >
> > build_arm_onx86$ bin/clang --version
> > clang version 12.0.0 (https://github.com/llvm/llvm-project.git
> 3948341fa5fab2695b0df95edc248fe959e40664)
> > Target: x86_64-unknown-linux-gnu
> > Thread model: posix
> > InstalledDir:
> /home_2t/workspace/llvm_clang/llvm-project/build_arm_onx86/bin
> >
> > Seems it failed from the configuration as attached cmake output log.
> > -- LLVM host triple: x86_64-unknown-linux-gnu
> > -- LLVM default target triple: x86_64-unknown-linux-gnu
> >
> >
> > Thanks in advance
> >
> >
> > Best Regards,
> > Andy
> >
> > On Thu, Aug 13, 2020 at 7:32 PM Dongdong Yang <
> contribute.kernel at gmail.com> wrote:
> >>
> >> May I ask how to build clang for arm on x86 linux ubuntu box? And how
> to use  sysroot  to cross compile clang  libc++  for arm on x86 linux?
>  Appreciate any guidance.
> >>
> >> Thanks,
> >> Andy
> >
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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