Clang Cross Compiling with Yocto SDK - clang

Question

Is it possible to use the toolchain and sysroot from the Yocto SDK to cross compile with Clang/LLVM for ARM?
The environment-setup script within the Yocto SDK has the following compiler options:
export CC="arm-poky-linux-gnueabi-gcc -march=armv7-a -mfloat-abi=hard -mfpu=neon -mtune=cortex-a7 --sysroot=$SDKTARGETSYSROOT"
I would like to cross compile this simple userspace application
testapp.c:
#include <stdio.h>
int main()
{
printf("Hello World\n");
return(0);
}
Following the guide http://clang.llvm.org/docs/CrossCompilation.html, I am trying to cross compile the app with
$ GCCFLAGS="-mcpu=cortex-a7 -mfpu=neon -mfloat-abi=hard"
$ GCCINCLUDE="/opt/poky/2.0.1/sysroots/cortexa7hf-vfp-neon-poky-linux-gnueabi/usr/include/c++/5.2.0/arm-poky-linux-gnueabi/
$ /usr/bin/clang -target arm-poky-linux-gnueabi $GCCFLAGS -I$GCCINCLUDE -ccc-gcc-name arm-poky-linux-gnueabi-gcc testapp.c -o testapp -v
However, this fails with
Ubuntu clang version 3.4.2-3ubuntu2~xedgers (tags/RELEASE_34/dot2-final) (based on LLVM 3.4.2)
Target: arm-poky-linux-gnueabi
Thread model: posix
Selected GCC installation:
"/usr/lib/llvm-3.4/bin/clang" -cc1 -triple armv7-poky-linux-gnueabi -S -disable-free -disable-llvm-verifier -main-file-name testapp.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -mconstructor-aliases -target-cpu cortex-a7 -target-feature +neon -target-abi aapcs-linux -mfloat-abi hard -target-linker-version 2.24 -v -resource-dir /usr/lib/llvm-3.4/bin/../lib/clang/3.4.2 -I /opt/poky/2.0.1/sysroots/cortexa7hf-vfp-neon-poky-linux-gnueabi/usr/include/c++/5.2.0/arm-poky-linux-gnueabi/ -internal-isystem /usr/include/clang/3.4.2/include/ -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-3.4/bin/../lib/clang/3.4.2/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fno-dwarf-directory-asm -fdebug-compilation-dir /home/adtec/workspace/yocto/testapp -ferror-limit 19 -fmessage-length 207 -mstackrealign -fno-signed-char -fobjc-runtime=gcc -fdiagnostics-show-option -fcolor-diagnostics -vectorize-slp -o /tmp/testapp-8517af.s -x c testapp.c
clang -cc1 version 3.4.2 based upon LLVM 3.4.2 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/include"
ignoring duplicate directory "/usr/include/clang/3.4.2/include"
#include "..." search starts here:
#include <...> search starts here:
/opt/poky/2.0.1/sysroots/cortexa7hf-vfp-neon-poky-linux-gnueabi/usr/include/c++/5.2.0/arm-poky-linux-gnueabi
/usr/include/clang/3.4.2/include
/usr/local/include
/usr/include
End of search list.
'cortex-a7' is not a recognized processor for this target (ignoring processor)
'cortex-a7' is not a recognized processor for this target (ignoring processor)
'cortex-a7' is not a recognized processor for this target (ignoring processor)
'cortex-a7' is not a recognized processor for this target (ignoring processor)
'cortex-a7' is not a recognized processor for this target (ignoring processor)
'cortex-a7' is not a recognized processor for this target (ignoring processor)
"/usr/bin/as" -mfloat-abi=hard -mcpu=cortex-a7 -mfpu=neon -o /tmp/testapp-f4f688.o /tmp/testapp-8517af.s
/usr/bin/as: unrecognized option '-mfloat-abi=hard'
clang: error: assembler command failed with exit code 1 (use -v to see invocation)
What would be the appropriate Clang flags necessary to cross compile this simple application for ARM?

Answer 1

If you have access to a full build rather than just the SDK, you could try using the clang layer, https://github.com/kraj/meta-clang. Whilst it should be possible eventually to build an SDK containing clang I don't think anyone has done that yet.

Answer 2

you can use meta-clang and also generate a SDK which contains both gcc and clang based cross compilers, once SDK is installed there are CLANGCC and CLANGCXX environment variables available to invoke clang instead of gcc. if you want to use it as default you can do
export CC=${CLANGCC}
export CXX=${CLANGCXX}
export CPP=${CLANGCPP}
There is also a writeup for Clang SDK here