llvm dev - Apr 2016 - bug: cross-compile Clang/LLVM for ARM using Clang/LLVM

​Hi all,

I'm new to LLVM project and now I need to cross-compile Clang/LLVM for ARM.
My host is Ubuntu 14.04 and I could compile Clang/LLVM for X86, however
when I cross-compile Clang/LLVM for ARM, I come across a few problems.

My build structure is in the picture. [llvm-source-code] is from github
llvm-mirror; [build4x86] is that I follow the guide  [Clang Getting
Started] and build clang/llvm to produce it; [build4arm] is that I try to
cross-compile clan/llvm for arm on x86 PC, the problem exist here.

Sorry to finish reading, thanks. My command is as follows:

CC='clang' CXX='clang++' cmake -G Ninja ../llvm-source-code
-DCMAKE_CROSSCOMPILING=True
-DCMAKE_INSTALL_PREFIX=./llvm
-DLLVM_TABLEGEN=/home/lab/workspace/llvm/build4x86/bin/llvm-tblgen
-DCLANG_TABLEGEN=/home/lab/workspace/llvm/build4x86/bin/clang-tblgen
-DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf
-DLLVM_TARGET_ARCH=ARM
-DLLVM_TARGETS_TO_BUILD=ARM
-DCMAKE_CXX_FLAGS='-target armv7a-linux-gnueabihf -mcpu=cortex-a9
-I/usr/arm-linux-gnueabihf/include/c++/4.7.3/arm-linux-gnueabihf/
-I/usr/arm-linux-gnueabihf/include/
-I/home/lab/workspace/llvm/llvm-source-code/lib4arm/
-I/home/lab/workspace/llvm/llvm-source-code/lib2ubuntu/
-I/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so
-I/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so
-L/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so
-L/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.a -mfloat-abi=hard
-ccc-gcc-name arm-linux-gnueabihf-gcc'
-DLLVM_ENABLE_LIBCXX=ON
-DLLVM_ENABLE_PIC=False

The error is as follows:
1. Host compiler appears to require libatomic, but cannot find it.
2. Host Clang must be able to find libstdc++4.7 or newer!

Could anyone give me some suggestions? Thanks in advance!

Best,
Liyang
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On 25 April 2016 at 10:39, 李阳 via llvm-dev <llvm-dev at lists.llvm.org>
wrote:
> CC='clang' CXX='clang++' cmake -G Ninja ../llvm-source-code
> -DCMAKE_CROSSCOMPILING=True
> -DCMAKE_INSTALL_PREFIX=./llvm
> -DLLVM_TABLEGEN=/home/lab/workspace/llvm/build4x86/bin/llvm-tblgen
> -DCLANG_TABLEGEN=/home/lab/workspace/llvm/build4x86/bin/clang-tblgen
> -DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf
> -DLLVM_TARGET_ARCH=ARM
> -DLLVM_TARGETS_TO_BUILD=ARM
> -DCMAKE_CXX_FLAGS='-target armv7a-linux-gnueabihf -mcpu=cortex-a9
> -I/usr/arm-linux-gnueabihf/include/c++/4.7.3/arm-linux-gnueabihf/
> -I/usr/arm-linux-gnueabihf/include/
> -I/home/lab/workspace/llvm/llvm-source-code/lib4arm/
> -I/home/lab/workspace/llvm/llvm-source-code/lib2ubuntu/
> -I/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so
> -I/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so
> -L/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so
> -L/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.a -mfloat-abi=hard
> -ccc-gcc-name arm-linux-gnueabihf-gcc'
> -DLLVM_ENABLE_LIBCXX=ON
> -DLLVM_ENABLE_PIC=False
>
Hi Liyang,

You have "LLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf"  and
"CMAKE_CXX_FLAGS='-target armv7a-linux-gnueabihf" which are
different, and
may confuse Clang when finding tools and libraries. Use whatever is the ARM
triple in your system (looks like it's "arm-linux-gnueabihf") and
make both
target triple and "-target" to be that exact same string.

Also, you're including x86_64 libraries
(ex. -L/usr/lib/gcc/x86_64-linux-gnu/4.8/libatomic.so) to an ARM
compilation job, that won't work. You have to find the ARM version (maybe
/usr/lib/gcc/arm-linux-gnueabihf/ ?).

Finally, the include paths with -I are only necessary if Clang cannot find
it. The first three should be ok, but using -I on a DSO object
(libatomic.so) makes no sense.

cheers,
--renato
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