Clang uses system include files instead of GCC-Toolchain ones

[marcovanleeuwen]

Hi Giulio, all,

I am trying to compile O2 on SL6 (unsupported, I know; the error below may however come to bite us on other platforms as well?) and get an error when aliBuild is doing a test compilation within the clang build script ($INSTALLROOT/bin-safe/clang++ -v -c test.cc).

The underlying issue seems to be that it picks up the system gcc and includes, instead of the GCC-Toolchain version:

Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /data1/software/alisoft/INSTALLROOT/ae8bbd59f63481f0629333a5d157d37be09c41a7/slc6_x86-64/Clang/v10.0.1-1/bin-safe
Found candidate GCC installation: /usr/lib/gcc/i686-redhat-linux/4.4.4
Found candidate GCC installation: /usr/lib/gcc/i686-redhat-linux/4.4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/3.4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.7
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.7
....
clang -cc1 version 10.0.1 based upon LLVM 10.0.1 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/x86_64-redhat-linux
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/backward
/usr/local/include
/data1/software/alisoft/INSTALLROOT/ae8bbd59f63481f0629333a5d157d37be09c41a7/slc6_x86-64/Clang/v10.0.1-1/lib/clang/10.0.1/include
/usr/include
End of search list.

I can counteract this by specifying the
--gcc-toolchain=/data1/software/alisoft/slc6_x86-64/GCC-Toolchain/v7.3.0-alice2-1
flag, but this would likely cause problems down the line. Do you know how to set this by default?

I check the compilation logs, and cmake correctly uses GCC 7.3.0:

++ cmake /data1/software/alisoft/SOURCES/Clang/v10.0.1/llvmorg-10.0.1/llvm '-DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra;compiler-rt' -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX:PATH=/data1/software/alisoft/INSTALLROOT/ae8bbd59f63481f0629333a5d157d37be09c41a7/slc6_x86-64/Clang/v10.0.1-1 -DLLVM_INSTALL_UTILS=ON -DPYTHON_EXECUTABLE=/data1/software/alisoft/slc6_x86-64/Python/v3.6.10-4/bin/python3 -DDEFAULT_SYSROOT= -DLLVM_BUILD_LLVM_DYLIB=ON -DBUILD_SHARED_LIBS=OFF
-- The C compiler identification is GNU 7.3.0
-- The CXX compiler identification is GNU 7.3.0
-- The ASM compiler identification is GNU
-- Found assembler: /data1/software/alisoft/slc6_x86-64/GCC-Toolchain/v7.3.0-alice2-1/bin/cc
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /data1/software/alisoft/slc6_x86-64/GCC-Toolchain/v7.3.0-alice2-1/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /data1/software/alisoft/slc6_x86-64/GCC-Toolchain/v7.3.0-alice2-1/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done

but somehow it seems that this is not used at run-time.

Best regards,

Marco.

[marcovanleeuwen]

Hi Guilio, all,

I looked a bit more into this and I see the same feature on SL7 with the clang and GCC 7.3 from CVMFS:

[marcovl@stbc-i1 ~]$ alienv enter VO_ALICE@O2::nightly-20201110-1
[O2/nightly-20201110-1] ~ > which clang
/cvmfs/alice.cern.ch/el7-x86_64/Packages/Clang/v10.0.1-17/bin-safe/clang
[O2/nightly-20201110-1] ~ > clang -v
clang version 10.0.1 (https://github.com/llvm/llvm-project ef32c611aa214dea855364efd7ba451ec5ec3f74)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /cvmfs/alice.cern.ch/el7-x86_64/Packages/Clang/v10.0.1-17/bin-safe
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.5
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.5
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
[O2/nightly-20201110-1] ~ > which gcc
/cvmfs/alice.cern.ch/el7-x86_64/Packages/GCC-Toolchain/v7.3.0-alice1-9/bin/gcc

clang picks up the include files from the system compiler instead of GCC 7.3.0