Compiler Specifics

Alpaka supports a large number of different compilers. Each of the compilers has its own special features. This page explains some of these specifics.

Choosing the correct Standard Library in Clang

Clang supports both, libstdc++ shipped with the GNU GCC toolchain and libc++, LLVM’s own implementation of the C++ standard library. By default, clang and all clang-based compilers, such as the hipcc, use libstdc++ (GNU GCC). If more than one GCC version is installed, it is not entirely clear which version of libstdc++ is selected. The following code can be used to check which standard library and version clang is using by default with the current setup.

#include <iostream>

int main(){
    #ifdef _GLIBCXX_RELEASE
    std::cout << "use libstdc++ (GNU GCC's standard library implementation)" << std::endl;
    std::cout << "version: " <<   _GLIBCXX_RELEASE << std::endl;
    #endif

    #ifdef _LIBCPP_VERSION
    std::cout << "use libc++ (LLVM's standard library implementation)" << std::endl;
    std::cout << "version: " <<   _LIBCPP_VERSION << std::endl;
    #endif
}

The command clang -v ... shows the include paths and also gives information about the standard library used.

Choose a specific libstdc++ version

Clang provides the argument --gcc-toolchain=<path> which allows you to select the path of a GCC installation. For example, if you built the GCC compiler from source, you can select the installation prefix, which is the base folder with the subfolders include, lib and so on.

If you are using CMake, you can set the --gcc-toolchain flag via the following CMake command line argument:

• -DCMAKE_CXX_FLAGS="--gcc-toolchain=<path>" if you use Clang as compiler for CPU backends or the HIP backend.

• -DCMAKE_CUDA_FLAGS="--gcc-toolchain=<path>" if you use Clang as CUDA compiler.

Hint

If you are using Ubuntu and install a new gcc version via apt, it is not possible to select a specific gcc version because apt installs all headers and shared libraries in subfolders of /usr/include and /usr/lib. Therefore, you can only use the /usr base path and Clang will automatically select one of the installed libstdc++ versions.

Hint

If you installed Clang/LLVM with spack and a gcc compiler, the Clang compiler will use the libstdc++ of the compiler used to build Clang/LLVM.

Selecting libc++

libc++ can be used if you set the compiler flag -stdlib=libc++.

If you are using CMake, you can select libc++ via the following CMake command line argument:

• -DCMAKE_CXX_FLAGS="-stdlib=libc++" if you use Clang as compiler for CPU backends or the HIP backend.

• -DCMAKE_CUDA_FLAGS="-stdlib=libc++" if you use Clang as CUDA compiler.