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README.md

LCG Toolchains

CMake toolchain files to build a standard CMake project on top of LCG releases.

Usage

To enable a toolchain it's enough to add the option -DCMAKE_TOOLCHAIN_FILE=path/to/the/toolchain.cmake to the call to cmake, e.g.:

cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=/opt/lcg-toolchains/LCG_96b/x86_64-centos7-gcc9-opt.cmake

The toolchain code will create a few helper scripts in the build tree, under the directory ${CMAKE_BINARY_DIR}/toolchain:

  • wrappers for the compilers
  • generic wrapper command
  • lib directory for pkgconfig workaround

Layout

The toolchain files are organized in one directory per supported LCG version plus one directory (fragments) with the implementation details.

The LCG version directory contains, one symlink to a CMake fragment common to all platforms (fragments/main.cmake) for each LCG binary tag, and one file for each special platform used by LHCb, which delegates to the corresponding LCG binary tag.

The main fragment then delegates to LCG version specific files for the configuration of the compiler, with the details in one file per specific version of the compiler. Then a file from the directory fragments/packages is included for the specific settings for the external packages.

The special file fragments/compilation_flags.cmake is then included to set the LHCb specific compilation flags (e.g. architecture flags, warnings settings, etc.).

Customize the toolchains

By default the value of LCG_PLATFORM (from the name of the CMake file) is used to set LCG_ARCHITECTURE and LCG_OPTIMIZATION, which are used to tune the compilation flags, but they can be overridden from the command line, for example to build for a custom architecture, not officially supported:

cmake -S . -B build -DLCG_ARCHITECTURE=skylake -DCMAKE_TOOLCHAIN_FILE=/opt/lcg-toolchains/LCG_96b/x86_64-centos7-gcc9-opt.cmake

Toolchain for LCG nightly builds

Since LCG nightly builds change configuration very often, we cannot provide static toolchain files, so a generic one is provided: special/lcg-nightly.cmake.

To use it, it's mandatory to specify the file defining the LCG nightly slot, via the variable LCG_EXTERNALS_FILE (either as CMake variable or environment variable):

cmake -S . -B build -DLCG_EXTERNALS_FILE=/cvmfs/sft-nightlies.cern.ch/lcg/nightlies/dev4/Mon/LCG_externals_x86_64-centos7-gcc9-opt.txt \
    -DCMAKE_TOOLCHAIN_FILE=/opt/lcg-toolchains/special/lcg-nightly.cmake