Compile¶

Access¶

The ICON repository is hosted on the C2SM GitHub organisation. If you do not have access, please follow the instructions under How to get Access.

If you do not already have an SSH key set up for GitHub, but would like to do so, follow the instructions .

Since 2024, ICON is open-source and comes with semi-annual releases, which can be accessed via this public repository .

If you are an ICON developer, you should have access to the DKRZ GitLab, where the original ICON repository is hosted. All developments related to GPU go into the icon-nwp repository .

Configure and compile¶

Below you find instructions on how to compile different flavors of ICON on C2SM-supported machines.

Clone the ICON repository:

C2SM (latest release)DKRZ (latest release)DKRZ (icon-nwp master)

git clone -b release-2025.10 --recurse-submodules git@github.com:C2SM/icon.git

git clone -b release-2025.10-public --recurse-submodules https://gitlab.dkrz.de/icon/icon-model.git

git clone --recurse-submodules git@gitlab.dkrz.de:icon/icon-nwp.git

Säntis¶

Run the following after navigating into ICON root folder:

CPU compilationGPU compilation

UENV_VERSION=$(cat config/cscs/SANTIS_ENV_TAG)
uenv run ${UENV_VERSION} -- ./config/cscs/santis.cpu.nvhpc

UENV_VERSION=$(cat config/cscs/SANTIS_ENV_TAG)
uenv run ${UENV_VERSION} -- ./config/cscs/santis.gpu.nvhpc

User environments and out-of-source builds

If you have never used a uenv on Säntis, you need to create a uenv repo first:

uenv repo create

In case you are using the uenv version for the first time, you need to pull the image first:

uenv image pull $UENV_VERSION

Building out-of-source¶

Out-of-source builds are useful if you want to have two or more compiled versions of ICON in the same repository. To achieve that, you simply need to create separate folders in the ICON root folder and run the configure wrapper from there.

For example, if you want to compile ICON both for cpu and gpu, create those directories:

mkdir nvhpc_cpu
mkdir nvhpc_gpu

Then, navigate into the corresponding folder and source the configure wrapper for compilation:

cpugpu

UENV_VERSION=$(cat config/cscs/SANTIS_ENV_TAG)
cd nvhpc_cpu
uenv run ${UENV_VERSION} -- ./../config/cscs/santis.cpu.nvhpc

UENV_VERSION=$(cat config/cscs/SANTIS_ENV_TAG)
cd nvhpc_gpu
uenv run ${UENV_VERSION} -- ./../config/cscs/santis.gpu.nvhpc

Euler¶

Navigate into the ICON root folder.

Now, set up your spack instance:

# Setup spack
SPACK_TAG=$(cat "config/ethz/SPACK_TAG_EULER")
git clone --depth 1 --recurse-submodules --shallow-submodules -b ${SPACK_TAG} https://github.com/C2SM/spack-c2sm.git
. spack-c2sm/setup-env.sh

Euler Support recommends to compile code on compute nodes. There, we can take advantage of multi-core compiling. However, we need to load the module eth_proxy, which enables connecting from a compute node to an external service, e.g. GitHub or GitLab.

module load eth_proxy

Now, activate the spack environment and build ICON:

# Build ICON
# For out-of-source builds: navigate into the build folder and 
# adapt the path to the Spack environment in the following
spack env activate -d config/ethz/spack/${SPACK_TAG}/euler_cpu_gcc
srun -N 1 -n 12 --mem-per-cpu=1G spack install -j 12

Eiger¶

Pull and start the Environment:

uenv image pull prgenv-gnu/24.11:v1
uenv start --view=spack prgenv-gnu/24.11:v1

Clone and setup Spack

git clone -b releases/v0.23 https://github.com/spack/spack
source spack/share/spack/setup-env.sh

Set spack system config path:

export SPACK_SYSTEM_CONFIG_PATH=/user-environment/config/

Modify compiler flags for ICON:

Edit the file: spack/var/spack/repos/builtin/packages/icon/package.py

Add the following under the line if self.compiler.name == "gcc"::

flags["LDFLAGS"].append("-Wl,--copy-dt-needed-entries")
flags["ICON_FCFLAGS"].append("-fallow-argument-mismatch")
flags["ICON_OCEAN_FCFLAGS"].extend([
    "-O3",
    "-fno-tree-loop-vectorize",
    "-fallow-argument-mismatch"
])

Create and configure Spack environment for ICON

spack env create my-icon

Modify the spack.yaml file in your my-icon environment. Example:

# This is a Spack Environment file.
#
# It describes a set of packages to be installed, along with
# configuration settings.
spack:
  specs:
  - icon+art+rte-rrtmgp+grib2 -ocean ~coupling
  view: true
  concretizer:
    unify: true
  develop:
    icon:
      spec: icon +rte-rrtmgp +art +grib2 -ocean ~coupling
      path: /capstor/store/cscs/empa/em05/ckeller/icon-kit
  compilers:
  - compiler:
      spec: gcc@=7.5.0
      paths:
        cc: /usr/bin/gcc
        cxx: /usr/bin/g++
        f77: /usr/bin/gfortran
        fc: /usr/bin/gfortran
      flags: {}
      operating_system: sles15
      target: x86_64
      modules: []
      environment: {}
      extra_rpaths: []
  - compiler:
      spec: gcc@=12.3.0
      paths:
        cc: /usr/bin/gcc-12
        cxx: /usr/bin/g++-12
        f77: /usr/bin/gfortran-12
        fc: /usr/bin/gfortran-12
      flags: {}
      operating_system: sles15
      target: x86_64
      modules: []
      environment: {}
      extra_rpaths: []

Path to ICON code

Ensure develop.icon.path points to your local ICON source code.

Activate the environment:

spack env activate path/to/my-icon

Concretize and install packages:

spack concretize
spack install

concretize resolves dependencies, and install builds the packages.