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proposal

Generate: spack:packages:all:prefer: ['%[when=%c] c=[{compilers[0]}] %[when=%cxx] cxx={compilers[0]} %[when=%fortran] fortran={compilers[0]}']

The user can then manually customise specs, e.g. %fortran=nvhpc, on a per-package basis.

This approach is a little ad-hoc, but gets the job done for now.

For a future change, I would propose adding a new toolchain field to the field that lets the recipe author provide a specific toolchain that would be implemented as a Spack toolchain or using a require

default:
  compiler: [gcc, nvhpc]
  toolchain:
    c: gcc
    cxx: gcc
    fortran: nvhpc
  mpi:
    spec: cray-mpich@8.1.30
    gpu: cuda
  specs:
    ...

By not using "spack syntax" for the toolchain spec, we would have the freedom to generate an elaborate require statement, or create a proper toolchain.

working

TODO:

There are two ways to set up a multi compiler toolchain

  • set spack:packages:all:prefer to prefer the first compiler (trying to make it select compilers doesn't have any effect)
    • then add %fortran=nvhpc to individual packages
    • if applied to one package, this will propogate to other fortran packages (probably via nvhpc)
    • note that applying this to cray-mpich does the trick
  • set spack:packages:all:require;-one_of to hard-code the specific c, cxx and fortran compilers (see below)
    • then the user does not override this on individual specs

The best spot to specify this is probably in the compiler field of environments.yaml in a recipe, e.g.:

  compiler:
        c: gcc
        cxx: gcc
        fortran: nvhpc
    mode: # one of prefer, require

This would be easy to match to '%[when=%c] c=gcc %[when=%cxx] cxx=gcc %[when=%fortran] fortran=nvhpc', and push into require:one_of

Note the docs on toolchains: https://spack.readthedocs.io/en/latest/advanced_topics.html#defining-and-using-toolchains

note from the PR suggests something like:

packages:
  all:
    requires:
    - spec: %[virtuals=c] gcc
      when: %c
    - spec: %[virtuals=cxx] gcc
      when: %cxx
    - spec: %[virtuals=fortran] nvhpc
      when: %fortran

The syntax in the PR might a bit out of date, but we could aim to generate similar text? Or, we can create a spack:toolchain: field that defines a toollchain, and use that toolchain everywhere.

spack:
  specs: []
  packages:
    all:
      # pick one of the following (require will force everything - no need to override anything
      require:
      - one_of: ['%[when=%c] c=gcc %[when=%cxx] cxx=gcc %[when=%fortran] fortran=nvhpc']
      # it is not clear whether this actually does anything (other than subtly pushing spack towards gcc)
      # we find that it setting, e.g., fortran=nvhpc, changes concretisation, but does not actaully use nvfortran
      # with this option, users can 
      prefer: ['%[when=%c] c=gcc %[when=%cxx] cxx=gcc %[when=%fortran] fortran=gcc']
      variants: ['+mpi', '+cuda', 'cuda_arch=90']
    mpi:
      require: ...