Address Issues with Face Selectors

[Thomas Bendall (tommbendall)]

PR Summary

Sci/Tech Reviewer: iboutle
Code Reviewer: Erica Neininger (@ericaneininger)

This PR revamps the face_selector infrastructure, which ensure that when looping over facse within columns, faces are only looped over once. This PR fixes a number of flaws and deficiencies with them. It does this by changing how the face_selector fields are computed, and changing the values they can take.

In particular:

• columns previously always iterated over W and S faces. Now they can just iterate over N, E or no faces if necessary
• every face will now only be iterated over once, irrespective of the MPI decomposition (there were previously decompositions in which faces were iterated over twice)

This fixes a number of problems:

• as recorded in The combination of threading and MPI regions that cross cubed-sphere panel edges is not bit-reproducible lfric_apps#294, the combination of threading with 3-panel cubed-sphere decompositions was not bit-reproducible
• it may now be possible to run safely without annexed DoFs (addressing the above issue was necessary if not sufficient)
• the face_selector fields now take sensible values in halos, which may allow some kernels to used with redundant computation

Linked-To

• LFRic-Apps #458

Details

• components/science/source/kernel/algebra/sci_face_selector_support_mod.F90:
  • introduces new method for making face_selector
  • adds a routine for decoding the face_selector values, to be used in kernels that loop over faces within a column
• components/science/source/kernel/algebra/sci_face_selector_kernel_mod.F90: rewritten to use the new method (from the support module)
• components/science/source/kernel/algebra/sci_face_selector_halo_kernel_mod.F90: a new kernel to allow calculation of face_selector fields into halo regions
• components/science/source/psy/sci_psykal_light_mod.f90: adds a simple deep_int_setval_c routine, setting the value of an integer field into the halos. This could hypothetically be done as an adjustment to int_setval_c through a psyclone optimisation script, but as this is not an optimisation and is necessary scientifically, I chose to add a specific psykal_light routine
• components/science/source/algorithm/sci_geometric_constants_mod.x90: updated for the new computation of the face selectors
• other kernels: use the new method to decode the face selectors

Documentation

The old and new methods are summarised and illustrated in the attached PDF:
main.pdf

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
• If required (e.g. API changes) I have also run the LFRic Apps test suite using this branch
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
• I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

trac.log

Test Suite Results - lfric_core - face_selectors_core/run3

Suite Information

Item	Value
Suite Name	face_selectors_core/run3
Suite User	thomas.bendall
Workflow Start	2026-04-28T10:55:51
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	tommbendall/lfric_core@TBendall/face_selectors	False
SimSys_Scripts	MetOffice/SimSys_Scripts@2025.12.1	True

Task Information

✅ succeeded tasks - 384

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• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted

AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

PSyclone Approval

• If you have edited any PSyclone-related code (e.g. PSyKAl-lite, Kernel interface, optimisation scripts, LFRic data structure code) then please contact the TCD Team

Sci/Tech Review

• I understand this area of code and the changes being added
• The proposed changes correspond to the pull request description
• Documentation is sufficient (do documentation papers need updating)
• Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
• Related Issues have been properly linked and addressed
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[iboutle]

Code changes look fine to me. Only question is having written a nice document describing how the new method works, is it worth adding this to the "documentation" folder on main, so that it's always there to describe how these work?

[Erica Neininger (ericaneininger)]

Looks good to me.
There are a just a couple of tidying up suggestions, commented in line.

[Thomas Bendall (tommbendall)]

Erica Neininger (@ericaneininger) thanks for the review and the helpful suggestions. I think I've made them all of them, so this is back to you for review