Update to c++20 for EAMxx and Homme by tcclevenger · Pull Request #8559 · E3SM-Project/E3SM

tcclevenger · 2026-07-14T20:08:30Z

Update EAMxx and Homme to C++20. Changes:

~~Update EKAT (now requires C++20)~~ (Updated as a part of Initial port to vista machine at TACC #8604)
Remove setting CXX_STANDARD in EAMxx
Remove setting CXX_STANDARD in machine/cime build files
Update homme standalone testing to use C++20

[non-BFB] Potentially for EAMxx/homme standalone cases

Copilot

Pull request overview

Update the build configuration across E3SM components to use C++20 (instead of C++17), aligning with the updated EKAT requirement and ensuring Homme/EAMxx default to the newer standard.

Changes:

Set the global components build to C++20.
Default EAMxx standalone builds to C++20.
Default Homme standalone builds to C++20.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
components/CMakeLists.txt	Switch global components build to C++20.
components/eamxx/CMakeLists.txt	Update EAMxx default C++ standard from 17 to 20.
components/homme/CMakeLists.txt	Set Homme standalone default C++ standard to 20.

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

Copilot

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

bartgol · 2026-07-15T22:47:15Z

-set(CMAKE_CXX_STANDARD 17)
+
+# Require C++20.
+set(CMAKE_CXX_STANDARD 20)


Do we need to set these? If a library uses c++20, they can use target_compile_features, no?

Ah, but we create targets in build_model.cmake, in a very code-agnostic way. So I guess for that reason we must set this here, in case some lib does use c++20?

I think this line should actually not be touch. I misunderstood it to mean we were setting everything to C++17, but actually EAMxx is free to take C++20 from EKAT, correct?

tcclevenger · 2026-07-16T20:51:29Z

I reverted back the changes in components/CMakeLists.txt. There is no reason each component must use C++20.

EAMxx will use C++20 due to it's dependence on EKAT.

As it stands, hommexx will not use C++20, but, if this PR is an effort to prepare us for updating to Kokkos 5, I'm now thinking again that we maybe should use C++20 here in hommexx instead of waiting till Kokkos 5 update. @bartgol thoughts?

bartgol · 2026-07-20T20:13:17Z

As it stands, hommexx will not use C++20, but, if this PR is an effort to prepare us for updating to Kokkos 5, I'm now thinking again that we maybe should use C++20 here in hommexx instead of waiting till Kokkos 5 update. @bartgol thoughts?

On one hand, I would say "no need to do it", but otoh C++20 adds new reserved keywords, so if linking against kk 5 will implicitly force to use c++20, we may as well test it now.

EAMxx will get CXX standard from it's dependance on EKAT core

…_config

This is unrelated to C++20, but exposed by updating to C++20 (perhaps different linking behavior?)

Update EAMxx and Homme to C++20. Changes: ~Update EKAT (now requires C++20)~ (Updated as a part of Initial port to vista machine at TACC #8604) Remove setting CXX_STANDARD in EAMxx Remove setting CXX_STANDARD in machine/cime build files Update homme standalone testing to use C++20 [non-BFB] Potentially for EAMxx/homme standalone cases

This bug was introduced in #8604 when updating ekat to use C++20. See #8559 (comment) for discussion of why this occurs. [BFB]

tcclevenger marked this pull request as draft July 14, 2026 20:08

tcclevenger requested a review from Copilot July 14, 2026 20:08

tcclevenger self-assigned this Jul 14, 2026

Copilot started reviewing on behalf of tcclevenger July 14, 2026 20:09

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Copilot AI mentioned this pull request Jul 14, 2026

Fix C++20 CMake build configuration: enforce standard and fix style #8560

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tcclevenger requested a review from Copilot July 14, 2026 20:28

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tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch from 304f23c to 817deee Compare July 14, 2026 20:29

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tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch from 817deee to 184fb10 Compare July 14, 2026 20:36

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tcclevenger requested a review from Copilot July 14, 2026 20:58

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tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch 3 times, most recently from 37d08d2 to 2f55695 Compare July 15, 2026 22:38

tcclevenger requested a review from bartgol July 15, 2026 22:38

tcclevenger marked this pull request as ready for review July 15, 2026 22:38

bartgol reviewed Jul 15, 2026

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bartgol mentioned this pull request Jul 15, 2026

Workflows: add more eamxx-v1 tests on ghci-snl runners #8564

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tcclevenger commented Jul 16, 2026

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Comment thread share/build/buildlib.ekat

tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch 2 times, most recently from 64ac70c to c905b19 Compare July 16, 2026 20:47

tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch from c905b19 to 12e6372 Compare July 20, 2026 18:40

rljacob added EAMxx C++ based E3SM atmosphere model (aka SCREAM) EKAT Machine Files labels Jul 21, 2026

tcclevenger changed the title ~~Update to c++20 (from c++17).~~ Update to c++20 for EAMxx and Homme Jul 29, 2026

tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch from 12e6372 to 3a616dc Compare July 29, 2026 18:40

tcclevenger requested a review from bartgol July 29, 2026 18:40

tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch 2 times, most recently from a1c5746 to 2bb7528 Compare July 29, 2026 20:39

tcclevenger commented Jul 30, 2026

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Comment thread components/eamxx/src/share/physics/tests/CMakeLists.txt

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tcclevenger requested a review from bartgol August 2, 2026 14:26

bartgol approved these changes Aug 3, 2026

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tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch from 0b3a6a9 to 8dbd570 Compare August 5, 2026 19:03

tcclevenger added 5 commits August 5, 2026 15:26

Remove CXX standarded being set in EAMxx

9dc1ddb

EAMxx will get CXX standard from it's dependance on EKAT core

Remove references to C++17 from machine files in homme/cmake and cime…

5df1be2

…_config

Remove default cxx standard from buildlib.ekat

120def3

Only set cxx standard in homme machine files

3978fad

Add missing EXCLUDE_MAIN_CPP from p3 test

705c5b7

This is unrelated to C++20, but exposed by updating to C++20 (perhaps different linking behavior?)

tcclevenger force-pushed the tcclevenger/update_to_cpp20 branch from fb6f47d to 705c5b7 Compare August 5, 2026 19:26

tcclevenger added the non-BFB PR makes roundoff changes to answers. label Aug 5, 2026

tcclevenger mentioned this pull request Aug 6, 2026

Add missing EXCLUDE_MAIN_CPP from p3 test #8611

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tcclevenger added a commit that referenced this pull request Aug 6, 2026

Merge branch 'tcclevenger/fix_p3_test_fail' into next (PR #8611)

2fa43da

This bug was introduced in #8604 when updating ekat to use C++20. See #8559 (comment) for discussion of why this occurs. [BFB]

tcclevenger added a commit that referenced this pull request Aug 6, 2026

Merge branch 'tcclevenger/fix_p3_test_fail' (PR #8611)

23311b9

This bug was introduced in #8604 when updating ekat to use C++20. See #8559 (comment) for discussion of why this occurs. [BFB]

tcclevenger merged commit 66395db into E3SM-Project:master Aug 6, 2026
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tcclevenger deleted the tcclevenger/update_to_cpp20 branch August 6, 2026 15:27

This was referenced Aug 13, 2026

Update Omega to C++20 E3SM-Project/Omega#405

Merged

Merge E3SM master into Omega develop E3SM-Project/Omega#521

Merged

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tcclevenger commented Jul 14, 2026 •

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