A migrator (or a hint) for moving from cross-compilation to native GHA builds

[mgorny]

Your question:

Now that we have native linux_aarch64, linux_ppc64le, osx_arm64 runners, I suppose it makes sense to start moving feedstocks away from cross-compiling and to native builds on these platforms. Especially that people seem to be still unaware that they don't need to add platform support via cross anymore.

What I'm thinking of is creating a migrator that would try switching feedstocks from cross compilation to native builds for the platforms that support it. For example, from:

provider:
  linux_64: github_actions
build_platform:
  linux_aarch64: linux_64

to:

provider:
  linux_64: github_actions
  linux_aarch64: github_actions

Alternatively, if migrator is too aggressive, we could have conda-smithy emit a hint that native builds could be used. However, I think migrator is better since it saves users from doing this manually.

[jaimergp]

What about the build script logic or source patches that were added to support cross compilation? How do we remove those automatically? I'm in favor of doing of promoting native builds (especially if it improves testing), but I'd be careful of doing this too quickly because there may be many edge cases.

[mfansler]

Perhaps a migration of the emulating ones might be a place to start - those QEMU runs are hugely inefficient. I think they shouldn't have logic/patch issues @jaimergp raises, and could serve as an initial test workload for the runners.

[jaimergp]

That's an excellent idea, yes!

[isuruf]

What about the build script logic or source patches that were added to support cross compilation?

These should stay as they are. They'll be useful for adding new platforms

[mgorny]

What about the build script logic or source patches that were added to support cross compilation? How do we remove those automatically?

I'm not sure if I understand correctly, but I'd say we keep them by default. If they are really cumbersome, the maintainers can make the call to drop them.

Just to be clear, I'm not talking of admin migration here, but either an explicit migrator or minimigrator, so that we test that we don't break anything in the process.

[mfansler]

"Now that we have native linux_aarch64, linux_ppc64le, ..."

Is the linux_ppc64le native? In the Start Docker build logs on GHA, I see:

WARNING: The requested image's platform (linux/ppc64le) does not match the detected host platform (linux/amd64/v3) and no specific platform was requested

and speed is a crawl 🐌

Seems cross-compilation is still the best option there (when it works).

[mgorny]

Yeah, sorry, I initially recalled wrong, then checked, skipped it from the example and forgot that I already mentioned it earlier in the text :-).