I am also wary of us to deferring checking whole platforms and what not.
I think that's just kicking the can down the road, and will result in
more variance and uncertainty. It might be alright for those authoring
PRs, but it will make Ben's job keeping the system running even more
grueling.
Before getting into these complex trade-offs, I think we should focus on
the cornerstone issue that CI isn't incremental.
1. Building and testing happen together. When tests failure spuriously,
we also have to rebuild GHC in addition to re-running the tests.
That's pure waste. https://gitlab.haskell.org/ghc/ghc/-/issues/13897
tracks this more or less.
2. We don't cache between jobs. Shake and Make do not enforce
dependency soundness, nor cache-correctness when the build plan
itself changes, and this had made this hard/impossible to do safely.
Naively this only helps with stage 1 and not stage 2, but if we have
separate stage 1 and --freeze1 stage 2 builds, both can be
incremental. Yes, this is also lossy, but I only see it leading to
false failures not false acceptances (if we can also test the stage
1 one), so I consider it safe. MRs that only work with a slow full
build because ABI can so indicate.
The second, main part is quite hard to tackle, but I strongly believe
incrementality is what we need most, and what we should remain focused on.
John
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