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Eliminate `stg_stackDecode` and implement stack decoding via iterator based decoder

Hannes Siebenhandl requested to merge wip/fendor/remove-stg_stackDecode into master

Closes #24013 (closed)

The stack decoding logic in ghc-heap is more sophisticated than the one currently employed in CloneStack. We want to use the stack decoding implementation from ghc-heap in base. We cannot simply depend on ghc-heap in base due do bootstrapping issues. Thus, we move the code that is necessary to implement stack decoding to ghc-internal. This is the right location, as we don't want to add a new API to base.

Moving the stack decoding logic and re-exposing it in ghc-heap is insufficient, though, as we have a dependency cycle between.

  • ghc-heap depends on stage1:ghc-internal
  • stage0:ghc depends on stage0:ghc-heap

To fix this, we remove ghc-heap from the set of stage0 dependencies.

This is not entirely straight-forward, as a couple of boot dependencies, such as ghci depend on ghc-heap. Luckily, the boot compiler of GHC is now >=9.10, so we can migrate ghci to use ghc-internal instead of ghc-heap, which already exports the relevant modules.

However, we cannot 100% remove ghc's dependency on ghc-heap, since when we compile stage0:ghc, stage1:ghc-internal is not yet available. Thus, when we compile with the boot-compiler, we still depend on an older version of ghc-heap, and only use the modules from ghc-internal, if the ghc-internal version is recent enough.

Edited by Hannes Siebenhandl

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