Removing Hoopl dependency?

[Michal Terepeta]

Hi all,

I was looking at removing the `BlockId` type synonym in favor of
Hoopl's `Label` (there was already a TODO and it is a bit confusing).
But once I've started making the changes, I've realized that in a
bunch of places this makes the code *less* readable. Mostly because of
`CLabel` (sounds similar but is something quite different and having
to rename local variables from `label` to `clabel` is not great).

I started to look at alternatives and noticed that in general the
interface between GHC and Hoopl is quite noisy and confusing:
- Hoopl has `Label` which is GHC's `BlockId` but different than
  GHC's `CLabel`
- Hoopl has `Unique` which is different than GHC's `Unique`
- Hoopl has `Unique{Map,Set}` which are different than GHC's
  `Uniq{FM,Set}`
- GHC has its own specialized copy of `Dataflow`, so `cmm/Hoopl` is
  needed just to filter the exposed functions (filter out some of the
  Hoopl's and add the GHC ones).
- Working in `cmm/` requires constant switching between GHC code and
  Hoopl (`CmmNode`/`CmmGraph`/`CmmBlock` and dataflow stuff is in GHC,
  the actual implementation of `Block`/`Graph` are defined in Hoopl,
  etc.)

GHC is actually using only a small subset of Hoopl (e.g., the fixpoint
computation is copied/specialized: `cmm/Hoopl/Dataflow`). So I was
wondering - maybe it's worth to simply drop the dependency on Hoopl?
(and copy the code that is actually necessary in GHC)
I've done an experiment in [1] (to see how much we'd need to actually
copy) and I really like the result:
- We can remove one external dependency and git submodule at the
  cost of only 5 new modules in `cmm/Hoopl` (net gain of only 4
  modules: we add 5 new but can remove `cmm/Hoopl`, which is no longer
  needed)
- We should be able to fix all of the above issues and make the code
  easier to understand (less code, everything in one repo, fewer
  concepts).
- It's going to be easier to change things since we don't need to
  worry about changing the public interface of Hoopl (it's a
  standalone package on Hackage and other people already depend on the
  current behavior).

What do you think? Does anyone think we shouldn't do this?

Thanks,
Michal

[1] Branch: https://github.com/michalt/ghc/tree/hoopl/no-hoopl
    Diff: https://github.com/ghc/ghc/compare/master...michalt:hoopl/no-hoopl
    For now I just copied the code/updated imports and didn't do any
    cleanups, but I'd be happy to do them in subsequent PRs
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