[Git][ghc/ghc][wip/refactor-demand] 2 commits: Demand: Interleave usage and strictness demands (#18903)

[Sebastian Graf]

Sebastian Graf pushed to branch wip/refactor-demand at Glasgow Haskell Compiler / GHC


Commits:
07723a60 by Sebastian Graf at 2020-11-19T13:48:17+01:00
Demand: Interleave usage and strictness demands (#18903)

As outlined in #18903, interleaving usage and strictness demands not
only means a more compact demand representation, but also allows us to
express demands that we weren't easily able to express before.

Call demands are *relative* in the sense that a call demand `Cn(cd)`
on `g` says "`g` is called `n` times. *Whenever `g` is called*, the
result is used according to `cd`". Example from #18903:

```hs
h :: Int -> Int
h m =
  let g :: Int -> (Int,Int)
      g 1 = (m, 0)
      g n = (2 * n, 2 `div` n)
      {-# NOINLINE g #-}
  in case m of
    1 -> 0
    2 -> snd (g m)
    _ -> uncurry (+) (g m)
```

Without the interleaved representation, we would just get `L` for the
strictness demand on `g`. Now we are able to express that whenever
`g` is called, its second component is used strictly in denoting `g`
by `1C1(P(1P(U),SP(U)))`. This would allow Nested CPR to unbox the
division, for example.

Fixes #18903.
While fixing regressions, I also discovered and fixed #18957.

Metric Decrease:
    T13253-spj

- - - - -
fd1fa0fa by Sebastian Graf at 2020-11-19T13:53:49+01:00
Update user's guide entry on demand analysis and worker/wrapper

The demand signature notation has been undocumented for a long time.
The only source to understand it, apart from reading the `Outputable`
instance, has been an outdated wiki page.

Since the previous commits have reworked the demand lattice, I took
it as an opportunity to also write some documentation about notation.

- - - - -


30 changed files:

- compiler/GHC/Builtin/primops.txt.pp
- compiler/GHC/Core/Opt/CprAnal.hs
- compiler/GHC/Core/Opt/DmdAnal.hs
- compiler/GHC/Core/Opt/Pipeline.hs
- compiler/GHC/Core/Opt/SetLevels.hs
- compiler/GHC/Core/Opt/Simplify.hs
- compiler/GHC/Core/Opt/Simplify/Utils.hs
- compiler/GHC/Core/Opt/SpecConstr.hs
- compiler/GHC/Core/Opt/WorkWrap/Utils.hs
- compiler/GHC/CoreToStg.hs
- compiler/GHC/CoreToStg/Prep.hs
- compiler/GHC/Iface/Syntax.hs
- compiler/GHC/Stg/Lift/Analysis.hs
- compiler/GHC/Types/Demand.hs
- compiler/GHC/Types/Id.hs
- compiler/GHC/Types/Id/Info.hs
- compiler/GHC/Types/Id/Make.hs
- compiler/GHC/Utils/Outputable.hs
- docs/users_guide/9.2.1-notes.rst
- docs/users_guide/debugging.rst
- docs/users_guide/exts/instances.rst
- docs/users_guide/using-optimisation.rst
- testsuite/tests/arityanal/should_compile/Arity01.stderr
- testsuite/tests/arityanal/should_compile/Arity02.stderr
- testsuite/tests/arityanal/should_compile/Arity03.stderr
- testsuite/tests/arityanal/should_compile/Arity04.stderr
- testsuite/tests/arityanal/should_compile/Arity05.stderr
- testsuite/tests/arityanal/should_compile/Arity09.stderr
- testsuite/tests/arityanal/should_compile/Arity11.stderr
- testsuite/tests/arityanal/should_compile/Arity14.stderr


The diff was not included because it is too large.


View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/0195df964354808e4c8574d518f21f35fef0033e...fd1fa0fa20f6b431ef1f0fd8d48313632385dcfb

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/0195df964354808e4c8574d518f21f35fef0033e...fd1fa0fa20f6b431ef1f0fd8d48313632385dcfb
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