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author | Sebastian Graf <sebastian.graf@kit.edu> | 2022-02-17 18:11:24 +0100 |
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committer | Sebastian Graf <sebastian.graf@kit.edu> | 2022-04-19 12:47:52 +0200 |
commit | 79d69ad7548ef103974e77df2db6444ea031dd4b (patch) | |
tree | d74bd5e795e7f5fa640a2233cf9f556a475eb82d /compiler/GHC/Types/Id.hs | |
parent | 6810752a0764f63c3577b223bbfdc6e70dfcb33a (diff) | |
download | haskell-79d69ad7548ef103974e77df2db6444ea031dd4b.tar.gz |
Assume at least one evaluation for nested SubDemands (#21081, #21133)
See the new `Note [SubDemand denotes at least one evaluation]`.
A demand `n :* sd` on a let binder `x=e` now means
> "`x` was evaluated `n` times and in any program trace it is evaluated, `e` is
> evaluated deeply in sub-demand `sd`."
The "any time it is evaluated" premise is what this patch adds. As a result,
we get better nested strictness. For example (T21081)
```hs
f :: (Bool, Bool) -> (Bool, Bool)
f pr = (case pr of (a,b) -> a /= b, True)
-- before: <MP(L,L)>
-- after: <MP(SL,SL)>
g :: Int -> (Bool, Bool)
g x = let y = let z = odd x in (z,z) in f y
```
The change in demand signature "before" to "after" allows us to case-bind `z`
here.
Similarly good things happen for the `sd` in call sub-demands `Cn(sd)`, which
allows for more eta-reduction (which is only sound with `-fno-pedantic-bottoms`,
albeit).
We also fix #21085, a surprising inconsistency with `Poly` to `Call` sub-demand
expansion.
In an attempt to fix a regression caused by less inlining due to eta-reduction
in T15426, I eta-expanded the definition of `elemIndex` and `elemIndices`, thus
fixing #21345 on the go.
The main point of this patch is that it fixes #21081 and #21133.
Diffstat (limited to 'compiler/GHC/Types/Id.hs')
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