[Git][ghc/ghc][wip/T23083] 5 commits: exprIsTrivial: Factor out shared implementation

[Sebastian Graf (@sgraf812)]

Sebastian Graf pushed to branch wip/T23083 at Glasgow Haskell Compiler / GHC


Commits:
c8000b90 by Sebastian Graf at 2023-04-13T10:09:31+02:00
exprIsTrivial: Factor out shared implementation

The duplication between `exprIsTrivial` and `getIdFromTrivialExpr_maybe` has
been bugging me for a long time.

This patch introduces an inlinable worker function `trivial_expr_fold` acting
as the single, shared decision procedure of triviality. It "returns" a
Church-encoded `Maybe (Maybe Id)`, so when it is inlined, it fuses to similar
code as before.
(Better code, even, in the case of `getIdFromTrivialExpr` which presently
allocates a `Just` constructor that cancels away after this patch.)

- - - - -
58ca811d by Sebastian Graf at 2023-04-13T11:12:49+02:00
Simplify: Simplification of arguments in a single function

The Simplifier had a function `simplArg` that wasn't called in `rebuildCall`,
which seems to be the main way to simplify args. Hence I consolidated the code
path to call `simplArg`, too, renaming to `simplLazyArg`.

- - - - -
eed7c4d1 by Sebastian Graf at 2023-04-13T11:12:49+02:00
Core.Ppr: Omit case binder for empty case alternatives

A minor improvement to pretty-printing

- - - - -
27f108ac by Sebastian Graf at 2023-04-13T11:12:49+02:00
CorePrep: Do not eliminate EmptyCase, do it in StgToCmm instead

We eliminate EmptyCase by way of `cgCase e _ _ [] = cgExpr e` now.
The main reason is that it plays far better in conjunction with eta expansion
(as we aim to do for arguments in CorePrep, #23083), because we can discard
any arguments, `(case e of {}) eta == case e of {}`, whereas in `(e |> co) eta`
it's impossible to discard the argument.

It is also both much simpler to describe than the previous mechanism of emitting
an unsafe coercion and simpler to implement, removing quite a bit of commentary
and the `Bool` field of `CorePrepProv`.

- - - - -
62f77bf1 by Sebastian Graf at 2023-04-13T11:12:49+02:00
CorePrep: Eta expand arguments (#23083)

Previously, we'd only eta expand let bindings and lambdas,
now we'll also eta expand arguments such as in T23083:
```hs
g f h = f (h `seq` (h $))
```
Unless `-fpedantic-bottoms` is set, we'll now transform to
```hs
g f h = f (\eta -> h eta)
```
in CorePrep.

See the new `Note [Eta expansion of arguments in CorePrep]` for the details.

We only do this optimisation with -O2 because we saw 2-3% ghc/alloc regressions
in T4801 and T5321FD.

Fixes #23083.

- - - - -


30 changed files:

- compiler/GHC/Core.hs
- compiler/GHC/Core/Coercion.hs
- compiler/GHC/Core/Coercion/Opt.hs
- compiler/GHC/Core/FVs.hs
- compiler/GHC/Core/Lint.hs
- compiler/GHC/Core/Opt/Simplify/Iteration.hs
- compiler/GHC/Core/Ppr.hs
- compiler/GHC/Core/TyCo/FVs.hs
- compiler/GHC/Core/TyCo/Rep.hs
- compiler/GHC/Core/TyCo/Subst.hs
- compiler/GHC/Core/TyCo/Tidy.hs
- compiler/GHC/Core/Type.hs
- compiler/GHC/Core/Utils.hs
- compiler/GHC/CoreToIface.hs
- compiler/GHC/CoreToStg.hs
- compiler/GHC/CoreToStg/Prep.hs
- compiler/GHC/Driver/Config/CoreToStg/Prep.hs
- compiler/GHC/Driver/Flags.hs
- compiler/GHC/Driver/Session.hs
- compiler/GHC/Iface/Syntax.hs
- compiler/GHC/Iface/Type.hs
- compiler/GHC/IfaceToCore.hs
- compiler/GHC/Stg/CSE.hs
- compiler/GHC/Stg/Unarise.hs
- compiler/GHC/StgToCmm/Expr.hs
- compiler/GHC/Tc/TyCl/Utils.hs
- compiler/GHC/Tc/Utils/TcMType.hs
- docs/users_guide/using-optimisation.rst
- testsuite/tests/corelint/T21115b.stderr
- testsuite/tests/deSugar/should_compile/T2431.stderr


The diff was not included because it is too large.


View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/4f0eb801500744770cc6bfef401229113c28a8c4...62f77bf143492f6d983b9533978c21ef920278bd

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/4f0eb801500744770cc6bfef401229113c28a8c4...62f77bf143492f6d983b9533978c21ef920278bd
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