[Git][ghc/ghc][wip/simplifier-tweaks] 2 commits: Simplifier improvements

Fri Mar 22 16:12:09 UTC 2024

Simon Peyton Jones pushed to branch wip/simplifier-tweaks at Glasgow Haskell Compiler / GHC

Commits:
c4eb9d78 by Simon Peyton Jones at 2024-03-22T15:57:29+00:00
Simplifier improvements

This MR started as: allow the simplifer to do more in one pass,
arising from places I could see the simplifier taking two iterations
where one would do.  But it turned into a larger project, because
these changes unexpectedly made inlining blow up, especially join
points in deeply-nested cases.

The main changes are below.  There are also many new or rewritten Notes.

Avoiding simplifying repeatedly
~~~~~~~~~~~~~~~
See Note [Avoiding simplifying repeatedly]

* The SimplEnv now has a seInlineDepth field, which says how deep
  in unfoldings we are.  See Note [Inline depth] in Simplify.Env.
  Currently used only for the next point: avoiding repeatedly
  simplifying coercions.

* Avoid repeatedly simplifying coercions.
  see Note [Avoid re-simplifying coercions] in Simplify.Iteration
  As you'll see from the Note, this makes use of the seInlineDepth.

* Allow Simplify.Iteration.simplAuxBind to inline used-once things.
  This is another part of Note [Post-inline for single-use things], and
  is really good for reducing simplifier iterations in situations like
      case K e of { K x -> blah }
  wher x is used once in blah.

* Make GHC.Core.SimpleOpt.exprIsConApp_maybe do some simple case
  elimination.  Note [Case elim in exprIsConApp_maybe]

postInlineUnconditionally
~~~~~~~~~~~~~~~~~~~~~~~~~
* Allow Simplify.Utils.postInlineUnconditionally to inline variables
  that are used exactly once. See Note [Post-inline for single-use things].

* Do not postInlineUnconditionally join point, ever.
  Doing so does not reduce allocation, which is the main point,
  and with join points that are used a lot it can bloat code.
  See point (1) of Note [Duplicating join points] in
  GHC.Core.Opt.Simplify.Iteration.

* Do not postInlineUnconditionally a strict (demanded) binding.
  It will not allocate a thunk (it'll turn into a case instead)
  so again the main point of inlining it doesn't hold.  Better
  to check per-call-site.

* Improve occurrence analyis for bottoming function calls, to help
  postInlineUnconditionally.  See Note [Bottoming function calls]
  in GHC.Core.Opt.OccurAnal

Inlining generally
~~~~~~~~~~~~~~~~~~
* In GHC.Core.Opt.Simplify.Utils.interestingCallContext,
  use RhsCtxt NonRecursive (not BoringCtxt) for a plain-seq case.
  See Note [Seq is boring]

* In GHC.Core.Opt.Simplify.Utils.interestingArg,
  - return ValueArg for OtherCon [c1,c2, ...], but
  - return NonTrivArg for OtherCon []
  This makes a function a little less likely to inline if all we
  know is that the argument is evaluated, but nothing else.

* isConLikeUnfolding is no longer true for OtherCon {}.
  This propagates to exprIsConLike.  Con-like-ness has /positive/
  information.

Join points
~~~~~~~~~~~
* Be very careful about inlining join points.
  See these two long Notes
    Note [Duplicating join points] in GHC.Core.Opt.Simplify.Iteration
    Note [Inlining join points] in GHC.Core.Opt.Simplify.Inline

* When making join points, don't do so if the join point is so small
  it will immediately be inlined; check uncondInlineJoin.

* In GHC.Core.Opt.Simplify.Inline.tryUnfolding, improve the inlining
  heuristics for join points. In general we /do not/ want to inline
  join points /even if they are small/.  See Note [Duplicating join points]
  GHC.Core.Opt.Simplify.Iteration.

  But sometimes we do: see Note [Inlining join points] in
  GHC.Core.Opt.Simplify.Inline.

* Do not add an unfolding to a join point at birth.  This is a tricky one
  and has a long Note [Do not add unfoldings to join points at birth]
  It shows up in two places
  - In `mkDupableAlt` do not add an inlining
  - (trickier) In `simplLetUnfolding` don't add an unfolding for a
    fresh join point
  I am not fully satisifed with this, but it works and is well documented.

* In GHC.Core.Unfold.sizeExpr, make jumps small, so that we don't penalise
  having a non-inlined join point.

- - - - -
d66a7c83 by Simon Peyton Jones at 2024-03-22T15:57:29+00:00
Testsuite message changes from simplifier improvements

- - - - -

30 changed files:

- compiler/GHC/Core.hs
- compiler/GHC/Core/Opt/OccurAnal.hs
- compiler/GHC/Core/Opt/Simplify.hs
- compiler/GHC/Core/Opt/Simplify/Env.hs
- compiler/GHC/Core/Opt/Simplify/Inline.hs
- compiler/GHC/Core/Opt/Simplify/Iteration.hs
- compiler/GHC/Core/Opt/Simplify/Utils.hs
- compiler/GHC/Core/Opt/Stats.hs
- compiler/GHC/Core/Unfold.hs
- compiler/GHC/Core/Unfold/Make.hs
- compiler/GHC/IfaceToCore.hs
- compiler/GHC/Types/Id.hs
- compiler/GHC/Types/Tickish.hs
- testsuite/tests/arityanal/should_compile/Arity02.stderr
- testsuite/tests/arityanal/should_compile/Arity09.stderr
- testsuite/tests/arityanal/should_compile/Arity13.stderr
- testsuite/tests/cpranal/should_compile/T18401.stderr
- testsuite/tests/dmdanal/sigs/T21081.stderr
- testsuite/tests/driver/inline-check.stderr
- testsuite/tests/lib/integer/Makefile
- testsuite/tests/numeric/should_compile/T19641.stderr
- testsuite/tests/perf/compiler/T15630.hs
- + testsuite/tests/perf/compiler/T15630a.hs
- testsuite/tests/perf/compiler/all.T
- testsuite/tests/simplCore/should_compile/T15631.stdout
- testsuite/tests/simplCore/should_compile/T18013.stderr
- − testsuite/tests/simplCore/should_compile/T18730.hs
- − testsuite/tests/simplCore/should_compile/T18730_A.hs
- testsuite/tests/simplCore/should_compile/T20103.stderr
- testsuite/tests/simplCore/should_compile/T22428.stderr

The diff was not included because it is too large.

View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/da989ef8dc838a0a78e94c43a3d04e5bd9789e69...d66a7c832871a7cb537f4f03d555a469ffcb6a31

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/da989ef8dc838a0a78e94c43a3d04e5bd9789e69...d66a7c832871a7cb537f4f03d555a469ffcb6a31
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