[Git][ghc/ghc][wip/T18078] Implement cast worker/wrapper properly

[Sebastian Graf]

Sebastian Graf pushed to branch wip/T18078 at Glasgow Haskell Compiler / GHC


Commits:
c221207a by Simon Peyton Jones at 2020-06-09T12:04:31+02:00
Implement cast worker/wrapper properly

The cast worker/wrapper transformation transforms
   x = e |> co
into
   y = e
   x = y |> co

This is done by the simplifier, but we were being
careless about transferring IdInfo from x to y,
and about what to do if x is a NOINLNE function.
This resulted in a series of bugs:
     #17673, #18093, #18078.

This patch fixes all that:

* Main change is in GHC.Core.Opt.Simplify, and
  the new prepareBinding function, which does this
  cast worker/wrapper transform.
  See Note [Cast worker/wrappers].

* There is quite a bit of refactoring around
  prepareRhs, makeTrivial etc.  It's nicer now.

* Some wrappers from strictness and cast w/w, notably those for
  a function with a NOINLINE, should inline very late. There
  wasn't really a mechanism for that, which was an existing bug
  really; so I invented a new finalPhase = Phase (-1).  It's used
  for all simplifier runs after the user-visible phase 2,1,0 have
  run.  (No new runs of the simplifier are introduced thereby.)

  See new Note [Compiler phases] in GHC.Types.Basic;
  the main changes are in GHC.Core.Opt.Driver

* Doing this made me trip over two places where the AnonArgFlag on a
  FunTy was being lost so we could end up with (Num a -> ty)
  rather than (Num a => ty)
    - In coercionLKind/coercionRKind
    - In contHoleType in the Simplifier

  I fixed the former by defining mkFunctionType and using it in
  coercionLKind/RKind.

  I could have done the same for the latter, but the information
  is almost to hand.  So I fixed the latter by
    - adding sc_hole_ty to ApplyToVal (like ApplyToTy),
    - adding as_hole_ty to ValArg (like TyArg)
    - adding sc_fun_ty to StrictArg
  Turned out I could then remove ai_type from ArgInfo.  This is
  just moving the deck chairs around, but it worked out nicely.

  See the new Note [AnonArgFlag] in GHC.Types.Var

* When looking at the 'arity decrease' thing (#18093) I discovered
  that stable unfoldings had a much lower arity than the actual
  optimised function.  That's what led to the arity-decrease
  message.  Simple solution: eta-expand.

  It's described in Note [Eta-expand stable unfoldings]
  in GHC.Core.Opt.Simplify

* I also discovered that unsafeCoerce wasn't being inlined if
  the context was boring.  So (\x. f (unsafeCoerce x)) would
  create a thunk -- yikes!  I fixed that by making inlineBoringOK
  a bit cleverer: see Note [Inline unsafeCoerce] in GHC.Core.Unfold.

  I also found that unsafeCoerceName was unused, so I removed it.

I made a test case for #18078, and a very similar one for #17673.

The net effect of all this on nofib is very modest, but positive:

--------------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
--------------------------------------------------------------------------------
           anna          -0.4%     -0.1%     -3.1%     -3.1%      0.0%
 fannkuch-redux          -0.4%     -0.3%     -0.1%     -0.1%      0.0%
       maillist          -0.4%     -0.1%     -7.8%     -1.0%    -14.3%
      primetest          -0.4%    -15.6%     -7.1%     -6.6%      0.0%
--------------------------------------------------------------------------------
            Min          -0.9%    -15.6%    -13.3%    -14.2%    -14.3%
            Max          -0.3%      0.0%    +12.1%    +12.4%      0.0%
 Geometric Mean          -0.4%     -0.2%     -2.3%     -2.2%     -0.1%

All following metric decreases are compile-time allocation decreases
between -1% and -3%:

Metric Decrease:
  T5631
  T13701
  T14697
  T15164

- - - - -


30 changed files:

- compiler/GHC/Builtin/Names.hs
- compiler/GHC/Core/Coercion.hs
- compiler/GHC/Core/Opt/CSE.hs
- compiler/GHC/Core/Opt/CprAnal.hs
- compiler/GHC/Core/Opt/Driver.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.hs
- compiler/GHC/Core/SimpleOpt.hs
- compiler/GHC/Core/Type.hs
- compiler/GHC/Core/Unfold.hs
- compiler/GHC/Core/Utils.hs
- compiler/GHC/CoreToStg/Prep.hs
- compiler/GHC/Types/Basic.hs
- compiler/GHC/Types/Id/Make.hs
- compiler/GHC/Types/Var.hs
- compiler/GHC/Utils/Binary.hs
- libraries/base/Unsafe/Coerce.hs
- testsuite/tests/codeGen/should_compile/debug.stdout
- testsuite/tests/deSugar/should_compile/T2431.stderr
- testsuite/tests/perf/compiler/T16473.stdout
- testsuite/tests/simplCore/should_compile/T13143.stderr
- + testsuite/tests/simplCore/should_compile/T17673.hs
- + testsuite/tests/simplCore/should_compile/T17673.stderr
- + testsuite/tests/simplCore/should_compile/T18078.hs
- + testsuite/tests/simplCore/should_compile/T18078.stderr
- testsuite/tests/simplCore/should_compile/T3772.stdout
- testsuite/tests/simplCore/should_compile/T7360.stderr
- testsuite/tests/simplCore/should_compile/T7865.stdout


The diff was not included because it is too large.


View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c221207aa69092e9dff21310e797d0d35c47afde

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c221207aa69092e9dff21310e797d0d35c47afde
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