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<h3>
Ben Gamari pushed to branch wip/T18078
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/ca7ff81c4b8b9888f6d415b9f7cbdd4e6b16e05e">ca7ff81c</a></strong>
<div>
<span>by Simon Peyton Jones</span>
<i>at 2020-05-27T11:31:39-04:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">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%

Metric Decrease:
  T15164
</pre>
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<h4>30 changed files:</h4>
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<a href="#0887cf39c5cdf9cf8d6758f410d7dab3023c0d77">
compiler/GHC/Builtin/Names.hs
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<a href="#783e5dae6e86931f06700fc088fb7d48c8a07386">
compiler/GHC/Core/Coercion.hs
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<a href="#446b4afbe6836b950115a619acea24ddde19dc90">
compiler/GHC/Core/Opt/CSE.hs
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<a href="#c1bf6b849619bc8d7ef37a97d993dbfaa9966f7b">
compiler/GHC/Core/Opt/CprAnal.hs
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compiler/GHC/Core/Opt/Driver.hs
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compiler/GHC/Core/Opt/Simplify.hs
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<a href="#48fbb5cdea308650de5756521feb28ec68819b9b">
compiler/GHC/Core/Opt/Simplify/Utils.hs
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compiler/GHC/Core/Opt/SpecConstr.hs
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compiler/GHC/Core/Opt/WorkWrap.hs
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compiler/GHC/Core/SimpleOpt.hs
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compiler/GHC/Core/Type.hs
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compiler/GHC/Core/Unfold.hs
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compiler/GHC/Core/Utils.hs
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compiler/GHC/CoreToStg/Prep.hs
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compiler/GHC/Types/Basic.hs
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compiler/GHC/Types/Id/Make.hs
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compiler/GHC/Utils/Binary.hs
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libraries/base/Unsafe/Coerce.hs
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testsuite/tests/codeGen/should_compile/debug.stdout
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<h5>The diff was not included because it is too large.</h5>

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