[GHC] #13890: Loss of inlining after strictness analysis

[GHC]

#13890: Loss of inlining after strictness analysis
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While investigating something else I saw this in the final `Tidy Core`
 after optimisation
 {{{
 join {
   wild_X5jU [Dmd=<S,1*U>]
     :: (# State# RealWorld, Array Int HValue #)
   [LclId[JoinId(0)],
    Unf=Unf{Src=InlineStable, TopLvl=False, Value=False,
            ConLike=False, WorkFree=False,
            Expandable=False,
            Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=False)
            Tmpl= jump $j1_s5RP w_s5Qs}]
   wild_X5jU = jump $j1_s5RP ww_s5Qw } in
 jump wild_X5jU;
 }}}
 That's ridiculous!  Why wasn't it inlined at its (only!) use site?

 It turns out that

 * The original `InlineStable` came from when `wild` was lambda-bound.
   See `Note [Case binders and join points]` in `Simplify.hs`

 * But then worker-wrapper let-binds that previously-lambda-boudn arg
   in `WwLib.mkWWstr_one`.  But it does not remove the unfolding.

   That's bad; in contrast, when we beta-reduce we are careful to
   remove the unfolding; see `Note [Zap unfolding when beta-reducing]`
   in `Simplify.hs`.

 * We don't `preInlineUnconditionally` things with stable unfoldings; see
 `Note [Stable unfoldings and preInlineUnconditionally]` in `SimplUtils`

 Net result: it doesn't get inlined at all!

 Easy fix: zap the unfolding in `mkWWstr_one` as well.

 I'm unhappy with the whole business of passing both boxed and
 unboxed versions, described in `Note [Case binders and join points]`.
 It smells wrong, and this ticket is another canary.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13890>
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