[GHC] #13379: Space leak / quadratic behavior when inlining

[GHC]

#13379: Space leak / quadratic behavior when inlining
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        Reporter:  jberryman         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  Inlining
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13586            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"a1b753e8b1475659440f524b3e66dfbea31c5787/ghc"
 a1b753e8/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a1b753e8b1475659440f524b3e66dfbea31c5787"
 Cure exponential behaviour in the simplifier

 This patch nails a Bad Bug exposed in Trac #13379. Roughly,
 a deeply-nested application like
    f (f (f ....) ) )
 could make the simplifier go exponential -- without producing
 an exponential-sized result!

 The reason was that we
   - simplified a (big) function argument
   - then decided to inline the function
   - then preInilneUnconditionally the argument
   - and then re-simplified the big argument

 And if the "big argument" itself had a similar structure
 things could get very bad.

 Once I'd understood, it was easy to fix:

 * See Note Note [Avoiding exponential behaviour] for an overview

 * The key change is that Simplify.simplLam now as a case for
   (isSimplified dup). This is what removes the perf bug.

 * But I also made simplCast more parsimonious about simplifying,
   avoiding doing so when the coercion is Refl

 * And similarly I now try to avoid simplifying arguments
   where possible before applying rules.
   See Note [Trying rewrite rules]

 The latter two points tackle common cases, and in those cases make the
 simplifier take fewer iterations.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13379#comment:17>
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