[GHC] #11029: Performance loss due to eta expansion

[GHC]

#11029: Performance loss due to eta expansion
-------------------------------------+-------------------------------------
           Reporter:  NeilMitchell   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given the attached file, at both {{{-O2}}} and {{{-O0}}}, GHC translates
 the function:

 {{{#!hs
 test1 [1,2,3,4,5,6,7,8,9,10] = \x -> x
 test1 _ = \x -> negate x
 }}}

 To be equivalent to:

 {{{#!hs
 test0 [1,2,3,4,5,6,7,8,9,10] x = x
 test0 _ x = negate x
 }}}

 When applied in a loop with something like:

 {{{#!hs
 map (test1 [1..]) [1..1000]
 }}}

 The eta-expanded variant is 3x slower. Adding a trace breaks that
 transformation, and then the code goes 3x faster. Specifically:

 {noformat}
 test2 [1,2,3,4,5,6,7,8,9,10] = \x -> x
 test2 _ = trace "here" $ \x -> negate x
 {noformat}

 Timings, as reported by Criterion under O2 with GHC 7.10.2, are:

 {{{
 benchmarking test0 = 40.99 ns   (40.96 ns .. 41.02 ns)
 benchmarking test1 = 41.09 ns   (41.06 ns .. 41.14 ns)
 benchmarking test2 = 17.74 ns   (17.68 ns .. 17.81 ns)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11029>
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