[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
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by NeilMitchell:

Old description:

> 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)
> }}}

New description:

 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:

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

 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#comment:1>
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