#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: -------------------------------------+------------------------------------- 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) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11029 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler