#7542: GHC doesn't optimize (strict) composition with id ---------------------------------+------------------------------------------ Reporter: shachaf | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Runtime performance bug Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Comment(by simonpj): Also this: {{{ commit 7a1480c7c590d4d2fa7a105a4eebf299e921e056 Author: Simon Peyton Jones <simonpj@microsoft.com> Date: Tue Jan 22 22:43:22 2013 +0000 Allow eta-reduction of eval'd functions if of arity 1 See Note [Eta reduction of an eval'd function] in CoreUtils. This doesn't fix Trac #7542, but that was the ticket that pointed out this infelicity.

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compiler/coreSyn/CoreUtils.lhs | 24 ++++++++++++++++++++++-- 1 files changed, 22 insertions(+), 2 deletions(-) diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs index 7017f70..9b527e7 100644 --- a/compiler/coreSyn/CoreUtils.lhs +++ b/compiler/coreSyn/CoreUtils.lhs @@ -1712,8 +1712,14 @@ tryEtaReduce bndrs body --------------- fun_arity fun -- See Note [Arity care] - | isLocalId fun && isStrongLoopBreaker (idOccInfo fun) = 0 - | otherwise = idArity fun + | isLocalId fun + , isStrongLoopBreaker (idOccInfo fun) = 0 + | arity > 0 = arity + | isEvaldUnfolding (idUnfolding fun) = 1 + -- See Note [Eta reduction of an eval'd function] + | otherwise = 0 + where + arity = idArity fun --------------- ok_lam v = isTyVar v || isEvVar v @@ -1737,6 +1743,20 @@ tryEtaReduce bndrs body ok_arg _ _ _ = Nothing \end{code} +Note [Eta reduction of an eval'd function] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In Haskell is is not true that f = \x. f x +because f might be bottom, and 'seq' can distinguish them. + +But it *is* true that f = f `seq` \x. f x +and we'd like to simplify the latter to the former. This amounts to +the rule that + * when there is just *one* value argument, + * f is not bottom +we can eta-reduce \x. f x ===> f + +This turned up in Trac #7542. }}} -- Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7542#comment:8> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler