#11710: Fusion of a simple listArray call is very fragile -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 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: -------------------------------------+------------------------------------- Consider the following (taken from ticket:11707#comment:2), {{{#!hs module Test where import Data.Array arr, arr2 :: Array Int Int arr = listArray (0,10) [ 1,1,1,1,1,1,1,1,1,1 ] arr2 = listArray (0,10) [ 1,1,1,1,1,1,1,1,1,-1 ] }}} Given that these are a small array, one might suspect it would be worthwhile for GHC to fuse the lists with `listArray`, giving rise to two nicely unrolled construction procedures. However, if you look at the Core produced by `-O1` this you'll find that this only happens in the case of `arr2`. `arr` on the other handle, is mysteriously not fused. The fact that these expressions are so similar and yet produce entirely different code is quite worrying. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11710 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler