my apologies, sorry for the terrible bug report
On Sun, Dec 11, 2016 at 11:05 AM GHC
#876: Length is not a good consumer -------------------------------------+------------------------------------- Reporter: ariep@… | Owner: Type: bug | Status: new Priority: lowest | Milestone: 7.6.2 Component: libraries/base | Version: 6.5 Resolution: | Keywords: length Operating System: Linux | Architecture: | Unknown/Multiple Type of failure: Runtime | Test Case: performance bug | perf/should_run/T876 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+-------------------------------------
Comment (by nomeata):
This code, compiled with `-O, does fuse, and allocates nothing (or constant amounts) {{{#!hs
module Foo where x :: Int -> Int x n = length [0..(10^n)::Int] }}}
{{{ $ ghci -fobject-code -O Foo GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Foo ( Foo.hs, Foo.o ) Ok, modules loaded: Foo. Prelude Foo> :set +s Prelude Foo> x 1 11 (0.03 secs, 14,976,744 bytes) Prelude Foo> x 7 10000001 (0.02 secs, 0 bytes) Prelude Foo> x 8 100000001 (0.04 secs, 0 bytes) }}}
(almost) HEAD: {{{ GHCi, version 8.1.20161117: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Foo (.hs -> .o) WARNING: file compiler/simplCore/SimplCore.hs, line 663 Simplifier bailing out after 4 iterations [58, 14, 2, 2] Size = {terms: 96, types: 32, coercions: 0} Ok, modules loaded: Foo (Foo.o). Prelude Foo> :set +s Prelude Foo> x 1 11 (0.19 secs, 94,792 bytes) Prelude Foo> x 2 101 (0.01 secs, 94,648 bytes) Prelude Foo> x 7 10000001 (0.01 secs, 98,568 bytes) Prelude Foo> x 8 100000001 (0.05 secs, 98,448 bytes) }}}
Testing this in with interpreted code is not sufficient, as the optimizer does less in that case. So so far, everything seems as expected to me.
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/876#comment:30 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler