#7850: Strangely high memory usage on optimized Ackermann function ---------------------------------+------------------------------------------ Reporter: dolio | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Runtime performance bug Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Comment(by applicative): No one seems to be pointing out that this this program ''does'' return and more importantly ''gives the correct answer'' on `ghc-7.6.2`. On this dubious macbook air (osx 10.8, 4gb , 2.4ghz, 2 cores etc), the world does not end, this wiki does not freeze, my files are not threatened; it takes as much memory as the os is willing to give it -- ~2gb -- and looks like so: {{{ $ ghc -O2 ackermanbug.hs -fforce-recomp # ghc-7.6.2 64bit $ time ./ackermanbug 65533 real 1m47.372s; user 0m26.577s; sys 0m17.641s }}} The distinction between `real` and `user` is of course suggestive of pathology -- but not a bug in the strict sense -- and is indeed handsomely repaired in {{{ $ khc -O2 ackermanbug.hs -fforce-recomp # ghc HEAD $ time ./ackermanbug 65533 real 0m24.081; user 0m23.920s; sys 0m0.133s }}} If I add the news that "ack n 1 = n + 2", ghc does fine in either version; and of course it clunks along never to return with `ack 4 2`. **With the same emendation http://hpaste.org/86212 the vaunted `gcc` gives a segmentation fault instantly**. What I wonder is whether gcc's segfault is a 'bug' we should be hysterically reporting on the gcc trak, or if the `gcc` devs just put in an ack-detector because they got tired of people whining about such things, preferring to focus on what matters. What is more interesting is that e.g. `ocamlopt` takes 5s for `ack 4 1` where `ghc-HEAD` takes 24s. I cannot replicate the superiority the SO guy found with the `gcc` . It seems about the same as `ocamlopt`. If I add ''any help at all'' to the program, e.g. that ack 1 = (+2) -- ghc becomes, for all actually calculable arguments, just as fast as gcc. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7850#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler