#7309: The Ix instance for (,) leaks space in range ----------------------------------+----------------------------------------- Reporter: nomeata | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: libraries (other) | Version: 7.6.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Runtime performance bug Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ----------------------------------+----------------------------------------- Description changed by igloo: Old description:
(at least if you take leak to mean unexpected space behaviour).
This was brought to my attention via http://stackoverflow.com/questions/12780497/puzzling-memory-behavior-in- haskell where someone created a 6×10^6-array and was surprised that showing the array caused an additional (3+2)*8*10^6 bytes to be used. The reason (as far as I could tell) was this code:
{{{ instance (Ix a, Ix b) => Ix (a, b) where range ((l1,l2),(u1,u2)) = [ (i1,i2) | i1 <- range (l1,u1), i2 <- range (l2,u2) ] }}} in Arr.GHC. The result of {{{range (l2,u2)}}} is shared between every step in the first component of the index.
I guess it is reasonable to expect that the result of range is never worth sharing (is it?). I am not entirely sure what the best, cleanest, alternative implementation is, though. (At least not without a working ```dontshare``` annotation ;-))
New description: (at least if you take leak to mean unexpected space behaviour). This was brought to my attention via http://stackoverflow.com/questions/12780497/puzzling-memory-behavior-in- haskell where someone created a `6×10^6`-array and was surprised that showing the array caused an additional `(3+2)*8*10^6` bytes to be used. The reason (as far as I could tell) was this code: {{{ instance (Ix a, Ix b) => Ix (a, b) where range ((l1,l2),(u1,u2)) = [ (i1,i2) | i1 <- range (l1,u1), i2 <- range (l2,u2) ] }}} in Arr.GHC. The result of {{{range (l2,u2)}}} is shared between every step in the first component of the index. I guess it is reasonable to expect that the result of range is never worth sharing (is it?). I am not entirely sure what the best, cleanest, alternative implementation is, though. (At least not without a working ```dontshare``` annotation ;-)) -- -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7309#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler