Ticket #7309 (new bug)
The Ix instance for (,) leaks space in range
|Reported by:||nomeata||Owned by:|
|Type of failure:||Runtime performance bug||Difficulty:||Unknown|
|Test Case:||Blocked By:|
Description (last modified by igloo) (diff)
(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 ;-))