#7309: The Ix instance for (,) leaks space in range
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Reporter: nomeata | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries (other)
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Runtime performance bug | Testcase:
Blockedby: | Blocking:
Related: |
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(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>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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