#2186: unsafePerformIO prevents space leak?
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Reporter: ravi | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.2 | Severity: normal
Keywords: | Testcase:
Architecture: Multiple | Os: Linux
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Sadly, I don't have a concrete testcase for this, but I've fixed this
issue in the Bluespec compiler more than once, so I have a strong
suspicion that there's a GHC bug (or lack of optimization) going on here.
The basic situation is that I have a deeply nested structures of IORefs (I
once measured one particular instance as including chains over 300 IORefs
deep). I want to walk this structure recursively (it does not contain
loops) without mutating it to compute a property of this structure.
Generally speaking I'm either looking to accumulate certain kinds of nodes
or I'm looking to detect the presence or absence of a certain kind of
node.
If I traverse this structure in the IO monad (using readIORef directly and
recursing and combining using things like mapM and foldM) I see a huge
space leak (which can be confirmed with profiling). If I take the same
traversal and wrap up the readIORef inside an unsafePerformIO (and change
mapM to map and the like) the space leak disappears. This surprised me
because I thought it would be the sort of thing GHC could figure out.
I'm sorry I can't easily give you a testcase, but I hope I've included
enough detail that you have a fighting chance of reproducing it. Please
let me know if you have any questions.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2186>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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