#4448: Another case of SpecConstr not specialising
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Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.0.2
Component: Compiler | Version: 7.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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Comment(by simonpj):
Nice example! The reason for this turns out to be simple. !SpecConstr
specialises a function `f` when
* It sees a call to `f (I# x, I# y)`
* AND it can see that `f` takes apart the pair, and takes apart the first
`Int`, and takes apart the second `Int`.
The second condition is to avoid over-specialising (see the
[http://research.microsoft.com/~simonpj/papers/spec-constr/index.htm
original paper]). The "taking apart" analysis is fiarly simple-minded,
and it fails in this example, because one of the arguments is passed on
(un-take-apart) to a recursive call, and ''that'' takes it apart.
One solution is presumably to use `ForceSpecConstr` on this loop?
(Another would be to use a more sophisticated analysis.)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4448#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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