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Better occurrence analysis for join points

    • Open Issue created by Simon Peyton Jones @simonpj

    Consider this

    let x = ... in
join j y = x+y in
case v of
  A -> x
  B -> j 1
  C -> j 2
  D -> 3

    What does OccAnal say about x's usage? It says ManyOccs!

    But it's plain as a pikestaff that x is used at most once:

    • Either in the A branch,
    • or in the B or C branches via j.

    If instead we had inlined j we'd have

    let x = ... in
join j y = x+y in
case v of
  A -> x
  B -> x + 1
  C -> x + 2
  D -> 3

    and now it's all more obvious: x's occurrence info should be OneOcc { occ_one_br = False }, not ManyOccs.

    Does this matter? Not a great deal, but there is a reason for having OneOcc with occ_one_br = False, and it seems a shame not to take advantage.

    One case in point is the definition of `SimplUtils.isExitJoinId

    isExitJoinId :: Var -> Bool
isExitJoinId id = isJoinId id && isOneOcc (idOccInfo id) && occ_in_lam (idOccInfo id)

    Something does not cease to be an exit-join-point if it is mentioned in multiple places as above.

    Another use for this info is postInlineUnconditionally.

    Could we improve this situation? I think it'd be quite easy. For non-recursive join points j = rhs

    • Gather occurrence info from the RHS
    • Bind j to its occurrence info
    • Unleash that occurrence info at each jump-site for j\, just as if it had been inlined.

    See Trac #14152 (closed), #15091.

    Trac metadata
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    Version 8.2.2
    Type Task
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
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