Better occurrence analysis for join points

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.

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