Opportunity to improve CSE

In nofib/spectral/mandel2, the function check_perim calls point_colour on various arguments. After inlining point_colour there is the opportunity to CSE among the sub-expressions the inlinings create.

In GHC 7.6, the join point didn't have a "one-shot" flag, so the full laziness pass floated these sub-expressions out, and they got CSEd.

As part of Ilya's new demand analyser changes, we get the "one-shot" flag right, so don't MFE those sub-expressions, so they aren't CSEd. As a result, allocation on mandel2 increases slightly (4.2%).

The solution is, I think, to do something a bit like like CorePrep before CSE. At the moment if we have

case f (I# y) of { (a,b) ->
case g (I# y) of { (p,q) -> ... }}

we stuipdly don't CSE that (I# y) even though it is manifestly sharable. Somehow we should.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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