Better CSE optimisation

GHC's CSE optimisation is pretty weedy. It looks for expressions like this:

   let x = e1 in e2

and replaces all occurrences of e1 in e2 with x. This doesn't do full CSE, but it does catch some cases. There have been case where full CSE would be significantly beneficial, though.

One possible way forward is to have a separate CSE pass that transformed expressions containing common subexpressions into the let-form above, and let the existing CSE pass do the final replacement.

We must be cautious, though: increasing sharing can introduce space leaks. Sometimes we can prove that this cannot happen, for example when the shared object is primitive, or has a bounded size.

Trac metadata

Trac field	Value
Version	6.4.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Architecture	Unknown