Arbitrarily large expressions built out of cheap primops are not floated out

While investigating #13338 (closed) I was tripped up by the fact that GHC won't float out even a large expression like the product in

\i# -> magic# (x# *# 2# *# 2# *# 2# *# 2# *# 2# *# 2# *# 2# *# 2# -# i#)

The test involved here is exprIsCheap. The fact that *# is a cheap primop makes exprIsCheap think the whole expression is cheap; but clearly there must come some point where it would be better to float out the expression.

Perhaps exprIsCheap should work more like exprIsDupable, and take the size of the expression into account. See related comments on primOpIsCheap, though this ticket is about saving runtime work, not code size.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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