#5075: CPR optimisation for sum types if only one constructor is used
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.0.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Inspired by #3138, it might be useful for StrAnal to detect functions such
as the following where only one of the data constructors for a sum type
are CPRed:
{{{
loop x = case x < 10 of True -> Left x; False -> loop (x*2)
}}}
We can usefully transform to:
{{{
$wloop x = case (case x < 10 of True -> Left x; False -> loop (x*2)) of
Left y -> (# y #)
loop x = case loop x of (# y #) -> Left y
}}}
Attached patch implements this behaviour. Most of the complication in the
new code occurs because adding a DataCon field to the Demand data type
means that we have to define a separate IfaceDemand type for storage in
interface files.
The patch validates but I haven't done any tests on nofib. I have
confirmed that the new optimisation hits on some examples, though.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5075>
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The Glasgow Haskell Compiler
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