+Thunk splitting
+~~~~~~~~~~~~~~~
+Suppose x is used strictly (never mind whether it has the CPR
+property).
+
+ let
+ x* = x-rhs
+ in body
+
+splitThunk transforms like this:
+
+ let
+ x* = case x-rhs of { I# a -> I# a }
+ in body
+
+Now simplifier will transform to
+
+ case x-rhs of
+ I# a -> let x* = I# b
+ in body
+
+which is what we want. Now suppose x-rhs is itself a case:
+
+ x-rhs = case e of { T -> I# a; F -> I# b }
+
+The join point will abstract over a, rather than over (which is
+what would have happened before) which is fine.
+
+Notice that x certainly has the CPR property now!
+
+In fact, splitThunk uses the function argument w/w splitting
+function, so that if x's demand is deeper (say U(U(L,L),L))
+then the splitting will go deeper too.
+
+\begin{code}
+-- splitThunk converts the *non-recursive* binding
+-- x = e
+-- into
+-- x = let x = e
+-- in case x of
+-- I# y -> let x = I# y in x }
+-- See comments above. Is it not beautifully short?
+
+splitThunk fn_id rhs
+ = mkWWstr [fn_id] `thenUs` \ (_, wrap_fn, work_fn) ->
+ returnUs [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Functions over Demands}
+%* *
+%************************************************************************
+
+\begin{code}
+worthSplittingFun :: [Demand] -> DmdResult -> Bool
+ -- True <=> the wrapper would not be an identity function
+worthSplittingFun ds res
+ = any worth_it ds || returnsCPR res
+ -- worthSplitting returns False for an empty list of demands,
+ -- and hence do_strict_ww is False if arity is zero and there is no CPR
+
+ -- We used not to split if the result is bottom.
+ -- [Justification: there's no efficiency to be gained.]
+ -- But it's sometimes bad not to make a wrapper. Consider
+ -- fw = \x# -> let x = I# x# in case e of
+ -- p1 -> error_fn x
+ -- p2 -> error_fn x
+ -- p3 -> the real stuff
+ -- The re-boxing code won't go away unless error_fn gets a wrapper too.
+ -- [We don't do reboxing now, but in general it's better to pass
+ -- an unboxed thing to f, and have it reboxed in the error cases....]
+ where
+ worth_it Abs = True -- Absent arg
+ worth_it (Eval (Prod ds)) = True -- Product arg to evaluate
+ worth_it other = False
+
+worthSplittingThunk :: Maybe Demand -- Demand on the thunk
+ -> DmdResult -- CPR info for the thunk
+ -> Bool
+worthSplittingThunk maybe_dmd res
+ = worth_it maybe_dmd || returnsCPR res
+ where
+ -- Split if the thing is unpacked
+ worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
+ worth_it other = False
+\end{code}
+