+ Interesting arguments
+%* *
+%************************************************************************
+
+Note [Interesting arguments]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+An argument is interesting if it deserves a discount for unfoldings
+with a discount in that argument position. The idea is to avoid
+unfolding a function that is applied only to variables that have no
+unfolding (i.e. they are probably lambda bound): f x y z There is
+little point in inlining f here.
+
+Generally, *values* (like (C a b) and (\x.e)) deserve discounts. But
+we must look through lets, eg (let x = e in C a b), because the let will
+float, exposing the value, if we inline. That makes it different to
+exprIsHNF.
+
+Before 2009 we said it was interesting if the argument had *any* structure
+at all; i.e. (hasSomeUnfolding v). But does too much inlining; see Trac #3016.
+
+But we don't regard (f x y) as interesting, unless f is unsaturated.
+If it's saturated and f hasn't inlined, then it's probably not going
+to now!
+
+\begin{code}
+data ArgSummary = TrivArg -- Nothing interesting
+ | NonTrivArg -- Arg has structure
+ | ValueArg -- Arg is a con-app or PAP
+
+interestingArg :: CoreExpr -> ArgSummary
+-- See Note [Interesting arguments]
+interestingArg e = go e 0
+ where
+ -- n is # value args to which the expression is applied
+ go (Lit {}) _ = ValueArg
+ go (Var v) n
+ | isDataConWorkId v = ValueArg
+ | idArity v > n = ValueArg -- Catches (eg) primops with arity but no unfolding
+ | n > 0 = NonTrivArg -- Saturated or unknown call
+ | evald_unfolding = ValueArg -- n==0; look for a value
+ | otherwise = TrivArg -- n==0, no useful unfolding
+ where
+ evald_unfolding = isEvaldUnfolding (idUnfolding v)
+
+ go (Type _) _ = TrivArg
+ go (App fn (Type _)) n = go fn n
+ go (App fn _) n = go fn (n+1)
+ go (Note _ a) n = go a n
+ go (Cast e _) n = go e n
+ go (Lam v e) n
+ | isTyVar v = go e n
+ | n>0 = go e (n-1)
+ | otherwise = ValueArg
+ go (Let _ e) n = case go e n of { ValueArg -> ValueArg; _ -> NonTrivArg }
+ go (Case {}) _ = NonTrivArg
+
+nonTriv :: ArgSummary -> Bool
+nonTriv TrivArg = False
+nonTriv _ = True
+\end{code}
+
+
+%************************************************************************
+%* *
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