Fix exprIsDupable

It turns out that exprIsDupable would return True for an expression of
*arbitrary* size, provide it was a nested bunch of applications in
which no function had more than three arguments.  That was never the
intention, and could give rise to massive code duplication.

This patch makes it much less aggressive.

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index 2eedd33..c50251d 100644 (file)
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -465,22 +465,24 @@ Note [exprIsDupable]
 
 \begin{code}
 exprIsDupable :: CoreExpr -> Bool
-exprIsDupable (Type _)   = True
-exprIsDupable (Var _)    = True
-exprIsDupable (Lit lit)  = litIsDupable lit
-exprIsDupable (Note _ e) = exprIsDupable e
-exprIsDupable (Cast e _) = exprIsDupable e
-exprIsDupable expr
-  = go expr 0
+exprIsDupable e
+  = isJust (go dupAppSize e)
   where
-    go (Var _)   _      = True
-    go (App f a) n_args =  n_args < dupAppSize
-                       && exprIsDupable a
-                       && go f (n_args+1)
-    go _         _      = False
+    go :: Int -> CoreExpr -> Maybe Int
+    go n (Type {}) = Just n
+    go n (Var {})  = decrement n
+    go n (Note _ e) = go n e
+    go n (Cast e _) = go n e
+    go n (App f a) | Just n' <- go n a = go n' f
+    go n (Lit lit) | litIsDupable lit = decrement n
+    go _ _ = Nothing
+
+    decrement :: Int -> Maybe Int
+    decrement 0 = Nothing
+    decrement n = Just (n-1)
 
 dupAppSize :: Int
-dupAppSize = 4         -- Size of application we are prepared to duplicate
+dupAppSize = 6  -- Size of term we are prepared to duplicate
 \end{code}
 
 %************************************************************************