X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FcoreSyn%2FCoreUtils.lhs;h=19ac62ff8a1b93753be558085d8dbb21d6c67c8b;hp=a0a229f6c66807e202d2a393d165aea16fd37373;hb=c5b178be60a5a44abd2f4ddf8c399857678326e2;hpb=fdf8656855d26105ff36bdd24d41827b05037b91

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index a0a229f..19ac62f 100644
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -589,12 +589,16 @@ exprIsCheap' good_app other_expr 	-- Applications and variables
     go _ _ = False
  
     --------------
-    go_pap args = all exprIsTrivial args
- 	-- For constructor applications and primops, check that all
- 	-- the args are trivial.  We don't want to treat as cheap, say,
- 	-- 	(1:2:3:4:5:[])
- 	-- We'll put up with one constructor application, but not dozens
- 	
+    go_pap args = all (exprIsCheap' good_app) args
+        -- Used to be "all exprIsTrivial args" due to concerns about
+        -- duplicating nested constructor applications, but see #4978.
+	-- The principle here is that 
+	--    let x = a +# b in c *# x
+        -- should behave equivalently to
+        --    c *# (a +# b)
+        -- Since lets with cheap RHSs are accepted, 
+        -- so should paps with cheap arguments
+
     --------------
     go_primop op args = primOpIsCheap op && all (exprIsCheap' good_app) args
  	-- In principle we should worry about primops