X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FcoreSyn%2FCoreUtils.lhs;fp=compiler%2FcoreSyn%2FCoreUtils.lhs;h=19ac62ff8a1b93753be558085d8dbb21d6c67c8b;hp=4146b621e158df90d53bcb01ad7346e0a3203726;hb=7e95df790b34e11d7308e43dab0a7175b69b70fc;hpb=c0687066474aa4ce4912f31a5c09c1bcd673fb06

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index 4146b62..19ac62f 100644
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -592,6 +592,12 @@ exprIsCheap' good_app other_expr 	-- Applications and variables
     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