From: simonpj@microsoft.com <unknown>
Date: Mon, 14 Aug 2006 16:54:24 +0000 (+0000)
Subject: Be more conservative about duplicating continuations
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=da10704518081f55e848095d6b49bc8e11a2a3bb

Be more conservative about duplicating continuations

Roman found that GHC was duplicating continuations that arose (essentially)
from uses of 'seq', or strict constructors.  This fixes the problem;
see the comments mkDupableCont (the Select case with a single alternative).

I'm a little concerned that this may also miss useful case-of-case
tranformations, so I'd like to know if anyone finds that this patch
makes performance worse.

To make it a bit more gung-ho, one could check for all the binders
being dead, before choosing this new, conservative alternative.
---

diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs
index 17a6bcc..fa5798f 100644
--- a/compiler/simplCore/Simplify.lhs
+++ b/compiler/simplCore/Simplify.lhs
@@ -1826,6 +1826,43 @@ mkDupableCont env (ApplyTo _ arg mb_se cont)
 	; (floats2, arg2) <- mkDupableArg env arg1
 	; return (floats2, (ApplyTo OkToDup arg2 Nothing dup_cont, nondup_cont)) }}
 
+mkDupableCont env cont@(Select _ case_bndr [_] _ _)
+  = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
+  where
+    scrut_ty = substTy env (idType case_bndr)
+	-- This case is just like the previous one.  Here's an example:
+	--	data T a = MkT !a
+	--	...(MkT (abs x))...
+	-- Then we get
+	--	case (case x of I# x' -> 
+	--	      case x' <# 0# of
+	--		True  -> I# (negate# x')
+	--		False -> I# x') of y {
+	--	  DEFAULT -> MkT y
+	-- Because the (case x) has only one alternative, we'll transform to
+	--	case x of I# x' ->
+	--	case (case x' <# 0# of
+	--		True  -> I# (negate# x')
+	--		False -> I# x') of y {
+	--	  DEFAULT -> MkT y
+	-- But now we do *NOT* want to make a join point etc, giving 
+	--	case x of I# x' ->
+	--	let $j = \y -> MkT y
+	--	in case x' <# 0# of
+	--		True  -> $j (I# (negate# x'))
+	--		False -> $j (I# x')
+	-- In this case the $j will inline again, but suppose there was a big
+	-- strict computation enclosing the orginal call to MkT.  Then, it won't
+	-- "see" the MkT any more, because it's big and won't get duplicated.
+	-- And, what is worse, nothing was gained by the case-of-case transform.
+	--
+	-- NB: Originally I matched [(DEFAULT,_,_)], but in the common
+	-- case of Int, the alternative-filling-in code turned the outer case into
+	--	case (...) of y { I# _ -> MkT y }
+	-- and that doesn't match the DEFAULT!
+	-- Now I match on any single-alternative case. 
+	-- I hope that is the right thing to do!
+
 mkDupableCont env (Select _ case_bndr alts se cont)
   = 	-- e.g.		(case [...hole...] of { pi -> ei })
 	--	===>