From: simonpj <unknown>
Date: Thu, 4 Jan 2001 08:42:21 +0000 (+0000)
Subject: [project @ 2001-01-04 08:42:21 by simonpj]
X-Git-Tag: Approximately_9120_patches~2972
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=790dea010e6068a0be319562858495273a94fcf3;p=ghc-hetmet.git

[project @ 2001-01-04 08:42:21 by simonpj]
Allow rules to apply for loop-breakers.
This backs out the previous (bogus) change.
There's a comment in Simplify.completeCall that explains.
---

diff --git a/ghc/compiler/simplCore/Simplify.lhs b/ghc/compiler/simplCore/Simplify.lhs
index 4683370..76da6e5 100644
--- a/ghc/compiler/simplCore/Simplify.lhs
+++ b/ghc/compiler/simplCore/Simplify.lhs
@@ -810,14 +810,21 @@ completeCall var occ_info cont
 	-- won't occur for things that have specialisations till a later phase, so
 	-- it's ok to try for inlining first.
 	--
-	-- Don't apply rules for a loop breaker: doing so might give rise
-	-- to an infinite loop, for the same reasons that inlining the ordinary
-	-- RHS of a loop breaker might.
+	-- You might think that we shouldn't apply rules for a loop breaker: 
+	-- doing so might give rise to an infinite loop, because a RULE is
+	-- rather like an extra equation for the function:
+	--	RULE:		f (g x) y = x+y
+	--	Eqn:		f a     y = a-y
+	--
+	-- But it's too drastic to disable rules for loop breakers.  
+	-- Even the foldr/build rule would be disabled, because foldr 
+	-- is recursive, and hence a loop breaker:
+	--	foldr k z (build g) = g k z
+	-- So it's up to the programmer: rules can cause divergence
 
     getSwitchChecker 	`thenSmpl` \ chkr ->
     let
-	maybe_rule |  switchIsOn chkr DontApplyRules 
-		   || isLoopBreaker occ_info	    = Nothing
+	maybe_rule | switchIsOn chkr DontApplyRules = Nothing
 		   | otherwise			    = lookupRule in_scope var args' 
     in
     case maybe_rule of {