[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 (file)
--- 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 {