From: simonpj@microsoft.com <unknown>
Date: Wed, 29 Nov 2006 21:24:40 +0000 (+0000)
Subject: Improve the loop-breaking heuristics
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=1dca158769225e58ec90d95526f968515e325378

Improve the loop-breaking heuristics

The loop-breaking heuristics were making it a high priority to
avoid choosing a variable as a loop breaker if its *type* was a
data type.  The reason is that it's very good to be able to "see"
constructor applications.

But it's only good if the constructor application is *visible*,
so that is what I test for now.  I found a case (when testing
SpecConstr) where I had a Rec like this:
	rec { lvl = foo Nothing
	      foo = ...
	        RULE foo Nothing = ...
   	    }

Even if lvl has a data type, it's much better to make lvl the loop
breaker, not foo, so that foo's RULE is visible in lvl's RHS.
---

diff --git a/compiler/simplCore/OccurAnal.lhs b/compiler/simplCore/OccurAnal.lhs
index 5bfb4b9..8b3d45e 100644
--- a/compiler/simplCore/OccurAnal.lhs
+++ b/compiler/simplCore/OccurAnal.lhs
@@ -23,14 +23,13 @@ import CoreUtils	( exprIsTrivial, isDefaultAlt )
 import Id		( isDataConWorkId, isOneShotBndr, setOneShotLambda, 
 			  idOccInfo, setIdOccInfo, isLocalId,
 			  isExportedId, idArity, idHasRules,
-			  idType, idUnique, Id
+			  idUnique, Id
 			)
 import BasicTypes	( OccInfo(..), isOneOcc, InterestingCxt )
 
 import VarSet
 import VarEnv
 
-import Type		( isFunTy, dropForAlls )
 import Maybes		( orElse )
 import Digraph		( stronglyConnCompR, SCC(..) )
 import PrelNames	( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
@@ -301,7 +300,7 @@ reOrderCycle bndrs (bind : binds)
 		-- where df is the exported dictionary. Then df makes a really
 		-- bad choice for loop breaker
 	  
-	| not_fun_ty (idType bndr) = 3	-- Data types help with cases
+	| is_con_app rhs = 3	-- Data types help with cases
 		-- This used to have a lower score than inlineCandidate, but
 		-- it's *really* helpful if dictionaries get inlined fast,
 		-- so I'm experimenting with giving higher priority to data-typed things
@@ -328,7 +327,11 @@ reOrderCycle bndrs (bind : binds)
 	-- we didn't stupidly choose d as the loop breaker.
 	-- But we won't because constructor args are marked "Many".
 
-    not_fun_ty ty = not (isFunTy (dropForAlls ty))
+	-- Cheap and cheerful; the simplifer moves casts out of the way
+    is_con_app (Var v)    = isDataConWorkId v
+    is_con_app (App f _)  = is_con_app f
+    is_con_app (Note _ e) = is_con_app e
+    is_con_app other      = False
 
 makeLoopBreaker :: VarSet		-- Binders of this group
 		-> UsageDetails		-- Usage of this rhs (neglecting rules)