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 (file)
--- 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)