[m-rev.] diff: switch handling optimization

[Zoltan Somogyi]

compiler/switch_detection.m:
	When recognizing that a unification of the form X = f in a disjunct
	is part of a switch, delete it from the switch arm. Simplification
	would later delete it anyway (since X is known to be f in that switch
	arm, so this is a pure det goal with no outputs), but simplify.m would
	set the flag requesting a requantification when it did so. Since
	requantification is slow for large predicate bodies, this is an
	improvement.

	This reduces the time to compile training_cars_full.m from 250 seconds
	to 176 (since quantification takes so long).

Zoltan.

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Index: compiler/switch_detection.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/switch_detection.m,v
retrieving revision 1.131
diff -u -b -r1.131 switch_detection.m
--- compiler/switch_detection.m	1 Nov 2006 06:33:21 -0000	1.131
+++ compiler/switch_detection.m	16 Dec 2006 09:16:33 -0000
@@ -529,20 +529,32 @@
     list(hlds_goal)::out, maybe(cons_id)::in, maybe(cons_id)::out,
     unit::in, unit::out) is det.
 
-find_bind_var_for_switch_in_deconstruct(_UnifyVar, Goal0, Goals,
+find_bind_var_for_switch_in_deconstruct(SwitchVar, Goal0, Goals,
         _Result0, Result, _, unit) :-
     (
         Goal0 = GoalExpr0 - GoalInfo,
         UnifyInfo0 = GoalExpr0 ^ unify_kind,
-        UnifyInfo0 = deconstruct(_, Functor, _, _, _, _)
+        UnifyInfo0 = deconstruct(UnifyVar, Functor, ArgVars, _, _, _)
     ->
         Result = yes(Functor),
+        (
+            ArgVars = [],
+            SwitchVar = UnifyVar
+        ->
+            % The test will get carried out in the switch, there are no
+            % argument values to pick up, and the test was on the switch
+            % variable (not on one of its aliases), so the unification
+            % serve no further purpose. We delete it here, so simplify
+            % doesn't have to.
+            Goals = []
+        ;
         % The deconstruction unification now becomes deterministic, since
         % the test will get carried out in the switch.
         UnifyInfo = UnifyInfo0 ^ deconstruct_can_fail := cannot_fail,
         GoalExpr = GoalExpr0 ^ unify_kind := UnifyInfo,
         Goal = GoalExpr - GoalInfo,
         Goals = [Goal]
+        )
     ;
         unexpected(this_file, "find_bind_var_for_switch_in_deconstruct")
     ).
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