[m-dev.] diff: disable buggy double-negation optimization

[Fergus Henderson]

Estimated hours taken: 0.75

compiler/simplify.m:
	Disable the double-negation elimination optimization,
	since it doesn't always preserve mode-correctness.
	In particular, if the goal has mode `mdi', then
	moving it outside the double-negation can cause
	a variable to be clobbered.

Workspace: /home/mercury0/fjh/mercury
Index: compiler/simplify.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/simplify.m,v
retrieving revision 1.74
diff -u -d -r1.74 simplify.m
--- simplify.m	1999/10/28 00:57:00	1.74
+++ simplify.m	2000/02/03 16:38:11
@@ -927,7 +927,15 @@
 	; CondSolns0 = at_most_zero ->
 		% Optimize away the condition and the `then' part.
 		det_negation_det(CondDetism0, MaybeNegDetism),
-		( Cond0 = not(NegCond) - _ ->
+		(
+			Cond0 = not(NegCond) - _,
+			% XXX BUG! This optimization is only safe if it
+			% preserves mode correctness, which means in particular
+			% that the the negated goal must not clobber any
+			% variables.
+			% For now I've just disabled the optimization.
+			semidet_fail
+		->
 			Cond = NegCond
 		;
 			(
@@ -1050,7 +1058,12 @@
 		Info = Info3
 	;
 		% remove double negation
-		Goal1 = not(SubGoal - SubGoalInfo) - _
+		Goal1 = not(SubGoal - SubGoalInfo) - _,
+		% XXX BUG! This optimization is only safe if it preserves
+		% mode correctness, which means in particular that the
+		% the negated goal must not clobber any variables.
+		% For now I've just disabled the optimization.
+		semidet_fail
 	->
 		simplify__maybe_wrap_goal(GoalInfo0, SubGoalInfo, SubGoal,
 			Goal, GoalInfo, Info3, Info)

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
--------------------------------------------------------------------------
mercury-developers mailing list
Post messages to:       mercury-developers at cs.mu.oz.au
Administrative Queries: owner-mercury-developers at cs.mu.oz.au
Subscriptions:          mercury-developers-request at cs.mu.oz.au
--------------------------------------------------------------------------