for review: infinite loops introduced by deforestation

[Simon Taylor]

This will need to be committed on both the main and alias branches.
David Overton, could you please review this.

Estimated hours taken: 0.5

compiler/deforest.m:
	Fix a bug reported by David Overton for the alias branch.
	Don't assume the first goal in a conjunction to be deforested
	can be unfolded -- if the unfolding fails an infinite loop
	is introduced into the generated code. 
	The test case for this is library/relation.m on the alias branch.
	This came up because of a bug in mode_util:recompute_instmap_delta
	on the alias branch -- it doesn't handle constructions of
	higher-order terms properly.


Index: deforest.m
===================================================================
RCS file: /home/staff/zs/imp/mercury/compiler/deforest.m,v
retrieving revision 1.6
diff -u -t -u -r1.6 deforest.m
--- deforest.m	1998/11/03 05:13:56	1.6
+++ deforest.m	1998/11/03 05:17:10
@@ -842,12 +842,12 @@
                 pd_debug__message("unfolding first call\n", []),
 
                 deforest__unfold_call(no, no, PredId1, ProcId1, Args1, 
-                        EarlierGoal, UnfoldedCall, _),
+                        EarlierGoal, UnfoldedCall, DidUnfold),
                 { deforest__create_conj(UnfoldedCall, BetweenGoals,
                         LaterGoal, NonLocals, DeforestGoal0) },
                 { set__to_sorted_list(NonLocals, NonLocalsList) },
 
-                ( { RunModes = yes } ->
+                ( { DidUnfold = yes, RunModes = yes } ->
 
                         %
                         % If we did a generalisation step when creating this
@@ -870,7 +870,11 @@
                         { FoldGoal = FoldGoal0 },
                         { Errors = [] }
                 ),
-                ( { Errors = [] } -> 
+
+                % We must have been able to unfold the first call to proceed
+                % with the optimization, otherwise we will introduce an
+                % infinite loop in the generated code.
+                ( { DidUnfold = yes, Errors = [] } -> 
 
                         %
                         % Create the new version.