[m-dev.] Improve scheduling of solver goals around impure goals.

[Ralph Becket]

Estimated hours taken: 20
Branches: main, release

compiler/modes.m:
	Improve goal scheduling around solver type goals and impure goals.
	Now, when the compiler has to schedule an impure goal, it first
	tries to schedule any delayed goals (that are not unifications
	with clause head-variables) by inserting solver initialisation
	calls.

Index: compiler/modes.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/modes.m,v
retrieving revision 1.292
diff -u -r1.292 modes.m
--- compiler/modes.m	1 Feb 2005 07:11:34 -0000	1.292
+++ compiler/modes.m	9 Mar 2005 00:48:52 -0000
@@ -1498,20 +1498,13 @@
 	delay_info__leave_conj(DelayInfo2, DelayedGoals0, DelayInfo3),
 	mode_info_set_delay_info(DelayInfo3, !ModeInfo),
 
-		% Try to handle any unscheduled goals by inserting solver
-		% initialisation calls, aiming for a deterministic
-		% schedule.
-		%
-	modecheck_conj_list_3(DelayedGoals0, DelayedGoals1, Goals2,
-		RevImpurityErrors0, RevImpurityErrors1, !ModeInfo, !IO),
-
-		% Try to handle any unscheduled goals by inserting solver
-		% initialisation calls, aiming for *any* workable schedule.
+		% Otherwise try scheduling by inserting solver
+		% initialisation calls where necessary.
 		%
-	modecheck_conj_list_4(DelayedGoals1, DelayedGoals, Goals3,
-		RevImpurityErrors1, RevImpurityErrors, !ModeInfo, !IO),
+	modecheck_delayed_solver_goals(Goals2, DelayedGoals0, DelayedGoals,
+		RevImpurityErrors0, RevImpurityErrors, !ModeInfo, !IO),
 
-	Goals = Goals1 ++ Goals2 ++ Goals3,
+	Goals = Goals1 ++ Goals2,
 
 	mode_info_get_errors(!.ModeInfo, NewErrors),
 	list__append(OldErrors, NewErrors, Errors),
@@ -1585,9 +1578,11 @@
 	Goal0 = _GoalExpr - GoalInfo0,
 	( goal_info_is_impure(GoalInfo0) ->
 		Impure = yes,
-		check_for_impurity_error(Goal0, !ImpurityErrors, !ModeInfo)
+		check_for_impurity_error(Goal0, ScheduledSolverGoals,
+			!ImpurityErrors, !ModeInfo, !IO)
 	;
-		Impure = no
+		Impure = no,
+		ScheduledSolverGoals = []
 	),
 
 		% Hang onto the original instmap, delay_info, and live_vars
@@ -1659,14 +1654,43 @@
 	( Errors = [] ->
 		% we successfully scheduled this goal, so insert
 		% it in the list of successfully scheduled goals
-		Goals = [Goal | Goals2]
+		Goals = ScheduledSolverGoals ++ [Goal | Goals2]
 	;
 		% we delayed this goal -- it will be stored in the delay_info
-		Goals = Goals2
+		Goals = ScheduledSolverGoals ++ Goals2
 	).
 
 	% We may still have some unscheduled goals.  This may be because some
 	% initialisation calls are needed to turn some solver type vars
+	% from inst free to inst any.  This predicate attempts to schedule
+	% such goals.
+	%
+:- pred modecheck_delayed_solver_goals(list(hlds_goal)::out,
+	list(delayed_goal)::in, list(delayed_goal)::out,
+	impurity_errors::in, impurity_errors::out,
+	mode_info::in, mode_info::out, io::di, io::uo) is det.
+
+modecheck_delayed_solver_goals(Goals, DelayedGoals0, DelayedGoals,
+		!ImpurityErrors, !ModeInfo, !IO) :-
+
+		% Try to handle any unscheduled goals by inserting solver
+		% initialisation calls, aiming for a deterministic
+		% schedule.
+		%
+	modecheck_conj_list_3(DelayedGoals0, DelayedGoals1, Goals0,
+		!ImpurityErrors, !ModeInfo, !IO),
+
+		% Try to handle any unscheduled goals by inserting solver
+		% initialisation calls, aiming for *any* workable schedule.
+		%
+	modecheck_conj_list_4(DelayedGoals1, DelayedGoals, Goals1,
+		!ImpurityErrors, !ModeInfo, !IO),
+
+	Goals = Goals0 ++ Goals1.
+
+
+	% We may still have some unscheduled goals.  This may be because some
+	% initialisation calls are needed to turn some solver type vars
 	% from inst free to inst any.  This pass attempts to identify a
 	% minimal subset of such vars to initialise that will allow the
 	% remaining goals to be scheduled in a deterministic fashion.
@@ -2067,26 +2091,37 @@
 %  case of output arguments, they cannot be scheduled until the variable value
 %  is known.  If headvar unifications couldn't be delayed past impure goals,
 %  impure predicates wouldn't be able to have outputs!
+%
+%  (Note that we first try to schedule any delayed solver goals waiting for
+%  initialisation.)
 
-:- pred check_for_impurity_error(hlds_goal::in,
+:- pred check_for_impurity_error(hlds_goal::in, list(hlds_goal)::out,
 	impurity_errors::in, impurity_errors::out,
-	mode_info::in, mode_info::out) is det.
+	mode_info::in, mode_info::out, io::di, io::uo) is det.
 
-check_for_impurity_error(Goal, !ImpurityErrors, !ModeInfo) :-
+check_for_impurity_error(Goal, Goals, !ImpurityErrors, !ModeInfo, !IO) :-
 	mode_info_get_delay_info(!.ModeInfo, DelayInfo0),
-	delay_info__leave_conj(DelayInfo0, DelayedGoals, DelayInfo1),
-	delay_info__enter_conj(DelayInfo1, DelayInfo),
+	delay_info__leave_conj(DelayInfo0, DelayedGoals0, DelayInfo1),
+	mode_info_set_delay_info(DelayInfo1, !ModeInfo),
 	mode_info_get_module_info(!.ModeInfo, ModuleInfo),
 	mode_info_get_predid(!.ModeInfo, PredId),
 	module_info_pred_info(ModuleInfo, PredId, PredInfo),
 	pred_info_clauses_info(PredInfo, ClausesInfo),
 	clauses_info_headvars(ClausesInfo, HeadVars),
-	( no_non_headvar_unification_goals(DelayedGoals, HeadVars) ->
+	filter_headvar_unification_goals(HeadVars, DelayedGoals0,
+		HeadVarUnificationGoals, NonHeadVarUnificationGoals0),
+	modecheck_delayed_solver_goals(Goals,
+		NonHeadVarUnificationGoals0, NonHeadVarUnificationGoals,
+		!ImpurityErrors, !ModeInfo, !IO),
+	mode_info_get_delay_info(!.ModeInfo, DelayInfo2),
+	delay_info__enter_conj(DelayInfo2, DelayInfo3),
+	redelay_goals(HeadVarUnificationGoals, DelayInfo3, DelayInfo),
+	mode_info_set_delay_info(DelayInfo, !ModeInfo),
+	( NonHeadVarUnificationGoals = [] ->
 		true
 	;
-		mode_info_set_delay_info(DelayInfo, !ModeInfo),
-		get_all_waiting_vars(DelayedGoals, Vars),
-		ModeError = mode_error_conj(DelayedGoals,
+		get_all_waiting_vars(NonHeadVarUnificationGoals, Vars),
+		ModeError = mode_error_conj(NonHeadVarUnificationGoals,
 			goals_followed_by_impure_goal(Goal)),
 		mode_info_get_context(!.ModeInfo, Context),
 		mode_info_get_mode_context(!.ModeInfo, ModeContext),
@@ -2095,20 +2130,28 @@
 		!:ImpurityErrors = [ImpurityError | !.ImpurityErrors]
 	).
 
-:- pred no_non_headvar_unification_goals(list(delayed_goal)::in,
-	list(prog_var)::in) is semidet.
 
-no_non_headvar_unification_goals([], _).
-no_non_headvar_unification_goals([delayed_goal(_, _, Goal - _) | Goals],
-		HeadVars) :-
+:- pred filter_headvar_unification_goals(list(prog_var)::in,
+	list(delayed_goal)::in, list(delayed_goal)::out,
+	list(delayed_goal)::out) is det.
+
+filter_headvar_unification_goals(HeadVars, DelayedGoals,
+		HeadVarUnificationGoals, NonHeadVarUnificationGoals) :-
+	list__filter(is_headvar_unification_goal(HeadVars), DelayedGoals,
+		HeadVarUnificationGoals, NonHeadVarUnificationGoals).
+
+
+:- pred is_headvar_unification_goal(list(prog_var)::in, delayed_goal::in)
+	is semidet.
+
+is_headvar_unification_goal(HeadVars, delayed_goal(_, _, Goal - _)) :-
 	Goal = unify(Var, RHS, _, _, _),
 	(
 		list__member(Var, HeadVars)
 	;
 		RHS = var(OtherVar),
 		list__member(OtherVar, HeadVars)
-	),
-	no_non_headvar_unification_goals(Goals, HeadVars).
+	).
 
 	% Given an association list of Vars - Goals,
 	% combine all the Vars together into a single set.
@@ -2126,6 +2169,17 @@
 get_all_waiting_vars_2([delayed_goal(Vars1, _, _) | Rest], Vars0, Vars) :-
 	set__union(Vars0, Vars1, Vars2),
 	get_all_waiting_vars_2(Rest, Vars2, Vars).
+
+
+:- pred redelay_goals(list(delayed_goal)::in, delay_info::in, delay_info::out)
+	is det.
+
+redelay_goals([], DelayInfo, DelayInfo).
+
+redelay_goals([DelayedGoal | DelayedGoals], DelayInfo0, DelayInfo) :-
+	DelayedGoal = delayed_goal(_WaitingVars, ModeErrorInfo, Goal),
+	delay_info__delay_goal(DelayInfo0, ModeErrorInfo, Goal, DelayInfo1),
+	redelay_goals(DelayedGoals, DelayInfo1, DelayInfo).
 
 %-----------------------------------------------------------------------------%
 
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