for review: liveness bug fix, take 2

[Zoltan Somogyi]

Estimated hours taken: 1

compiler/liveness.m:
	Reapply the earlier change after fixing it. The problem was that
	the procedure body goal with the updated nonlocal annotation was
	not put back into the procinfo before the computation of the initial
	liveness, which therefore used the out-of-date nonlocal annotation.

Zoltan.

Index: compiler/liveness.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/liveness.m,v
retrieving revision 1.83
diff -u -r1.83 liveness.m
--- liveness.m	1997/10/17 01:53:46	1.83
+++ liveness.m	1997/10/17 05:47:29
@@ -52,11 +52,13 @@
 % code at that resume point as well as the nature of the required
 % entry labels.
 %
-% Accurate Garbage Collection Notes:
+% Accurate garbage collection notes:
 %
-% Note that when using accurate gc, liveness is computed slightly
-% differently. The garbage collector needs access to the typeinfo
-% variables of any variable that could be garbage collected. 
+% When using accurate gc, liveness is computed slightly differently.
+% The garbage collector needs access to the typeinfo variables of any
+% variable that could be live at a garbage collection point. In the
+% present design of the garbage collector, garbage collection takes place
+% at procedure returns.
 % 
 % Hence, the invariant needed for accurate GC is:
 % 	a variable holding a typeinfo must be live at any continuation
@@ -75,6 +77,7 @@
 % 
 % A typeinfo variable becomes dead after both the following conditions
 % are true: 
+%
 % 	(1) The typeinfo variable is not used again (when it is no
 % 	    longer part of the nonlocals)
 %	(2) No other nonlocal variable's type is described by that typeinfo
@@ -91,6 +94,30 @@
 % die only when no other variable needing them will be live, so the
 % invariant holds.
 %
+% Quantification notes:
+%
+% If a variable is not live on entry to a goal, but the goal gives it a value,
+% the code of this module assumes that
+%
+% (a) any parallel goals also give it a value, or
+% (b) the variable is local to this goal and hence does not occur in parallel
+%     goals.
+%
+% If a variable occurs in the nonlocal set of the goal, the code of this
+% assumes that (b) is not true, and will therefore require (a) to be true.
+% If some of the parallel goals cannot succeed, the first pass will include
+% the variable in their post-birth sets.
+%
+% If a variable occurs in the nonlocal set of the goal, but is actually
+% local to the goal, then any occurrence of that variable in the postbirth
+% sets of parallel goals will lead to an inconsistency, because the variable
+% will not die on those parallel paths, but will die on the path that
+% actually gives a value to the variable.
+%
+% The nonlocal set of a goal is in general allowed to overapproximate the
+% true set of nonlocal variables of the goal. Since this module requires
+% *exact* information about nonlocals, it must recompute the nonlocal sets
+% before starting.
 
 %-----------------------------------------------------------------------------%
 
@@ -118,30 +145,31 @@
 :- implementation.
 
 :- import_module hlds_goal, hlds_data, llds, quantification, (inst), instmap.
-:- import_module hlds_out, mode_util, code_util.
+:- import_module hlds_out, mode_util, code_util, quantification.
 :- import_module prog_data, globals, passes_aux.
 :- import_module bool, list, map, set, std_util, term, assoc_list, require.
 :- import_module varset, string.
 
 detect_liveness_proc(ProcInfo0, ModuleInfo, ProcInfo) :-
-	proc_info_goal(ProcInfo0, Goal0),
-	proc_info_variables(ProcInfo0, Varset),
-	proc_info_vartypes(ProcInfo0, VarTypes),
-	live_info_init(ModuleInfo, ProcInfo0, VarTypes, Varset, LiveInfo),
+	requantify_proc(ProcInfo0, ProcInfo1),
+	proc_info_goal(ProcInfo1, Goal0),
+	proc_info_variables(ProcInfo1, Varset),
+	proc_info_vartypes(ProcInfo1, VarTypes),
+	live_info_init(ModuleInfo, ProcInfo1, VarTypes, Varset, LiveInfo),
 
-	initial_liveness(ProcInfo0, ModuleInfo, Liveness0),
+	initial_liveness(ProcInfo1, ModuleInfo, Liveness0),
 	detect_liveness_in_goal(Goal0, Liveness0, LiveInfo,
 		_, Goal1),
 
-	initial_deadness(ProcInfo0, ModuleInfo, Deadness0),
+	initial_deadness(ProcInfo1, ModuleInfo, Deadness0),
 	detect_deadness_in_goal(Goal1, Deadness0, LiveInfo, _, Goal2),
 
 	set__init(ResumeVars0),
 	detect_resume_points_in_goal(Goal2, Liveness0, LiveInfo,
 		ResumeVars0, Goal, _),
 
-	proc_info_set_goal(ProcInfo0, Goal, ProcInfo1),
-	proc_info_set_liveness_info(ProcInfo1, Liveness0, ProcInfo).
+	proc_info_set_goal(ProcInfo1, Goal, ProcInfo2),
+	proc_info_set_liveness_info(ProcInfo2, Liveness0, ProcInfo).
 
 %-----------------------------------------------------------------------------%
 %-----------------------------------------------------------------------------%