Add %define lr.keep_unreachable_states.

* NEWS (2.3a+): Mention it in the entry for unreachable state removal.
* doc/bison.texinfo (Decl Summary): Mention it in the %define entry.
* src/main.c (main): Implement it.
* tests/conflicts.at (Unreachable States After Conflict Resolution):
Extend to test it, and fix a typo.

ChangeLog

@@ -1,6 +1,15 @@
 2007-10-19  Joel E. Denny  <jdenny@ces.clemson.edu>
 
-	* NEWS: Add entry for recent .output file lookahead set fix.
+	Add %define lr.keep_unreachable_states.
+	* NEWS (2.3a+): Mention it in the entry for unreachable state removal.
+	* doc/bison.texinfo (Decl Summary): Mention it in the %define entry.
+	* src/main.c (main): Implement it.
+	* tests/conflicts.at (Unreachable States After Conflict Resolution):
+	Extend to test it, and fix a typo.
+
+2007-10-19  Joel E. Denny  <jdenny@ces.clemson.edu>
+
+	* NEWS (2.3a+): Add entry for recent .output file lookahead set fix.
 	* doc/bison.texinfo (Understanding): Remove a bogus lookahead set in
 	the example .output text.
 	* tests/regression.at (Extra lookahead sets in report): Improve wording

NEWS

@@ -3,8 +3,9 @@ Bison News
 
 Changes in version 2.3a+ (????-??-??):
 
-* Previously, Bison sometimes generated parser tables with states that were
-  unreachable due to conflicts in predecessor states.  Bison now:
+* Previously, Bison sometimes generated parser tables containing unreachable
+  states.  A state can become unreachable during conflict resolution if Bison
+  disables a shift action leading to it from a predecessor state.  Bison now:
 
     1. Removes unreachable states.
 
@@ -15,6 +16,13 @@ Changes in version 2.3a+ (????-??-??):
     3. For any rule used only in such states, Bison now reports the rule as
        "never reduced because of conflicts".
 
+  This feature can be disabled with the following directive:
+
+    %define lr.keep_unreachable_states
+
+  See the %define entry in the `Bison Declaration Summary' in the Bison manual
+  for further discussion.
+
 * When instructed to generate a `.output' file including lookahead sets
   (using `--report=lookahead', for example), Bison now prints each reduction's
   lookahead set only next to the associated state's one item that (1) is

doc/bison.texinfo

@@ -4851,6 +4851,48 @@ Some of the accepted @var{variable}s are:
 @item Default Value: @code{"pull"}
 @end itemize
 
+@item lr.keep_unreachable_states
+@findex %define lr.keep_unreachable_states
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Requests that Bison allow unreachable parser states to remain in
+the parser tables.
+Bison considers a state to be unreachable if there exists no sequence of
+transitions from the start state to that state.
+A state can become unreachable during conflict resolution if Bison disables a
+shift action leading to it from a predecessor state.
+Keeping unreachable states is sometimes useful for analysis purposes, but they
+are useless in the generated parser.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{"false"}
+
+@item Caveats:
+
+@itemize @bullet
+@item Unreachable states may contain conflicts and may reduce rules not
+reduced in any other state.
+Thus, keeping unreachable states may induce warnings that are irrelevant to
+your parser's behavior, and it may eliminate warnings that are relevant.
+Of course, the change in warnings may actually be relevant to a parser table
+analysis that wants to keep unreachable states, so this behavior will likely
+remain in future Bison releases.
+
+@item While Bison is able to remove unreachable states, it is not guaranteed to
+remove other kinds of useless states.
+Specifically, when Bison disables reduce actions during conflict resolution,
+some goto actions may become useless, and thus some additional states may
+become useless.
+If Bison were to compute which goto actions were useless and then disable those
+actions, it could identify such states as unreachable and then remove those
+states.
+However, Bison does not compute which goto actions are useless.
+@end itemize
+@end itemize
+
 @item namespace
 @findex %define namespace
 

src/main.c

@@ -114,6 +114,8 @@ main (int argc, char *argv[])
      declarations.  */
   timevar_push (TV_CONFLICTS);
   conflicts_solve ();
+  muscle_percent_define_default ("lr.keep_unreachable_states", "false");
+  if (!muscle_percent_define_flag_if ("lr.keep_unreachable_states"))
     {
       state_number *old_to_new = xnmalloc (nstates, sizeof *old_to_new);
       state_number nstates_old = nstates;

tests/conflicts.at

@@ -636,7 +636,7 @@ AT_DATA([[input.y]],
 
 start: resolved_conflict 'a' reported_conflicts 'a' ;
 
-/* S/R conflict resolved as shift, so the state with item
+/* S/R conflict resolved as reduce, so the state with item
  * (resolved_conflict: 'a' . unreachable1) and all it transition successors are
  * unreachable, and the associated production is useless.  */
 resolved_conflict:
@@ -812,6 +812,19 @@ state 7
     $default  reduce using rule 1 (start)
 ]])
 
+AT_DATA([[input-keep.y]],
+[[%define lr.keep_unreachable_states
+]])
+AT_CHECK([[cat input.y >> input-keep.y]])
+
+AT_CHECK([[bison input-keep.y]], 0, [],
+[[input-keep.y: conflicts: 2 shift/reduce, 2 reduce/reduce
+input-keep.y:22.4: warning: rule never reduced because of conflicts: unreachable1: /* empty */
+input-keep.y:26.16: warning: rule never reduced because of conflicts: unreachable2: /* empty */
+input-keep.y:32.5-7: warning: rule never reduced because of conflicts: reported_conflicts: 'a'
+input-keep.y:33.4: warning: rule never reduced because of conflicts: reported_conflicts: /* empty */
+]])
+
 AT_CLEANUP