X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/d5eb0826afb8ede89bfe314671f2b37db55bd817..041308d0c03783c51ef362f888c957cd2a0e129a:/src/tables.h

diff --git a/src/tables.h b/src/tables.h
index 9b52f092..8b5c1fa4 100644
--- a/src/tables.h
+++ b/src/tables.h
@@ -1,5 +1,6 @@
 /* Prepare the LALR and GLR parser tables.
-   Copyright (C) 2002, 2004, 2009 Free Software Foundation, Inc.
+
+   Copyright (C) 2002, 2004, 2009-2012 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -48,7 +49,7 @@
    YYFINAL = the state number of the termination state.
 
    YYTABLE = a vector filled with portions for different uses, found
-   via YYPACT and YYPGOTO.
+   via YYPACT and YYPGOTO, described below.
 
    YYLAST ( = high) the number of the last element of YYTABLE, i.e.,
    sizeof (YYTABLE) - 1.
@@ -87,7 +88,7 @@
 
    If the value is negative, it is minus a rule number to reduce by.
 
-   If the value is zero or YYTABLE_NINF, it's a syntax error.
+   If the value is YYTABLE_NINF, it's a syntax error.
 
    YYPGOTO[I] = the index in YYTABLE of the portion describing what to
    do after reducing a rule that derives variable I + NTOKENS.  This
@@ -99,6 +100,16 @@
    YYCHECK[YYPGOTO[I] + S] != S), then the default state (that is,
    YYDEFGOTO[I]) should be used instead of YYTABLE.  Otherwise,
    YYTABLE[YYPGOTO[I] + S] is the state to go to even if YYPGOTO[I] < 0.
+
+   When the above YYPACT, YYPGOTO, and YYCHECK tests determine that a
+   value from YYTABLE should be used, that value is never zero, so it is
+   useless to check for zero.  When those tests indicate that the value
+   from YYDEFACT or YYDEFGOTO should be used instead, the value from
+   YYTABLE *might* be zero, which, as a consequence of the way in which
+   the tables are constructed, also happens to indicate that YYDEFACT or
+   YYDEFGOTO should be used.  However, the YYTABLE value cannot be
+   trusted when the YYDEFACT or YYDEFGOTO value should be used.  In
+   summary, forget about zero values in YYTABLE.
 */
 
 extern int nvectors;