src/nullable.c

   1 /* Calculate which nonterminals can expand into the null string for Bison.
   2
   3    Copyright (C) 1984, 1989, 2000, 2001, 2002, 2003, 2004, 2005, 2006
   4    Free Software Foundation, Inc.
   5
   6    This file is part of Bison, the GNU Compiler Compiler.
   7
   8    Bison is free software; you can redistribute it and/or modify
   9    it under the terms of the GNU General Public License as published by
  10    the Free Software Foundation; either version 2, or (at your option)
  11    any later version.
  12
  13    Bison is distributed in the hope that it will be useful,
  14    but WITHOUT ANY WARRANTY; without even the implied warranty of
  15    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16    GNU General Public License for more details.
  17
  18    You should have received a copy of the GNU General Public License
  19    along with Bison; see the file COPYING.  If not, write to
  20    the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  21    Boston, MA 02110-1301, USA.  */
  22
  23
  24 /* Set up NULLABLE, a vector saying which nonterminals can expand into
  25    the null string.  NULLABLE[I - NTOKENS] is nonzero if symbol I can
  26    do so.  */
  27
  28 #include <config.h>
  29 #include "system.h"
  30
  31 #include "getargs.h"
  32 #include "gram.h"
  33 #include "nullable.h"
  34 #include "reduce.h"
  35 #include "symtab.h"
  36
  37 /* Linked list of rules.  */
  38 typedef struct rule_list
  39 {
  40   struct rule_list *next;
  41   rule *value;
  42 } rule_list;
  43
  44 bool *nullable = NULL;
  45
  46 static void
  47 nullable_print (FILE *out)
  48 {
  49   int i;
  50   fputs ("NULLABLE\n", out);
  51   for (i = ntokens; i < nsyms; i++)
  52     fprintf (out, "\t%s: %s\n", symbols[i]->tag,
  53              nullable[i - ntokens] ? "yes" : "no");
  54   fputs ("\n\n", out);
  55 }
  56
  57 void
  58 nullable_compute (void)
  59 {
  60   rule_number ruleno;
  61   symbol_number *s1;
  62   symbol_number *s2;
  63   rule_list *p;
  64
  65   symbol_number *squeue = xnmalloc (nvars, sizeof *squeue);
  66   size_t *rcount = xcalloc (nrules, sizeof *rcount);
  67   /* RITEM contains all the rules, including useless productions.
  68      Hence we must allocate room for useless nonterminals too.  */
  69   rule_list **rsets = xcalloc (nvars, sizeof *rsets);
  70   /* This is said to be more elements than we actually use.
  71      Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
  72   rule_list *relts = xnmalloc (nritems + nvars + 1, sizeof *relts);
  73
  74   nullable = xcalloc (nvars, sizeof *nullable);
  75
  76   s1 = s2 = squeue;
  77   p = relts;
  78
  79   for (ruleno = 0; ruleno < nrules; ++ruleno)
  80     if (rules[ruleno].useful)
  81       {
  82         rule *rules_ruleno = &rules[ruleno];
  83         if (rules_ruleno->rhs[0] >= 0)
  84           {
  85             /* This rule has a non empty RHS. */
  86             item_number *rp = NULL;
  87             bool any_tokens = false;
  88             for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
  89               if (ISTOKEN (*rp))
  90                 any_tokens = true;
  91
  92             /* This rule has only nonterminals: schedule it for the second
  93                pass.  */
  94             if (!any_tokens)
  95               for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
  96                 {
  97                   rcount[ruleno]++;
  98                   p->next = rsets[*rp - ntokens];
  99                   p->value = rules_ruleno;
 100                   rsets[*rp - ntokens] = p;
 101                   p++;
 102                 }
 103           }
 104         else
 105           {
 106             /* This rule has an empty RHS. */
 107             aver (item_number_as_rule_number (rules_ruleno->rhs[0])
 108                   == ruleno);
 109             if (rules_ruleno->useful
 110                 && ! nullable[rules_ruleno->lhs->number - ntokens])
 111               {
 112                 nullable[rules_ruleno->lhs->number - ntokens] = true;
 113                 *s2++ = rules_ruleno->lhs->number;
 114               }
 115           }
 116       }
 117
 118   while (s1 < s2)
 119     for (p = rsets[*s1++ - ntokens]; p; p = p->next)
 120       {
 121         rule *r = p->value;
 122         if (--rcount[r->number] == 0)
 123           if (r->useful && ! nullable[r->lhs->number - ntokens])
 124             {
 125               nullable[r->lhs->number - ntokens] = true;
 126               *s2++ = r->lhs->number;
 127             }
 128       }
 129
 130   free (squeue);
 131   free (rcount);
 132   free (rsets);
 133   free (relts);
 134
 135   if (trace_flag & trace_sets)
 136     nullable_print (stderr);
 137 }
 138
 139
 140 void
 141 nullable_free (void)
 142 {
 143   free (nullable);
 144 }