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 Free Software
   4    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., 59 Temple Place - Suite 330,
  21    Boston, MA 02111-1307, 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 "system.h"
  29
  30 #include "getargs.h"
  31 #include "gram.h"
  32 #include "nullable.h"
  33 #include "reduce.h"
  34 #include "symtab.h"
  35
  36 /* Linked list of rules.  */
  37 typedef struct rule_list
  38 {
  39   struct rule_list *next;
  40   rule *value;
  41 } rule_list;
  42
  43 bool *nullable = NULL;
  44
  45 static void
  46 nullable_print (FILE *out)
  47 {
  48   int i;
  49   fputs ("NULLABLE\n", out);
  50   for (i = ntokens; i < nsyms; i++)
  51     fprintf (out, "\t%s: %s\n", symbols[i]->tag,
  52              nullable[i - ntokens] ? "yes" : "no");
  53   fputs ("\n\n", out);
  54 }
  55
  56 void
  57 nullable_compute (void)
  58 {
  59   rule_number ruleno;
  60   symbol_number *s1;
  61   symbol_number *s2;
  62   rule_list *p;
  63
  64   symbol_number *squeue = CALLOC (squeue, nvars);
  65   short *rcount = CALLOC (rcount, nrules);
  66   /* RITEM contains all the rules, including useless productions.
  67      Hence we must allocate room for useless nonterminals too.  */
  68   rule_list **rsets = CALLOC (rsets, nvars);
  69   /* This is said to be more elements than we actually use.
  70      Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
  71   rule_list *relts = CALLOC (relts, nritems + nvars + 1);
  72
  73   CALLOC (nullable, nvars);
  74
  75   s1 = s2 = squeue;
  76   p = relts;
  77
  78   for (ruleno = 0; ruleno < nrules; ++ruleno)
  79     if (rules[ruleno].useful)
  80       {
  81         rule *rules_ruleno = &rules[ruleno];
  82         if (rules_ruleno->rhs[0] >= 0)
  83           {
  84             /* This rule has a non empty RHS. */
  85             item_number *rp = NULL;
  86             bool any_tokens = false;
  87             for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
  88               if (ISTOKEN (*rp))
  89                 any_tokens = true;
  90
  91             /* This rule has only nonterminals: schedule it for the second
  92                pass.  */
  93             if (!any_tokens)
  94               for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
  95                 {
  96                   rcount[ruleno]++;
  97                   p->next = rsets[*rp - ntokens];
  98                   p->value = rules_ruleno;
  99                   rsets[*rp - ntokens] = p;
 100                   p++;
 101                 }
 102           }
 103         else
 104           {
 105             /* This rule has an empty RHS. */
 106             if (item_number_as_rule_number (rules_ruleno->rhs[0]) != ruleno)
 107               abort ();
 108             if (rules_ruleno->useful
 109                 && ! nullable[rules_ruleno->lhs->number - ntokens])
 110               {
 111                 nullable[rules_ruleno->lhs->number - ntokens] = true;
 112                 *s2++ = rules_ruleno->lhs->number;
 113               }
 114           }
 115       }
 116
 117   while (s1 < s2)
 118     for (p = rsets[*s1++ - ntokens]; p; p = p->next)
 119       {
 120         rule *r = p->value;
 121         if (--rcount[r->number] == 0)
 122           if (r->useful && ! nullable[r->lhs->number - ntokens])
 123             {
 124               nullable[r->lhs->number - ntokens] = true;
 125               *s2++ = r->lhs->number;
 126             }
 127       }
 128
 129   XFREE (squeue);
 130   XFREE (rcount);
 131   XFREE (rsets);
 132   XFREE (relts);
 133
 134   if (trace_flag & trace_sets)
 135     nullable_print (stderr);
 136 }
 137
 138
 139 void
 140 nullable_free (void)
 141 {
 142   XFREE (nullable);
 143 }