1  /* Calculate which nonterminals can expand into the null string for Bison.
       2  
       3     Copyright (C) 1984, 1989, 2000-2006, 2009-2015, 2018-2021 Free
       4     Software Foundation, Inc.
       5  
       6     This file is part of Bison, the GNU Compiler Compiler.
       7  
       8     This program 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 3 of the License, or
      11     (at your option) any later version.
      12  
      13     This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.  */
      20  
      21  
      22  /* Set up NULLABLE, a vector saying which nonterminals can expand into
      23     the null string.  NULLABLE[I - NTOKENS] is nonzero if symbol I can
      24     do so.  */
      25  
      26  #include <config.h>
      27  #include "system.h"
      28  
      29  #include "getargs.h"
      30  #include "gram.h"
      31  #include "nullable.h"
      32  #include "reduce.h"
      33  #include "symtab.h"
      34  
      35  /* Linked list of rules.  */
      36  typedef struct rule_list
      37  {
      38    struct rule_list *next;
      39    const rule *value;
      40  } rule_list;
      41  
      42  bool *nullable = NULL;
      43  
      44  static void
      45  nullable_print (FILE *out)
      46  {
      47    fputs ("NULLABLE\n", out);
      48    for (int i = ntokens; i < nsyms; i++)
      49      fprintf (out, "  %s: %s\n", symbols[i]->tag,
      50               nullable[i - ntokens] ? "yes" : "no");
      51    fputs ("\n\n", out);
      52  }
      53  
      54  void
      55  nullable_compute (void)
      56  {
      57    nullable = xcalloc (nnterms, sizeof *nullable);
      58  
      59    size_t *rcount = xcalloc (nrules, sizeof *rcount);
      60    /* RITEM contains all the rules, including useless productions.
      61       Hence we must allocate room for useless nonterminals too.  */
      62    rule_list **rsets = xcalloc (nnterms, sizeof *rsets);
      63    /* This is said to be more elements than we actually use.
      64       Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
      65    rule_list *relts = xnmalloc (nritems + nnterms + 1, sizeof *relts);
      66  
      67    symbol_number *squeue = xnmalloc (nnterms, sizeof *squeue);
      68    symbol_number *s2 = squeue;
      69    {
      70      rule_list *p = relts;
      71      for (rule_number ruleno = 0; ruleno < nrules; ++ruleno)
      72        if (rules[ruleno].useful)
      73          {
      74            const rule *r = &rules[ruleno];
      75            if (r->rhs[0] >= 0)
      76              {
      77                /* This rule has a non empty RHS. */
      78                bool any_tokens = false;
      79                for (item_number *rp = r->rhs; *rp >= 0; ++rp)
      80                  if (ISTOKEN (*rp))
      81                    any_tokens = true;
      82  
      83                /* This rule has only nonterminals: schedule it for the second
      84                   pass.  */
      85                if (!any_tokens)
      86                  for (item_number *rp = r->rhs; *rp >= 0; ++rp)
      87                    {
      88                      rcount[ruleno]++;
      89                      p->next = rsets[*rp - ntokens];
      90                      p->value = r;
      91                      rsets[*rp - ntokens] = p;
      92                      p++;
      93                    }
      94              }
      95            else
      96              {
      97                /* This rule has an empty RHS. */
      98                if (r->useful
      99                    && ! nullable[r->lhs->number - ntokens])
     100                  {
     101                    nullable[r->lhs->number - ntokens] = true;
     102                    *s2++ = r->lhs->number;
     103                  }
     104              }
     105          }
     106    }
     107  
     108    symbol_number *s1 = squeue;
     109    while (s1 < s2)
     110      for (rule_list *p = rsets[*s1++ - ntokens]; p; p = p->next)
     111        {
     112          const rule *r = p->value;
     113          if (--rcount[r->number] == 0)
     114            if (r->useful && ! nullable[r->lhs->number - ntokens])
     115              {
     116                nullable[r->lhs->number - ntokens] = true;
     117                *s2++ = r->lhs->number;
     118              }
     119        }
     120  
     121    free (squeue);
     122    free (rcount);
     123    free (rsets);
     124    free (relts);
     125  
     126    if (trace_flag & trace_sets)
     127      nullable_print (stderr);
     128  }
     129  
     130  
     131  void
     132  nullable_free (void)
     133  {
     134    free (nullable);
     135  }