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Diffstat (limited to 'src/nullable.c')
| -rw-r--r-- | src/nullable.c | 144 |
1 files changed, 144 insertions, 0 deletions
diff --git a/src/nullable.c b/src/nullable.c new file mode 100644 index 00000000..83a90e95 --- /dev/null +++ b/src/nullable.c @@ -0,0 +1,144 @@ +/* Calculate which nonterminals can expand into the null string for Bison. + + Copyright (C) 1984, 1989, 2000, 2001, 2002, 2003, 2004, 2005, 2006 + Free Software Foundation, Inc. + + This file is part of Bison, the GNU Compiler Compiler. + + Bison is free software; you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 2, or (at your option) + any later version. + + Bison is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with Bison; see the file COPYING. If not, write to + the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, + Boston, MA 02110-1301, USA. */ + + +/* Set up NULLABLE, a vector saying which nonterminals can expand into + the null string. NULLABLE[I - NTOKENS] is nonzero if symbol I can + do so. */ + +#include <config.h> +#include "system.h" + +#include "getargs.h" +#include "gram.h" +#include "nullable.h" +#include "reduce.h" +#include "symtab.h" + +/* Linked list of rules. */ +typedef struct rule_list +{ + struct rule_list *next; + rule *value; +} rule_list; + +bool *nullable = NULL; + +static void +nullable_print (FILE *out) +{ + int i; + fputs ("NULLABLE\n", out); + for (i = ntokens; i < nsyms; i++) + fprintf (out, "\t%s: %s\n", symbols[i]->tag, + nullable[i - ntokens] ? "yes" : "no"); + fputs ("\n\n", out); +} + +void +nullable_compute (void) +{ + rule_number ruleno; + symbol_number *s1; + symbol_number *s2; + rule_list *p; + + symbol_number *squeue = xnmalloc (nvars, sizeof *squeue); + size_t *rcount = xcalloc (nrules, sizeof *rcount); + /* RITEM contains all the rules, including useless productions. + Hence we must allocate room for useless nonterminals too. */ + rule_list **rsets = xcalloc (nvars, sizeof *rsets); + /* This is said to be more elements than we actually use. + Supposedly NRITEMS - NRULES is enough. But why take the risk? */ + rule_list *relts = xnmalloc (nritems + nvars + 1, sizeof *relts); + + nullable = xcalloc (nvars, sizeof *nullable); + + s1 = s2 = squeue; + p = relts; + + for (ruleno = 0; ruleno < nrules; ++ruleno) + if (rules[ruleno].useful) + { + rule *rules_ruleno = &rules[ruleno]; + if (rules_ruleno->rhs[0] >= 0) + { + /* This rule has a non empty RHS. */ + item_number *rp = NULL; + bool any_tokens = false; + for (rp = rules_ruleno->rhs; *rp >= 0; ++rp) + if (ISTOKEN (*rp)) + any_tokens = true; + + /* This rule has only nonterminals: schedule it for the second + pass. */ + if (!any_tokens) + for (rp = rules_ruleno->rhs; *rp >= 0; ++rp) + { + rcount[ruleno]++; + p->next = rsets[*rp - ntokens]; + p->value = rules_ruleno; + rsets[*rp - ntokens] = p; + p++; + } + } + else + { + /* This rule has an empty RHS. */ + assert (item_number_as_rule_number (rules_ruleno->rhs[0]) + == ruleno); + if (rules_ruleno->useful + && ! nullable[rules_ruleno->lhs->number - ntokens]) + { + nullable[rules_ruleno->lhs->number - ntokens] = true; + *s2++ = rules_ruleno->lhs->number; + } + } + } + + while (s1 < s2) + for (p = rsets[*s1++ - ntokens]; p; p = p->next) + { + rule *r = p->value; + if (--rcount[r->number] == 0) + if (r->useful && ! nullable[r->lhs->number - ntokens]) + { + nullable[r->lhs->number - ntokens] = true; + *s2++ = r->lhs->number; + } + } + + free (squeue); + free (rcount); + free (rsets); + free (relts); + + if (trace_flag & trace_sets) + nullable_print (stderr); +} + + +void +nullable_free (void) +{ + free (nullable); +} |