bison/src/lalr.c

/* Compute lookahead criteria for Bison.

   Copyright (C) 1984, 1986, 1989, 2000, 2001, 2002, 2003, 2004, 2005,
   2006, 2007, 2008, 2009 Free Software Foundation, Inc.

   This file is part of Bison, the GNU Compiler Compiler.

   This program 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 3 of the License, or
   (at your option) any later version.

   This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.  */


/* Compute how to make the finite state machine deterministic; find
   which rules need lookahead in each state, and which lookahead
   tokens they accept.  */

#include <config.h>
#include "system.h"

#include <bitset.h>
#include <bitsetv.h>
#include <quotearg.h>

#include "LR0.h"
#include "complain.h"
#include "derives.h"
#include "getargs.h"
#include "gram.h"
#include "lalr.h"
#include "muscle-tab.h"
#include "nullable.h"
#include "reader.h"
#include "relation.h"
#include "symtab.h"

goto_number *goto_map;
goto_number ngotos;
state_number *from_state;
state_number *to_state;
bitsetv goto_follows = NULL;

/* Linked list of goto numbers.  */
typedef struct goto_list
{
  struct goto_list *next;
  goto_number value;
} goto_list;


/* LA is an NLA by NTOKENS matrix of bits.  LA[l, i] is 1 if the rule
   LArule[l] is applicable in the appropriate state when the next
   token is symbol i.  If LA[l, i] and LA[l, j] are both 1 for i != j,
   it is a conflict.  */

static bitsetv LA = NULL;
size_t nLA;


static goto_number **includes;
static goto_list **lookback;




void
set_goto_map (void)
{
  state_number s;
  goto_number *temp_map;

  goto_map = xcalloc (nvars + 1, sizeof *goto_map);
  temp_map = xnmalloc (nvars + 1, sizeof *temp_map);

  ngotos = 0;
  for (s = 0; s < nstates; ++s)
    {
      transitions *sp = states[s]->transitions;
      int i;
      for (i = sp->num - 1; i >= 0 && TRANSITION_IS_GOTO (sp, i); --i)
	{
	  ngotos++;

	  /* Abort if (ngotos + 1) would overflow.  */
	  aver (ngotos != GOTO_NUMBER_MAXIMUM);

	  goto_map[TRANSITION_SYMBOL (sp, i) - ntokens]++;
	}
    }

  {
    goto_number k = 0;
    int i;
    for (i = ntokens; i < nsyms; i++)
      {
	temp_map[i - ntokens] = k;
	k += goto_map[i - ntokens];
      }

    for (i = ntokens; i < nsyms; i++)
      goto_map[i - ntokens] = temp_map[i - ntokens];

    goto_map[nsyms - ntokens] = ngotos;
    temp_map[nsyms - ntokens] = ngotos;
  }

  from_state = xcalloc (ngotos, sizeof *from_state);
  to_state = xcalloc (ngotos, sizeof *to_state);

  for (s = 0; s < nstates; ++s)
    {
      transitions *sp = states[s]->transitions;
      int i;
      for (i = sp->num - 1; i >= 0 && TRANSITION_IS_GOTO (sp, i); --i)
	{
	  goto_number k = temp_map[TRANSITION_SYMBOL (sp, i) - ntokens]++;
	  from_state[k] = s;
	  to_state[k] = sp->states[i]->number;
	}
    }

  free (temp_map);
}


goto_number
map_goto (state_number s0, symbol_number sym)
{
  goto_number high;
  goto_number low;
  goto_number middle;
  state_number s;

  low = goto_map[sym - ntokens];
  high = goto_map[sym - ntokens + 1] - 1;

  for (;;)
    {
      aver (low <= high);
      middle = (low + high) / 2;
      s = from_state[middle];
      if (s == s0)
	return middle;
      else if (s < s0)
	low = middle + 1;
      else
	high = middle - 1;
    }
}


static void
initialize_F (void)
{
  goto_number **reads = xnmalloc (ngotos, sizeof *reads);
  goto_number *edge = xnmalloc (ngotos + 1, sizeof *edge);
  goto_number nedges = 0;

  goto_number i;

  goto_follows = bitsetv_create (ngotos, ntokens, BITSET_FIXED);

  for (i = 0; i < ngotos; i++)
    {
      state_number stateno = to_state[i];
      transitions *sp = states[stateno]->transitions;

      int j;
      FOR_EACH_SHIFT (sp, j)
	bitset_set (goto_follows[i], TRANSITION_SYMBOL (sp, j));

      for (; j < sp->num; j++)
	{
	  symbol_number sym = TRANSITION_SYMBOL (sp, j);
	  if (nullable[sym - ntokens])
	    edge[nedges++] = map_goto (stateno, sym);
	}

      if (nedges == 0)
	reads[i] = NULL;
      else
	{
	  reads[i] = xnmalloc (nedges + 1, sizeof reads[i][0]);
	  memcpy (reads[i], edge, nedges * sizeof edge[0]);
	  reads[i][nedges] = END_NODE;
	  nedges = 0;
	}
    }

  relation_digraph (reads, ngotos, &goto_follows);

  for (i = 0; i < ngotos; i++)
    free (reads[i]);

  free (reads);
  free (edge);
}


static void
add_lookback_edge (state *s, rule *r, goto_number gotono)
{
  int ri = state_reduction_find (s, r);
  goto_list *sp = xmalloc (sizeof *sp);
  sp->next = lookback[(s->reductions->lookahead_tokens - LA) + ri];
  sp->value = gotono;
  lookback[(s->reductions->lookahead_tokens - LA) + ri] = sp;
}



static void
build_relations (void)
{
  goto_number *edge = xnmalloc (ngotos + 1, sizeof *edge);
  state_number *states1 = xnmalloc (ritem_longest_rhs () + 1, sizeof *states1);
  goto_number i;

  includes = xnmalloc (ngotos, sizeof *includes);

  for (i = 0; i < ngotos; i++)
    {
      int nedges = 0;
      symbol_number symbol1 = states[to_state[i]]->accessing_symbol;
      rule **rulep;

      for (rulep = derives[symbol1 - ntokens]; *rulep; rulep++)
	{
	  bool done;
	  int length = 1;
	  item_number const *rp;
	  state *s = states[from_state[i]];
	  states1[0] = s->number;

	  for (rp = (*rulep)->rhs; ! item_number_is_rule_number (*rp); rp++)
	    {
	      s = transitions_to (s->transitions,
				  item_number_as_symbol_number (*rp));
	      states1[length++] = s->number;
	    }

	  if (!s->consistent)
	    add_lookback_edge (s, *rulep, i);

	  length--;
	  done = false;
	  while (!done)
	    {
	      done = true;
	      /* Each rhs ends in a rule number, and there is a
		 sentinel (ritem[-1]=0) before the first rhs, so it is safe to
		 decrement RP here.  */
	      rp--;
	      if (ISVAR (*rp))
		{
		  /* Downcasting from item_number to symbol_number.  */
		  edge[nedges++] = map_goto (states1[--length],
					     item_number_as_symbol_number (*rp));
		  if (nullable[*rp - ntokens])
		    done = false;
		}
	    }
	}

      if (nedges == 0)
	includes[i] = NULL;
      else
	{
	  int j;
	  includes[i] = xnmalloc (nedges + 1, sizeof includes[i][0]);
	  for (j = 0; j < nedges; j++)
	    includes[i][j] = edge[j];
	  includes[i][nedges] = END_NODE;
	}
    }

  free (edge);
  free (states1);

  relation_transpose (&includes, ngotos);
}



static void
compute_FOLLOWS (void)
{
  goto_number i;

  relation_digraph (includes, ngotos, &goto_follows);

  for (i = 0; i < ngotos; i++)
    free (includes[i]);

  free (includes);
}


static void
compute_lookahead_tokens (void)
{
  size_t i;
  goto_list *sp;

  for (i = 0; i < nLA; i++)
    for (sp = lookback[i]; sp; sp = sp->next)
      bitset_or (LA[i], LA[i], goto_follows[sp->value]);

  /* Free LOOKBACK. */
  for (i = 0; i < nLA; i++)
    LIST_FREE (goto_list, lookback[i]);

  free (lookback);
}


/*----------------------------------------------------.
| Count the number of lookahead tokens required for S |
| (N_LOOKAHEAD_TOKENS member).                        |
`----------------------------------------------------*/

static int
state_lookahead_tokens_count (state *s, bool default_reduction_only_for_accept)
{
  int n_lookahead_tokens = 0;
  reductions *rp = s->reductions;
  transitions *sp = s->transitions;

  /* Transitions are only disabled during conflict resolution, and that
     hasn't happened yet, so there should be no need to check that
     transition 0 hasn't been disabled before checking if it is a shift.
     However, this check was performed at one time, so we leave it as an
     aver.  */
  aver (sp->num == 0 || !TRANSITION_IS_DISABLED (sp, 0));

  /* We need a lookahead either to distinguish different reductions
     (i.e., there are two or more), or to distinguish a reduction from a
     shift.  Otherwise, it is straightforward, and the state is
     `consistent'.  However, do not treat a state with any reductions as
     consistent unless it is the accepting state (because there is never
     a lookahead token that makes sense there, and so no lookahead token
     should be read) if the user has otherwise disabled default
     reductions.  */
  if (rp->num > 1
      || (rp->num == 1 && sp->num && TRANSITION_IS_SHIFT (sp, 0))
      || (rp->num == 1 && rp->rules[0]->number != 0
          && default_reduction_only_for_accept))
    n_lookahead_tokens += rp->num;
  else
    s->consistent = 1;

  return n_lookahead_tokens;
}


/*----------------------------------------------------.
| Compute LA, NLA, and the lookahead_tokens members.  |
`----------------------------------------------------*/

void
initialize_LA (void)
{
  state_number i;
  bitsetv pLA;
  bool default_reduction_only_for_accept;
  {
    char *default_reductions =
      muscle_percent_define_get ("lr.default-reductions");
    default_reduction_only_for_accept =
      0 == strcmp (default_reductions, "accepting");
    free (default_reductions);
  }

  /* Compute the total number of reductions requiring a lookahead.  */
  nLA = 0;
  for (i = 0; i < nstates; i++)
    nLA +=
      state_lookahead_tokens_count (states[i],
                                    default_reduction_only_for_accept);
  /* Avoid having to special case 0.  */
  if (!nLA)
    nLA = 1;

  pLA = LA = bitsetv_create (nLA, ntokens, BITSET_FIXED);

  /* Initialize the members LOOKAHEAD_TOKENS for each state whose reductions
     require lookahead tokens.  */
  for (i = 0; i < nstates; i++)
    {
      int count =
        state_lookahead_tokens_count (states[i],
                                      default_reduction_only_for_accept);
      if (count)
	{
	  states[i]->reductions->lookahead_tokens = pLA;
	  pLA += count;
	}
    }
}


/*---------------------------------------------.
| Output the lookahead tokens for each state.  |
`---------------------------------------------*/

static void
lookahead_tokens_print (FILE *out)
{
  state_number i;
  int j, k;
  fprintf (out, "Lookahead tokens: BEGIN\n");
  for (i = 0; i < nstates; ++i)
    {
      reductions *reds = states[i]->reductions;
      bitset_iterator iter;
      int n_lookahead_tokens = 0;

      if (reds->lookahead_tokens)
	for (k = 0; k < reds->num; ++k)
	  if (reds->lookahead_tokens[k])
	    ++n_lookahead_tokens;

      fprintf (out, "State %d: %d lookahead tokens\n",
	       i, n_lookahead_tokens);

      if (reds->lookahead_tokens)
	for (j = 0; j < reds->num; ++j)
	  BITSET_FOR_EACH (iter, reds->lookahead_tokens[j], k, 0)
	  {
	    fprintf (out, "   on %d (%s) -> rule %d\n",
		     k, symbols[k]->tag,
		     reds->rules[j]->number);
	  };
    }
  fprintf (out, "Lookahead tokens: END\n");
}

void
lalr (void)
{
  initialize_LA ();
  set_goto_map ();
  initialize_F ();
  lookback = xcalloc (nLA, sizeof *lookback);
  build_relations ();
  compute_FOLLOWS ();
  compute_lookahead_tokens ();

  if (trace_flag & trace_sets)
    lookahead_tokens_print (stderr);
}


void
lalr_update_state_numbers (state_number old_to_new[], state_number nstates_old)
{
  goto_number ngotos_reachable = 0;
  symbol_number nonterminal = 0;
  aver (nsyms == nvars + ntokens);
  {
    goto_number i;
    for (i = 0; i < ngotos; ++i)
      {
        while (i == goto_map[nonterminal])
          goto_map[nonterminal++] = ngotos_reachable;
        /* If old_to_new[from_state[i]] = nstates_old, remove this goto
           entry.  */
        if (old_to_new[from_state[i]] != nstates_old)
          {
            /* from_state[i] is not removed, so it and thus to_state[i] are
               reachable, so to_state[i] != nstates_old.  */
            aver (old_to_new[to_state[i]] != nstates_old);
            from_state[ngotos_reachable] = old_to_new[from_state[i]];
            to_state[ngotos_reachable] = old_to_new[to_state[i]];
            ++ngotos_reachable;
          }
      }
  }
  while (nonterminal <= nvars)
    {
      aver (ngotos == goto_map[nonterminal]);
      goto_map[nonterminal++] = ngotos_reachable;
    }
  ngotos = ngotos_reachable;
}


void
lalr_free (void)
{
  state_number s;
  for (s = 0; s < nstates; ++s)
    states[s]->reductions->lookahead_tokens = NULL;
  bitsetv_free (LA);
}