This is reduce.c in view mode; [Download] [Up]
/* Grammar reduction for Bison.
Copyright (C) 1988, 1989 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, 675 Mass Ave, Cambridge, MA 02139, USA. */
/*
* Reduce the grammar: Find and eliminate unreachable terminals,
* nonterminals, and productions. David S. Bakin.
*/
/*
* Don't eliminate unreachable terminals: They may be used by the user's
* parser.
*/
#include <stdio.h>
#include "system.h"
#include "files.h"
#include "gram.h"
#include "machine.h"
#include "new.h"
extern char **tags; /* reader.c */
extern int verboseflag; /* getargs.c */
static int statisticsflag; /* XXXXXXX */
#ifndef TRUE
#define TRUE (1)
#define FALSE (0)
#endif
typedef int bool;
typedef unsigned *BSet;
typedef short *rule;
/*
* N is set of all nonterminals which are not useless. P is set of all rules
* which have no useless nonterminals in their RHS. V is the set of all
* accessible symbols.
*/
static BSet N, P, V, V1;
static int nuseful_productions, nuseless_productions,
nuseful_nonterminals, nuseless_nonterminals;
static void useless_nonterminals();
static void inaccessable_symbols();
static void reduce_grammar_tables();
static void print_results();
static void print_notices();
void dump_grammar();
extern void fatals ();
bool
bits_equal (L, R, n)
BSet L;
BSet R;
int n;
{
int i;
for (i = n - 1; i >= 0; i--)
if (L[i] != R[i])
return FALSE;
return TRUE;
}
int
nbits (i)
unsigned i;
{
int count = 0;
while (i != 0) {
i ^= (i & -i);
++count;
}
return count;
}
int
bits_size (S, n)
BSet S;
int n;
{
int i, count = 0;
for (i = n - 1; i >= 0; i--)
count += nbits(S[i]);
return count;
}
void
reduce_grammar ()
{
bool reduced;
/* Allocate the global sets used to compute the reduced grammar */
N = NEW2(WORDSIZE(nvars), unsigned);
P = NEW2(WORDSIZE(nrules + 1), unsigned);
V = NEW2(WORDSIZE(nsyms), unsigned);
V1 = NEW2(WORDSIZE(nsyms), unsigned);
useless_nonterminals();
inaccessable_symbols();
reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0);
if (verboseflag)
print_results();
if (reduced == FALSE)
goto done_reducing;
print_notices();
if (!BITISSET(N, start_symbol - ntokens))
fatals("Start symbol %s does not derive any sentence.",
tags[start_symbol]);
reduce_grammar_tables();
/* if (verboseflag) {
fprintf(foutput, "REDUCED GRAMMAR\n\n");
dump_grammar();
}
*/
/**/ statisticsflag = FALSE; /* someday getopts should handle this */
if (statisticsflag == TRUE)
fprintf(stderr,
"reduced %s defines %d terminal%s, %d nonterminal%s\
, and %d production%s.\n", infile,
ntokens, (ntokens == 1 ? "" : "s"),
nvars, (nvars == 1 ? "" : "s"),
nrules, (nrules == 1 ? "" : "s"));
done_reducing:
/* Free the global sets used to compute the reduced grammar */
FREE(N);
FREE(V);
FREE(P);
}
/*
* Another way to do this would be with a set for each production and then do
* subset tests against N, but even for the C grammar the whole reducing
* process takes only 2 seconds on my 8Mhz AT.
*/
static bool
useful_production (i, N)
int i;
BSet N;
{
rule r;
short n;
/*
* A production is useful if all of the nonterminals in its RHS
* appear in the set of useful nonterminals.
*/
for (r = &ritem[rrhs[i]]; *r > 0; r++)
if (ISVAR(n = *r))
if (!BITISSET(N, n - ntokens))
return FALSE;
return TRUE;
}
/* Remember that rules are 1-origin, symbols are 0-origin. */
static void
useless_nonterminals ()
{
BSet Np, Ns;
int i, n;
/*
* N is set as built. Np is set being built this iteration. P is set
* of all productions which have a RHS all in N.
*/
Np = NEW2(WORDSIZE(nvars), unsigned);
/*
* The set being computed is a set of nonterminals which can derive
* the empty string or strings consisting of all terminals. At each
* iteration a nonterminal is added to the set if there is a
* production with that nonterminal as its LHS for which all the
* nonterminals in its RHS are already in the set. Iterate until the
* set being computed remains unchanged. Any nonterminals not in the
* set at that point are useless in that they will never be used in
* deriving a sentence of the language.
*
* This iteration doesn't use any special traversal over the
* productions. A set is kept of all productions for which all the
* nonterminals in the RHS are in useful. Only productions not in
* this set are scanned on each iteration. At the end, this set is
* saved to be used when finding useful productions: only productions
* in this set will appear in the final grammar.
*/
n = 0;
while (1)
{
for (i = WORDSIZE(nvars) - 1; i >= 0; i--)
Np[i] = N[i];
for (i = 1; i <= nrules; i++)
{
if (!BITISSET(P, i))
{
if (useful_production(i, N))
{
SETBIT(Np, rlhs[i] - ntokens);
SETBIT(P, i);
}
}
}
if (bits_equal(N, Np, WORDSIZE(nvars)))
break;
Ns = Np;
Np = N;
N = Ns;
}
FREE(N);
N = Np;
}
static void
inaccessable_symbols ()
{
BSet Vp, Vs, Pp;
int i, n;
short t;
rule r;
/*
* Find out which productions are reachable and which symbols are
* used. Starting with an empty set of productions and a set of
* symbols which only has the start symbol in it, iterate over all
* productions until the set of productions remains unchanged for an
* iteration. For each production which has a LHS in the set of
* reachable symbols, add the production to the set of reachable
* productions, and add all of the nonterminals in the RHS of the
* production to the set of reachable symbols.
*
* Consider only the (partially) reduced grammar which has only
* nonterminals in N and productions in P.
*
* The result is the set P of productions in the reduced grammar, and
* the set V of symbols in the reduced grammar.
*
* Although this algorithm also computes the set of terminals which are
* reachable, no terminal will be deleted from the grammar. Some
* terminals might not be in the grammar but might be generated by
* semantic routines, and so the user might want them available with
* specified numbers. (Is this true?) However, the nonreachable
* terminals are printed (if running in verbose mode) so that the user
* can know.
*/
Vp = NEW2(WORDSIZE(nsyms), unsigned);
Pp = NEW2(WORDSIZE(nrules + 1), unsigned);
/* If the start symbol isn't useful, then nothing will be useful. */
if (!BITISSET(N, start_symbol - ntokens))
goto end_iteration;
SETBIT(V, start_symbol);
n = 0;
while (1)
{
for (i = WORDSIZE(nsyms) - 1; i >= 0; i--)
Vp[i] = V[i];
for (i = 1; i <= nrules; i++)
{
if (!BITISSET(Pp, i) && BITISSET(P, i) &&
BITISSET(V, rlhs[i]))
{
for (r = &ritem[rrhs[i]]; *r >= 0; r++)
{
if (ISTOKEN(t = *r)
|| BITISSET(N, t - ntokens))
{
SETBIT(Vp, t);
}
}
SETBIT(Pp, i);
}
}
if (bits_equal(V, Vp, WORDSIZE(nsyms)))
{
break;
}
Vs = Vp;
Vp = V;
V = Vs;
}
end_iteration:
FREE(V);
V = Vp;
/* Tokens 0, 1, and 2 are internal to Bison. Consider them useful. */
SETBIT(V, 0); /* end-of-input token */
SETBIT(V, 1); /* error token */
SETBIT(V, 2); /* illegal token */
FREE(P);
P = Pp;
nuseful_productions = bits_size(P, WORDSIZE(nrules + 1));
nuseless_productions = nrules - nuseful_productions;
nuseful_nonterminals = 0;
for (i = ntokens; i < nsyms; i++)
if (BITISSET(V, i))
nuseful_nonterminals++;
nuseless_nonterminals = nvars - nuseful_nonterminals;
/* A token that was used in %prec should not be warned about. */
for (i = 1; i < nrules; i++)
if (rprecsym[i] != 0)
SETBIT(V1, rprecsym[i]);
}
static void
reduce_grammar_tables ()
{
/* This is turned off because we would need to change the numbers
in the case statements in the actions file. */
#if 0
/* remove useless productions */
if (nuseless_productions > 0)
{
short np, pn, ni, pi;
np = 0;
ni = 0;
for (pn = 1; pn <= nrules; pn++)
{
if (BITISSET(P, pn))
{
np++;
if (pn != np)
{
rlhs[np] = rlhs[pn];
rline[np] = rline[pn];
rprec[np] = rprec[pn];
rassoc[np] = rassoc[pn];
rrhs[np] = rrhs[pn];
if (rrhs[np] != ni)
{
pi = rrhs[np];
rrhs[np] = ni;
while (ritem[pi] >= 0)
ritem[ni++] = ritem[pi++];
ritem[ni++] = -np;
}
} else {
while (ritem[ni++] >= 0);
}
}
}
ritem[ni] = 0;
nrules -= nuseless_productions;
nitems = ni;
/*
* Is it worth it to reduce the amount of memory for the
* grammar? Probably not.
*/
}
#endif /* 0 */
/* Disable useless productions,
since they may contain useless nonterms
that would get mapped below to -1 and confuse everyone. */
if (nuseless_productions > 0)
{
int pn;
for (pn = 1; pn <= nrules; pn++)
{
if (!BITISSET(P, pn))
{
rlhs[pn] = -1;
}
}
}
/* remove useless symbols */
if (nuseless_nonterminals > 0)
{
int i, n;
/* short j; JF unused */
short *nontermmap;
rule r;
/*
* create a map of nonterminal number to new nonterminal
* number. -1 in the map means it was useless and is being
* eliminated.
*/
nontermmap = NEW2(nvars, short) - ntokens;
for (i = ntokens; i < nsyms; i++)
nontermmap[i] = -1;
n = ntokens;
for (i = ntokens; i < nsyms; i++)
if (BITISSET(V, i))
nontermmap[i] = n++;
/* Shuffle elements of tables indexed by symbol number. */
for (i = ntokens; i < nsyms; i++)
{
n = nontermmap[i];
if (n >= 0)
{
sassoc[n] = sassoc[i];
sprec[n] = sprec[i];
tags[n] = tags[i];
} else {
free(tags[i]);
}
}
/* Replace all symbol numbers in valid data structures. */
for (i = 1; i <= nrules; i++)
{
/* Ignore the rules disabled above. */
if (rlhs[i] >= 0)
rlhs[i] = nontermmap[rlhs[i]];
if (ISVAR (rprecsym[i]))
/* Can this happen? */
rprecsym[i] = nontermmap[rprecsym[i]];
}
for (r = ritem; *r; r++)
if (ISVAR(*r))
*r = nontermmap[*r];
start_symbol = nontermmap[start_symbol];
nsyms -= nuseless_nonterminals;
nvars -= nuseless_nonterminals;
free(&nontermmap[ntokens]);
}
}
static void
print_results ()
{
int i;
/* short j; JF unused */
rule r;
bool b;
if (nuseless_nonterminals > 0)
{
fprintf(foutput, "Useless nonterminals:\n\n");
for (i = ntokens; i < nsyms; i++)
if (!BITISSET(V, i))
fprintf(foutput, " %s\n", tags[i]);
}
b = FALSE;
for (i = 0; i < ntokens; i++)
{
if (!BITISSET(V, i) && !BITISSET(V1, i))
{
if (!b)
{
fprintf(foutput, "\n\nTerminals which are not used:\n\n");
b = TRUE;
}
fprintf(foutput, " %s\n", tags[i]);
}
}
if (nuseless_productions > 0)
{
fprintf(foutput, "\n\nUseless rules:\n\n");
for (i = 1; i <= nrules; i++)
{
if (!BITISSET(P, i))
{
fprintf(foutput, "#%-4d ", i);
fprintf(foutput, "%s :\t", tags[rlhs[i]]);
for (r = &ritem[rrhs[i]]; *r >= 0; r++)
{
fprintf(foutput, " %s", tags[*r]);
}
fprintf(foutput, ";\n");
}
}
}
if (nuseless_nonterminals > 0 || nuseless_productions > 0 || b)
fprintf(foutput, "\n\n");
}
void
dump_grammar ()
{
int i;
rule r;
fprintf(foutput,
"ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n",
ntokens, nvars, nsyms, nrules, nitems);
fprintf(foutput, "Variables\n---------\n\n");
fprintf(foutput, "Value Sprec Sassoc Tag\n");
for (i = ntokens; i < nsyms; i++)
fprintf(foutput, "%5d %5d %5d %s\n",
i, sprec[i], sassoc[i], tags[i]);
fprintf(foutput, "\n\n");
fprintf(foutput, "Rules\n-----\n\n");
for (i = 1; i <= nrules; i++)
{
fprintf(foutput, "%-5d(%5d%5d)%5d : (@%-5d)",
i, rprec[i], rassoc[i], rlhs[i], rrhs[i]);
for (r = &ritem[rrhs[i]]; *r > 0; r++)
fprintf(foutput, "%5d", *r);
fprintf(foutput, " [%d]\n", -(*r));
}
fprintf(foutput, "\n\n");
fprintf(foutput, "Rules interpreted\n-----------------\n\n");
for (i = 1; i <= nrules; i++)
{
fprintf(foutput, "%-5d %s :", i, tags[rlhs[i]]);
for (r = &ritem[rrhs[i]]; *r > 0; r++)
fprintf(foutput, " %s", tags[*r]);
fprintf(foutput, "\n");
}
fprintf(foutput, "\n\n");
}
static void
print_notices ()
{
extern int fixed_outfiles;
if (fixed_outfiles && nuseless_productions)
fprintf(stderr, "%d rules never reduced\n", nuseless_productions);
fprintf(stderr, "%s contains ", infile);
if (nuseless_nonterminals > 0)
{
fprintf(stderr, "%d useless nonterminal%s",
nuseless_nonterminals,
(nuseless_nonterminals == 1 ? "" : "s"));
}
if (nuseless_nonterminals > 0 && nuseless_productions > 0)
fprintf(stderr, " and ");
if (nuseless_productions > 0)
{
fprintf(stderr, "%d useless rule%s",
nuseless_productions,
(nuseless_productions == 1 ? "" : "s"));
}
fprintf(stderr, ".\n");
fflush(stderr);
}
These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.