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Avoid determining regexp subexpression matches, when possible.

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Identifying the precise match locations for parenthesized subexpressions
is a fairly expensive task given the way our regexp engine works, both
at regexp compile time (where we must create an optimized NFA for each
parenthesized subexpression) and at runtime (where determining exact
match locations requires laborious search).

Up to now we've made little attempt to optimize this situation.  This
patch identifies cases where we know at compile time that we won't
need to know subexpression match locations, and teaches the regexp
compiler to not bother creating per-subexpression regexps for
parenthesis pairs that are not referenced by backrefs elsewhere in
the regexp.  (To preserve semantics, we obviously still have to
pin down the match locations of backref references.)  Users could
have obtained the same results before this by being careful to
write "non capturing" parentheses wherever possible, but few people
bother with that.

Discussion: https://postgr.es/m/2219936.1628115334@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2021-08-09 11:26:34 -04:00
parent 76ad24400d
commit 0e6aa8747d
10 changed files with 154 additions and 45 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
src/backend/regex/regc_lex.c
View File

@ -528,10 +528,7 @@ next(struct vars *v)
}
assert(NOTREACHED);
}
if (v->cflags & REG_NOSUB)
RETV('(', 0); /* all parens non-capturing */
else
RETV('(', 1);
RETV('(', 1);
break;
case CHR(')'):
if (LASTTYPE('('))

68
src/backend/regex/regcomp.c
View File

@ -65,7 +65,7 @@ static struct subre *subre(struct vars *, int, int, struct state *, struct state
static void freesubre(struct vars *, struct subre *);
static void freesubreandsiblings(struct vars *, struct subre *);
static void freesrnode(struct vars *, struct subre *);
static void optst(struct vars *, struct subre *);
static void removecaptures(struct vars *, struct subre *);
static int numst(struct subre *, int);
static void markst(struct subre *);
static void cleanst(struct vars *);
@ -431,7 +431,8 @@ pg_regcomp(regex_t *re,
dumpst(v->tree, debug, 1);
}
#endif
optst(v, v->tree);
if (v->cflags & REG_NOSUB)
removecaptures(v, v->tree);
v->ntree = numst(v->tree, 1);
markst(v->tree);
cleanst(v);
@ -1013,14 +1014,16 @@ parseqatom(struct vars *v,
break;
case BACKREF: /* the Feature From The Black Lagoon */
INSIST(type != LACON, REG_ESUBREG);
INSIST(v->nextvalue < v->nsubs, REG_ESUBREG);
INSIST(v->subs[v->nextvalue] != NULL, REG_ESUBREG);
subno = v->nextvalue;
assert(subno > 0);
INSIST(subno < v->nsubs, REG_ESUBREG);
NOERRN();
INSIST(v->subs[subno] != NULL, REG_ESUBREG);
NOERRN();
assert(v->nextvalue > 0);
atom = subre(v, 'b', BACKR, lp, rp);
NOERRN();
subno = v->nextvalue;
atom->backno = subno;
v->subs[subno]->flags |= BRUSE;
EMPTYARC(lp, rp); /* temporarily, so there's something */
NEXT();
break;
@ -2141,19 +2144,54 @@ freesrnode(struct vars *v, /* might be NULL */
}
/*
* optst - optimize a subRE subtree
* removecaptures - remove unnecessary capture subREs
*
* If the caller said that it doesn't care about subexpression match data,
* we may delete the "capture" markers on subREs that are not referenced
* by any backrefs, and then simplify anything that's become non-messy.
* Call this only if REG_NOSUB flag is set.
*/
static void
optst(struct vars *v,
struct subre *t)
removecaptures(struct vars *v,
struct subre *t)
{
struct subre *t2;
assert(t != NULL);
/*
* DGP (2007-11-13): I assume it was the programmer's intent to eventually
* come back and add code to optimize subRE trees, but the routine coded
* just spends effort traversing the tree and doing nothing. We can do
* nothing with less effort.
* If this isn't itself a backref target, clear capno and tentatively
* clear CAP flag.
*/
return;
if (!(t->flags & BRUSE))
{
t->capno = 0;
t->flags &= ~CAP;
}
/* Now recurse to children */
for (t2 = t->child; t2 != NULL; t2 = t2->sibling)
{
removecaptures(v, t2);
/* Propagate child CAP flag back up, if it's still set */
if (t2->flags & CAP)
t->flags |= CAP;
}
/*
* If t now contains neither captures nor backrefs, there's no longer any
* need to care where its sub-match boundaries are, so we can reduce it to
* a simple DFA node. (Note in particular that MIXED child greediness is
* not a hindrance here, so we don't use the MESSY() macro.)
*/
if ((t->flags & (CAP | BACKR)) == 0)
{
if (t->child)
freesubreandsiblings(v, t->child);
t->child = NULL;
t->op = '=';
t->flags &= ~MIXED;
}
}
/*
@ -2501,6 +2539,8 @@ stdump(struct subre *t,
fprintf(f, " hascapture");
if (t->flags & BACKR)
fprintf(f, " hasbackref");
if (t->flags & BRUSE)
fprintf(f, " isreferenced");
if (!(t->flags & INUSE))
fprintf(f, " UNUSED");
if (t->latype != (char) -1)

36
src/backend/regex/regexec.c
View File

@ -213,12 +213,9 @@ pg_regexec(regex_t *re,
return REG_NOMATCH;
backref = (v->g->info & REG_UBACKREF) ? 1 : 0;
v->eflags = flags;
if (v->g->cflags & REG_NOSUB)
nmatch = 0; /* override client */
v->nmatch = nmatch;
if (backref)
if (backref && nmatch <= v->g->nsub)
{
/* need work area */
/* need larger work area */
if (v->g->nsub + 1 <= LOCALMAT)
v->pmatch = mat;
else
@ -229,7 +226,17 @@ pg_regexec(regex_t *re,
v->nmatch = v->g->nsub + 1;
}
else
{
/* we can store results directly in caller's array */
v->pmatch = pmatch;
/* ensure any extra entries in caller's array are filled with -1 */
if (nmatch > 0)
zapallsubs(pmatch, nmatch);
/* then forget about extra entries, to avoid useless work in find() */
if (nmatch > v->g->nsub + 1)
nmatch = v->g->nsub + 1;
v->nmatch = nmatch;
}
v->details = details;
v->start = (chr *) string;
v->search_start = (chr *) string + search_start;
@ -290,12 +297,20 @@ pg_regexec(regex_t *re,
else
st = find(v, &v->g->tree->cnfa, &v->g->cmap);
/* copy (portion of) match vector over if necessary */
if (st == REG_OKAY && v->pmatch != pmatch && nmatch > 0)
/* on success, ensure caller's match vector is filled correctly */
if (st == REG_OKAY && nmatch > 0)
{
zapallsubs(pmatch, nmatch);
n = (nmatch < v->nmatch) ? nmatch : v->nmatch;
memcpy(VS(pmatch), VS(v->pmatch), n * sizeof(regmatch_t));
if (v->pmatch != pmatch)
{
/* copy portion of match vector over from (larger) work area */
assert(nmatch <= v->nmatch);
memcpy(VS(pmatch), VS(v->pmatch), nmatch * sizeof(regmatch_t));
}
if (v->g->cflags & REG_NOSUB)
{
/* don't expose possibly-partial sub-match results to caller */
zapallsubs(pmatch, nmatch);
}
}
/* clean up */
@ -752,7 +767,6 @@ cdissect(struct vars *v,
break;
case '(': /* no-op capture node */
assert(t->child != NULL);
assert(t->capno > 0);
er = cdissect(v, t->child, begin, end);
break;
default: