Improve performance of regular expression back-references.

In some cases, at the time that we're doing an NFA-based precheck
of whether a backref subexpression can match at a particular place
in the string, we already know which substring the referenced
subexpression matched.  If so, we might as well forget about the NFA
and just compare the substring; this is faster and it gives an exact
rather than approximate answer.

In general, this optimization can help while we are prechecking within
the second child expression of a concat node, while the capture was
within the first child expression; then the substring was saved during
cdissect() of the first child and will be available to NFA checks done
while cdissect() recurses into the second child.  It can help quite a
lot if the tree looks like

              concat
             /      \
      capture        concat
                    /      \
     expensive stuff        backref

as we will be able to avoid recursively dissecting the "expensive
stuff" before discovering that the backref isn't satisfied with a
particular midpoint that the lower concat node is testing.  This
doesn't help if the concat tree is left-deep, as the capture node
won't get set soon enough (and it's hard to fix that without changing
the engine's match behavior).  Fortunately, right-deep concat trees
are the common case.

Patch by me, reviewed by Joel Jacobson

Discussion: https://postgr.es/m/661609.1614560029@sss.pgh.pa.us

diff --git a/src/backend/regex/rege_dfa.c b/src/backend/regex/rege_dfa.c
index 1db52d1005c415b06c7556a24693111544baf5b5..18f87c69d4754f6a4e17a55b4e7b65875633154d 100644 (file)
--- a/src/backend/regex/rege_dfa.c
+++ b/src/backend/regex/rege_dfa.c
@@ -58,6 +58,19 @@ longest(struct vars *v,
    if (hitstopp != NULL)
        *hitstopp = 0;
 
+   /* if this is a backref to a known string, just match against that */
+   if (d->backno >= 0)
+   {
+       assert((size_t) d->backno < v->nmatch);
+       if (v->pmatch[d->backno].rm_so >= 0)
+       {
+           cp = dfa_backref(v, d, start, start, stop, false);
+           if (cp == v->stop && stop == v->stop && hitstopp != NULL)
+               *hitstopp = 1;
+           return cp;
+       }
+   }
+
    /* fast path for matchall NFAs */
    if (d->cnfa->flags & MATCHALL)
    {
@@ -210,6 +223,20 @@ shortest(struct vars *v,
    if (hitstopp != NULL)
        *hitstopp = 0;
 
+   /* if this is a backref to a known string, just match against that */
+   if (d->backno >= 0)
+   {
+       assert((size_t) d->backno < v->nmatch);
+       if (v->pmatch[d->backno].rm_so >= 0)
+       {
+           cp = dfa_backref(v, d, start, min, max, true);
+           if (cp != NULL && coldp != NULL)
+               *coldp = start;
+           /* there is no case where we should set *hitstopp */
+           return cp;
+       }
+   }
+
    /* fast path for matchall NFAs */
    if (d->cnfa->flags & MATCHALL)
    {
@@ -467,6 +494,94 @@ matchuntil(struct vars *v,
    return 1;
 }
 
+/*
+ * dfa_backref - find best match length for a known backref string
+ *
+ * When the backref's referent is already available, we can deliver an exact
+ * answer with considerably less work than running the backref node's NFA.
+ *
+ * Return match endpoint for longest or shortest valid repeated match,
+ * or NULL if there is no valid match.
+ *
+ * Should be in sync with cbrdissect(), although that has the different task
+ * of checking a match to a predetermined section of the string.
+ */
+static chr *
+dfa_backref(struct vars *v,
+           struct dfa *d,
+           chr *start,         /* where the match should start */
+           chr *min,           /* match must end at or after here */
+           chr *max,           /* match must end at or before here */
+           bool shortest)
+{
+   int         n = d->backno;
+   int         backmin = d->backmin;
+   int         backmax = d->backmax;
+   size_t      numreps;
+   size_t      minreps;
+   size_t      maxreps;
+   size_t      brlen;
+   chr        *brstring;
+   chr        *p;
+
+   /* get the backreferenced string (caller should have checked this) */
+   if (v->pmatch[n].rm_so == -1)
+       return NULL;
+   brstring = v->start + v->pmatch[n].rm_so;
+   brlen = v->pmatch[n].rm_eo - v->pmatch[n].rm_so;
+
+   /* special-case zero-length backreference to avoid divide by zero */
+   if (brlen == 0)
+   {
+       /*
+        * matches only a zero-length string, but any number of repetitions
+        * can be considered to be present
+        */
+       if (min == start && backmin <= backmax)
+           return start;
+       return NULL;
+   }
+
+   /*
+    * convert min and max into numbers of possible repetitions of the backref
+    * string, rounding appropriately
+    */
+   if (min <= start)
+       minreps = 0;
+   else
+       minreps = (min - start - 1) / brlen + 1;
+   maxreps = (max - start) / brlen;
+
+   /* apply bounds, then see if there is any allowed match length */
+   if (minreps < backmin)
+       minreps = backmin;
+   if (backmax != DUPINF && maxreps > backmax)
+       maxreps = backmax;
+   if (maxreps < minreps)
+       return NULL;
+
+   /* quick exit if zero-repetitions match is valid and preferred */
+   if (shortest && minreps == 0)
+       return start;
+
+   /* okay, compare the actual string contents */
+   p = start;
+   numreps = 0;
+   while (numreps < maxreps)
+   {
+       if ((*v->g->compare) (brstring, p, brlen) != 0)
+           break;
+       p += brlen;
+       numreps++;
+       if (shortest && numreps >= minreps)
+           break;
+   }
+
+   if (numreps >= minreps)
+       return p;
+   return NULL;
+}
+
 /*
  * lastcold - determine last point at which no progress had been made
  */
@@ -563,6 +678,8 @@ newdfa(struct vars *v,
    d->lastpost = NULL;
    d->lastnopr = NULL;
    d->search = d->ssets;
+   d->backno = -1;             /* may be set by caller */
+   d->backmin = d->backmax = 0;
 
    /* initialization of sset fields is done as needed */
 

diff --git a/src/backend/regex/regexec.c b/src/backend/regex/regexec.c
index cf95989948964414b6c6069863011a8bc5b07c2c..8d7777f8c62ff4eb0a8a8502a998e155df83b8a4 100644 (file)
--- a/src/backend/regex/regexec.c
+++ b/src/backend/regex/regexec.c
@@ -77,6 +77,9 @@ struct dfa
    chr        *lastpost;       /* location of last cache-flushed success */
    chr        *lastnopr;       /* location of last cache-flushed NOPROGRESS */
    struct sset *search;        /* replacement-search-pointer memory */
+   int         backno;         /* if DFA for a backref, subno it refers to */
+   short       backmin;        /* min repetitions for backref */
+   short       backmax;        /* max repetitions for backref */
    bool        ismalloced;     /* should this struct dfa be freed? */
    bool        arraysmalloced; /* should its subsidiary arrays be freed? */
 };
@@ -154,6 +157,7 @@ static int  creviterdissect(struct vars *, struct subre *, chr *, chr *);
 static chr *longest(struct vars *, struct dfa *, chr *, chr *, int *);
 static chr *shortest(struct vars *, struct dfa *, chr *, chr *, chr *, chr **, int *);
 static int matchuntil(struct vars *, struct dfa *, chr *, struct sset **, chr **);
+static chr *dfa_backref(struct vars *, struct dfa *, chr *, chr *, chr *, bool);
 static chr *lastcold(struct vars *, struct dfa *);
 static struct dfa *newdfa(struct vars *, struct cnfa *, struct colormap *, struct smalldfa *);
 static void freedfa(struct dfa *);
@@ -342,13 +346,23 @@ static struct dfa *
 getsubdfa(struct vars *v,
          struct subre *t)
 {
-   if (v->subdfas[t->id] == NULL)
+   struct dfa *d = v->subdfas[t->id];
+
+   if (d == NULL)
    {
-       v->subdfas[t->id] = newdfa(v, &t->cnfa, &v->g->cmap, DOMALLOC);
+       d = newdfa(v, &t->cnfa, &v->g->cmap, DOMALLOC);
        if (ISERR())
            return NULL;
+       /* set up additional info if this is a backref node */
+       if (t->op == 'b')
+       {
+           d->backno = t->backno;
+           d->backmin = t->min;
+           d->backmax = t->max;
+       }
+       v->subdfas[t->id] = d;
    }
-   return v->subdfas[t->id];
+   return d;
 }
 
 /*
@@ -369,6 +383,7 @@ getladfa(struct vars *v,
        v->ladfas[n] = newdfa(v, &sub->cnfa, &v->g->cmap, DOMALLOC);
        if (ISERR())
            return NULL;
+       /* a LACON can't contain a backref, so nothing else to do */
    }
    return v->ladfas[n];
 }
@@ -927,6 +942,9 @@ crevcondissect(struct vars *v,
 
 /*
  * cbrdissect - dissect match for backref node
+ *
+ * The backref match might already have been verified by dfa_backref(),
+ * but we don't know that for sure so must check it here.
  */
 static int                     /* regexec return code */
 cbrdissect(struct vars *v,