regcomp.c: Refactor join_exact() to eliminate extra passes

The strings in every EXACTFish node are examined for certain problematic
sequences and code points.  Prior to this patch, this was done in
several passes, but this refactors the routine to do it in a single
pass.

1 files changed, 144 insertions, 90 deletions

diff --git a/regcomp.c b/regcomp.c
index 7f51d8729a..3d0121c90b 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -2522,6 +2522,9 @@ S_make_trie_failtable(pTHX_ RExC_state_t *pRExC_state, regnode *source,  regnode
  * node type of the result is changed to reflect that it contains these
  * sequences.
  *
+ * And *has_exactf_sharp_s is set to indicate if the node is EXACTF and
+ * contains LATIN SMALL LETTER SHARP S
+ *
  * This is as good a place as any to discuss the design of handling these
  * problematic sequences.  It's been wrong in Perl for a very long time.  There
  * are three code points in Unicode whose folded lengths differ so much from
@@ -2592,8 +2595,9 @@ S_make_trie_failtable(pTHX_ RExC_state_t *pRExC_state, regnode *source,  regnode
  *      cases are either 1-1 folds when no UTF-8 is involved; or is true by
  *      virtue of having this file pre-fold UTF-8 patterns.   I'm
  *      reluctant to try to change this assumption, so instead the code punts.
- *      Elsewhere in this file, each EXACTF node is examined for the sharp s.
- *      If found, a flag is set that later causes the optimizer in this file to
+ *      This routine examines EXACTF nodes for the sharp s, and returns whether
+ *      the node is an EXACTF node that contains one or not.  When it is true,
+ *      the caller sets a flag that later causes the optimizer in this file to
  *      not set values for the floating and fixed string lengths, and thus
  *      avoid the optimizer code in regexec.c that makes this invalid
  *      assumption.  Thus, there is no optimization based on string lengths for
@@ -2601,12 +2605,12 @@ S_make_trie_failtable(pTHX_ RExC_state_t *pRExC_state, regnode *source,  regnode
  *      (which means the pattern isn't in UTF-8).
  */
 
-#define JOIN_EXACT(scan,min_change,flags) \
+#define JOIN_EXACT(scan,min_change,has_exactf_sharp_s, flags) \
     if (PL_regkind[OP(scan)] == EXACT) \
-        join_exact(pRExC_state,(scan),(min_change),(flags),NULL,depth+1)
+        join_exact(pRExC_state,(scan),(min_change),has_exactf_sharp_s, (flags),NULL,depth+1)
 
 STATIC U32
-S_join_exact(pTHX_ RExC_state_t *pRExC_state, regnode *scan, IV *min_change, U32 flags,regnode *val, U32 depth) {
+S_join_exact(pTHX_ RExC_state_t *pRExC_state, regnode *scan, IV *min_change, bool *has_exactf_sharp_s, U32 flags,regnode *val, U32 depth) {
     /* Merge several consecutive EXACTish nodes into one. */
     regnode *n = regnext(scan);
     U32 stringok = 1;
@@ -2689,22 +2693,35 @@ S_join_exact(pTHX_ RExC_state_t *pRExC_state, regnode *scan, IV *min_change, U32
 #define UPSILON_D_T	GREEK_SMALL_LETTER_UPSILON_WITH_DIALYTIKA_AND_TONOS
 
     *min_change = 0;
+    *has_exactf_sharp_s = FALSE;
 
     /* Here, all the adjacent mergeable EXACTish nodes have been merged.  We
      * can now analyze for sequences of problematic code points.  (Prior to
      * this final joining, sequences could have been split over boundaries, and
      * hence missed).  The sequences only happen in folding */
     if (OP(scan) != EXACT) {
-        char *s, *t;
-        char * s0 = STRING(scan);
-        char * const s_end = s0 + STR_LEN(scan);
-
-	/* First we look at the sequences that can occur only in UTF-8 strings.
-	 * The sequences are of length 6 */
-	if (UTF && STR_LEN(scan) >= 6) {
+        U8 *s;
+        U8 * s0 = (U8*) STRING(scan);
+        U8 * const s_end = s0 + STR_LEN(scan);
+
+	/* The below is perhaps overboard, but this allows us to save a test
+	 * each time through the loop at the expense of a mask.  This is
+	 * because on both EBCDIC and ASCII machines, 'S' and 's' differ by a
+	 * single bit.  On ASCII they are 32 apart; on EBCDIC, they are 64.
+	 * This uses an exclusive 'or' to find that bit and then inverts it to
+	 * form a mask, with just a single 0, in the bit position where 'S' and
+	 * 's' differ. */
+	const U8 S_or_s_mask = ~ ('S' ^ 's');
+	const U8 s_masked = 's' & S_or_s_mask;
+
+	/* One pass is made over the node's string looking for all the
+	 * possibilities.  to avoid some tests in the loop, there are two main
+	 * cases, for UTF-8 patterns (which can't have EXACTF nodes) and
+	 * non-UTF-8 */
+	if (UTF) {
 
-	    /* Two problematic code points in Unicode casefolding of EXACT
-	     * nodes:
+	    /* There are two problematic Greek code points in Unicode
+	     * casefolding
 	     *
 	     * U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
 	     * U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
@@ -2724,86 +2741,122 @@ S_join_exact(pTHX_ RExC_state_t *pRExC_state, regnode *scan, IV *min_change, U32
 	     * minimum length computation.  (there are other code points that
 	     * also fold to these two sequences, but the delta is smaller)
 	     *
-	     * What we'll do is to look for the tail four bytes, and then peek
-	     * at the preceding two bytes to see whether we need to decrease
-	     * the minimum length by four (six minus two).
+	     * If these sequences are found, the minimum length is decreased by
+	     * four (six minus two).
 	     *
-	     * Thanks to the design of UTF-8, there cannot be false matches:
-	     * A sequence of valid UTF-8 bytes cannot be a subsequence of
-	     * another valid sequence of UTF-8 bytes. */
+	     * Similarly, 'ss' may match the single char and byte LATIN SMALL
+	     * LETTER SHARP S.  We decrease the min length by 1 for each
+	     * occurrence of 'ss' found */
 
 #ifdef EBCDIC /* RD tunifold greek 0390 and 03B0 */
-	    const char U390_first_byte = '\xb4';
-	    const char U390_2nd_byte = '\x68';
-	    const char U3B0_first_byte = '\xb5';
-	    const char U3B0_2nd_byte = '\x46';
-	    const char tail[] = "\xaf\x49\xaf\x42";
+#	    define U390_first_byte 0xb4
+	    const U8 U390_tail[] = "\x68\xaf\x49\xaf\x42";
+#	    define U3B0_first_byte 0xb5
+	    const U8 U3B0_tail[] = "\x46\xaf\x49\xaf\x42";
 #else
-	    const char U390_first_byte = '\xce';
-	    const char U390_2nd_byte = '\xb9';
-	    const char U3B0_first_byte = '\xcf';
-	    const char U3B0_2nd_byte = '\x85';
-	    const char tail[] = "\xcc\x88\xcc\x81";
+#	    define U390_first_byte 0xce
+	    const U8 U390_tail[] = "\xb9\xcc\x88\xcc\x81";
+#	    define U3B0_first_byte 0xcf
+	    const U8 U3B0_tail[] = "\x85\xcc\x88\xcc\x81";
 #endif
-	    const STRLEN tail_len = sizeof(tail) - 1;
-	    for (s = s0 + 2;    /* +2 is to skip the non-tail */
-		 s <= s_end - tail_len
-		   && (t = ninstr(s, s_end, tail, tail + tail_len));
-		 s = t + tail_len)
+	    const U8 len = sizeof(U390_tail); /* (-1 for NUL; +1 for 1st byte;
+						 yields a net of 0 */
+	    /* Examine the string for one of the problematic sequences */
+	    for (s = s0;
+		 s < s_end - 1; /* Can stop 1 before the end, as minimum length
+				 * sequence we are looking for is 2 */
+		 s += UTF8SKIP(s))
 	    {
-		if ((t[-1] == U390_2nd_byte && t[-2] == U390_first_byte)
-		    || (t[-1] == U3B0_2nd_byte && t[-2] == U3B0_first_byte))
-		{
-		    *min_change -= 4;
 
-		    /* This can't currently be handled by tries, so change the
-		     * node type to indicate this. */
-		    if (OP(scan) == EXACTFU) {
-			OP(scan) = EXACTFU_NO_TRIE;
-		    }
+		/* Look for the first byte in each problematic sequence */
+		switch (*s) {
+		    /* We don't have to worry about other things that fold to
+		     * 's' (such as the long s, U+017F), as all above-latin1
+		     * code points have been pre-folded */
+		    case 's':
+		    case 'S':
+
+			if (((*(s+1) & S_or_s_mask) == s_masked)
+			    /* These two node types don't have special handling
+			     * for 'ss' */
+			    && OP(scan) != EXACTFL && OP(scan) != EXACTFA)
+			{
+			    *min_change -= 1;
+			    OP(scan) = EXACTFU_SS;
+			    s++;    /* No need to look at this character again */
+			}
+			break;
+
+		    case U390_first_byte:
+			if (s_end - s >= len
+
+			    /* The 1's are because are skipping comparing the
+			     * first byte */
+			    && memEQ(s + 1, U390_tail, len - 1))
+			{
+			    goto greek_sequence;
+			}
+			break;
+
+		    case U3B0_first_byte:
+			if (! (s_end - s >= len
+			       && memEQ(s + 1, U3B0_tail, len - 1)))
+			{
+			    break;
+			}
+		      greek_sequence:
+			*min_change -= 4;
+
+			/* This can't currently be handled by trie's, so change
+			 * the node type to indicate this.  If EXACTFA and
+			 * EXACTFL were ever to be handled by trie's, this
+			 * would have to be changed.  If this node has already
+			 * been changed to EXACTFU_SS in this loop, leave it as
+			 * is.  (I (khw) think it doesn't matter in regexec.c
+			 * for UTF patterns, but no need to change it */
+			if (OP(scan) == EXACTFU) {
+			    OP(scan) = EXACTFU_NO_TRIE;
+			}
+			s += 6;	/* We already know what this sequence is.  Skip
+				   the rest of it */
+			break;
 		}
 	    }
 	}
+	else if (OP(scan) != EXACTFL && OP(scan) != EXACTFA) {
 
-	/* The third problematic sequence is 'ss', which can match just the
-	 * single byte LATIN SMALL LETTER SHARP S, and it can do it in both
-	 * non- and UTF-8.  Code elsewhere in this file makes sure, however,
-	 * that the sharp s gets folded to 'ss' under Unicode rules even if not
-	 * UTF-8. */
-	if (STR_LEN(scan) >= 2
-	    && (OP(scan) == EXACTFU
-		|| OP(scan) == EXACTFU_NO_TRIE	/* The code above could have
-						   set to this node type */
-	        || OP(scan) == EXACTF))
-	{
-	    /* The string will be folded to 'ss' if it's in UTF-8, but it could
-             * include capital 'S' instead of lower case when not UTF-8.  We
-             * could have different code to handle the two cases, but this is
-             * not necessary since both S and s are invariants under UTF-8; and
-             * not worth it, especially because we can use just one test for
-             * either 'S' or 's' each * time through the loop (plus a mask).
-             * Ths is because on both EBCDIC and ASCII machines, 'S' and 's'
-             * differ by a single bit.  On ASCII they are 32 apart; on EBCDIC,
-             * they are 64.  This uses an exclusive 'or' to find that bit and
-             * then inverts it to form a mask, with just a single 0, in the bit
-             * position where 'S' and 's' differ. */
-	    const char S_or_s_mask = ~ ('S' ^ 's');
-	    const char s_masked = 's' & S_or_s_mask;
-
-	    for (s = s0; s < s_end - 1; s++) {
-		if (((*s & S_or_s_mask) == s_masked)
-		    && ((*(s+1) & S_or_s_mask) == s_masked))
-		{
-		    s++;
-		    *min_change -= 1;
-
-		    /* EXACTFU_SS also isn't trie'able, so don't have to
-		     * preserve EXACTFU_NO_TRIE.  EXACTF is also not trie'able,
-		     * and because we essentially punt the optimizations in its
-		     * case, we don't need to indicate that it has an ss */
-		    if (OP(scan) == EXACTFU || OP(scan) == EXACTFU_NO_TRIE) {
-			OP(scan) = EXACTFU_SS;
-		    }
+	    /* Here, the pattern is not UTF-8.  We need to look only for the
+	     * 'ss' sequence, and in the EXACTF case, the sharp s, which can be
+	     * in the final position.  Otherwise we can stop looking 1 byte
+	     * earlier because have to find both the first and second 's' */
+	    const U8* upper = (OP(scan) == EXACTF) ? s_end : s_end -1;
+
+	    for (s = s0; s < upper; s++) {
+		switch (*s) {
+		    case 'S':
+		    case 's':
+			if (s_end - s > 1
+			    && ((*(s+1) & S_or_s_mask) == s_masked))
+			{
+			    *min_change -= 1;
+
+			    /* EXACTF nodes need to know that the minimum
+			     * length changed so that a sharp s in the string
+			     * can match this ss in the pattern, but they
+			     * remain EXACTF nodes, as they are not trie'able,
+			     * so don't have to invent a new node type to
+			     * exclude them from the trie code */
+			    if (OP(scan) != EXACTF) {
+				OP(scan) = EXACTFU_SS;
+			    }
+			    s++;
+			}
+			break;
+		    case LATIN_SMALL_LETTER_SHARP_S:
+			if (OP(scan) == EXACTF) {
+			    *has_exactf_sharp_s = TRUE;
+			}
+			break;
 		}
 	    }
 	}
@@ -2923,10 +2976,11 @@ S_study_chunk(pTHX_ RExC_state_t *pRExC_state, regnode **scanp,
   fake_study_recurse:
     while ( scan && OP(scan) != END && scan < last ){
         IV min_change = 0;
+	bool has_exactf_sharp_s = FALSE;
 	/* Peephole optimizer: */
 	DEBUG_STUDYDATA("Peep:", data,depth);
 	DEBUG_PEEP("Peep",scan,depth);
-        JOIN_EXACT(scan,&min_change,0);
+        JOIN_EXACT(scan,&min_change, &has_exactf_sharp_s, 0);
 
 	/* Follow the next-chain of the current node and optimize
 	   away all the NOTHINGs from it.  */
@@ -3440,10 +3494,8 @@ S_study_chunk(pTHX_ RExC_state_t *pRExC_state, regnode **scanp,
 		l = utf8_length(s, s + l);
 		uc = utf8_to_uvchr(s, NULL);
 	    }
-	    else if (OP(scan) == EXACTF) {
-		if (memchr(STRING(scan), LATIN_SMALL_LETTER_SHARP_S, l)) {
-		    RExC_seen |= REG_SEEN_EXACTF_SHARP_S;
-		}
+	    else if (has_exactf_sharp_s) {
+		RExC_seen |= REG_SEEN_EXACTF_SHARP_S;
 	    }
 	    min += l + min_change;
             if (min < 0) {
@@ -11628,9 +11680,11 @@ S_regtail_study(pTHX_ RExC_state_t *pRExC_state, regnode *p, const regnode *val,
     for (;;) {
         regnode * const temp = regnext(scan);
 #ifdef EXPERIMENTAL_INPLACESCAN
-        if (PL_regkind[OP(scan)] == EXACT)
-            if (join_exact(pRExC_state,scan,&min,1,val,depth+1))
+        if (PL_regkind[OP(scan)] == EXACT) {
+	    bool has_exactf_sharp_s;	/* Unexamined in this routine */
+            if (join_exact(pRExC_state,scan,&min, &has_exactf_sharp_s, 1,val,depth+1))
                 return EXACT;
+	}
 #endif
         if ( exact ) {
             switch (OP(scan)) {