Perl_sv_len_utf8 can use the UTF-8 offset cache to reduce its linear scan.

Previously, if the scalar's character length wasn't yet known, but an offset
midway was, the offset would be ignored, and the linear scan of UTF-8 was for
the entire length of the scalar.

1 files changed, 12 insertions, 2 deletions

diff --git a/sv.c b/sv.c
index 3a25abcf10..6cfb488c19 100644
--- a/sv.c
+++ b/sv.c
@@ -6047,8 +6047,18 @@ Perl_sv_len_utf8(pTHX_ register SV *const sv)
 	    STRLEN ulen;
 	    MAGIC *mg = SvMAGICAL(sv) ? mg_find(sv, PERL_MAGIC_utf8) : NULL;
 
-	    if (mg && mg->mg_len != -1) {
-		ulen = mg->mg_len;
+	    if (mg && (mg->mg_len != -1 || mg->mg_ptr)) {
+		if (mg->mg_len != -1)
+		    ulen = mg->mg_len;
+		else {
+		    /* We can use the offset cache for a headstart.
+		       The longer value is stored in the first pair.  */
+		    STRLEN *cache = (STRLEN *) mg->mg_ptr;
+
+		    ulen = cache[0] + Perl_utf8_length(aTHX_ s + cache[1],
+						       s + len);
+		}
+		
 		if (PL_utf8cache < 0) {
 		    const STRLEN real = Perl_utf8_length(aTHX_ s, s + len);
 		    if (real != ulen) {