re_intuit_start(): don't decrease rx_origin

When calculating the new rx_origin after a successful check match,
don't set it to a lower value than it already is. This can avoid
having to do repeated HOP(check_at, -max_offset) over the same
section of string, which makes the following take milliseconds rather than
10's of seconds:

    $s = "-a-bc" x 250_000;
    $s .= "1a1bc";
    utf8::upgrade($s);
    $s =~ /\da\d{0,30000}bc/ or die;

1 files changed, 4 insertions, 4 deletions

diff --git a/regexec.c b/regexec.c
index b04c29920f..37253a54f1 100644
--- a/regexec.c
+++ b/regexec.c
@@ -883,12 +883,12 @@ Perl_re_intuit_start(pTHX_
     DEBUG_EXECUTE_r(PerlIO_printf(Perl_debug_log, "%ld...\n", (long)(check_at - i_strpos)) );
 
     /* set rx_origin to the minimum position where the regex could start
-     * matching, given the constraint of the just-matched check substring
+     * matching, given the constraint of the just-matched check substring.
+     * But don't set it lower than previously.
      */
 
-    rx_origin = (check_at - strpos <= prog->check_offset_max)
-        ? strpos
-        : HOP3c(check_at, -prog->check_offset_max, strpos);
+    if (check_at - rx_origin > prog->check_offset_max)
+        rx_origin = HOP3c(check_at, -prog->check_offset_max, rx_origin);
 
 
     /* XXX dmq: first branch is for positive lookbehind...