Re: speed up verifying UTF-8

On Mon, Jul 26, 2021 at 7:55 AM Vladimir Sitnikov <
sitnikov(dot)vladimir(at)gmail(dot)com> wrote:
>
> Just wondering, do you have the code in a GitHub/Gitlab branch?
>
> >+ utf8_advance(s, state, len);
> >+
> >+ /*
> >+ * If we saw an error during the loop, let the caller handle it. We
treat
> >+ * all other states as success.
> >+ */
> >+ if (state == ERR)
> >+ return 0;
>
> Did you mean state = utf8_advance(s, state, len); there? (reassign state
variable)

Yep, that's a bug, thanks for catching!

> >I wanted to try different strides for the DFA
>
> Does that (and "len >= 32" condition) mean the patch does not improve
validation of the shorter strings (the ones less than 32 bytes)?

Right. Also, the 32 byte threshold was just a temporary need for testing
32-byte stride -- testing different thresholds wouldn't hurt. I'm not
terribly concerned about short strings, though, as long as we don't
regress. That said, Heikki had something in his v14 [1] that we could use:

+/*
+ * Subroutine of pg_utf8_verifystr() to check on char. Returns the length
of the
+ * character at *s in bytes, or 0 on invalid input or premature end of
input.
+ *
+ * XXX: could this be combined with pg_utf8_verifychar above?
+ */
+static inline int
+pg_utf8_verify_one(const unsigned char *s, int len)

It would be easy to replace pg_utf8_verifychar with this. It might even
speed up the SQL function length_in_encoding() -- that would be a better
reason to do it.

[1]
https://www.postgresql.org/message-id/2f95e70d-4623-87d4-9f24-ca534155f179%40iki.fi
--
John Naylor
EDB: http://www.enterprisedb.com