Re: [PATCH] Optimize json_lex_string by batching character copying

On Thu, Aug 25, 2022 at 01:35:45PM +0700, John Naylor wrote:
> - For the following comment, pgindent will put spaced operands on a
> separate line which is not great for readability. and our other
> reference to the Stanford bithacks page keeps the in-page link, and I
> see no reason to exclude it -- if it goes missing, the whole page will
> still load. So I put back those two details.
>
> + * To find bytes <= c, we can use bitwise operations to find
> bytes < c+1,
> + * but it only works if c+1 <= 128 and if the highest bit in v
> is not set.
> + * Adapted from
> + * https://graphics.stanford.edu/~seander/bithacks.html#HasLessInWord

This was just unnecessary fiddling on my part, sorry about that.

> +test_lfind8_internal(uint8 key)
> +{
> + uint8 charbuf[LEN_WITH_TAIL(Vector8)];
> + const int len_no_tail = LEN_NO_TAIL(Vector8);
> + const int len_with_tail = LEN_WITH_TAIL(Vector8);
> +
> + memset(charbuf, 0xFF, len_with_tail);
> + /* search tail to test one-byte-at-a-time path */
> + charbuf[len_with_tail - 1] = key;
> + if (key > 0x00 && pg_lfind8(key - 1, charbuf, len_with_tail))
> + elog(ERROR, "pg_lfind8() found nonexistent element <= '0x%x'", key - 1);
> + if (key < 0xFF && !pg_lfind8(key, charbuf, len_with_tail))
> + elog(ERROR, "pg_lfind8() did not find existing element <= '0x%x'", key);
> + if (key < 0xFE && pg_lfind8(key + 1, charbuf, len_with_tail))
> + elog(ERROR, "pg_lfind8() found nonexistent element <= '0x%x'", key + 1);
> +
> + memset(charbuf, 0xFF, len_with_tail);
> + /* search with vector operations */
> + charbuf[len_no_tail - 1] = key;
> + if (key > 0x00 && pg_lfind8(key - 1, charbuf, len_no_tail))
> + elog(ERROR, "pg_lfind8() found nonexistent element <= '0x%x'", key - 1);
> + if (key < 0xFF && !pg_lfind8(key, charbuf, len_no_tail))
> + elog(ERROR, "pg_lfind8() did not find existing element <= '0x%x'", key);
> + if (key < 0xFE && pg_lfind8(key + 1, charbuf, len_no_tail))
> + elog(ERROR, "pg_lfind8() found nonexistent element <= '0x%x'", key + 1);
> +}

nitpick: Shouldn't the elog() calls use "==" instead of "<=" for this one?

Otherwise, 0001 looks good to me.

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com