Suboptimal code in my_strnxfrm_simple()

[Alexey Kopytov]

This ancient code in my_strnxfrm_simple() is suboptimal:

  if (dest != src)
  {
    const uchar *end;
    for ( end=src+len; src < end ; )
      *dest++= map[*src++];
  }
  else
  {
    const uchar *end;
    for ( end=dest+len; dest < end ; dest++)
      *dest= (char) map[(uchar) *dest];
  }

I can't see any difference between these branches. It's probably
remnants from _bin collations, where we don't have to do table
translation and thus the "dest == src" case is degenerate, e.g.:

static size_t my_strnxfrm_bin(CHARSET_INFO *cs __attribute__((unused)),
                              uchar *dest, size_t dstlen,
                              const uchar *src, size_t srclen)
{
  if (dest != src)
    memcpy(dest, src, min(dstlen,srclen));
  if (dstlen > srclen)
    bfill(dest + srclen, dstlen - srclen, 0);
  return dstlen;
}

But the "if()" statement in my_strnxfrm_simple() is redundant. It is
executed for each row processed by filesort and shows high branch
misprediction numbers in sysbench RO runs.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1320