[Bug 772864] audioconvert: mask calculation optimization

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=772864

--- Comment #1 from Petr Kulhavy <brain at jikos.cz> ---
Just to make clear why this is an issue:
In a common case with a stereo mask (0x03) it takes about 4k (!) cycles to
calculate find_suitable_mask(). This is because n_bits_set() always takes 64
cycles and it is called about 61-times in find_suitable_mask().


A better approach would be something like this:

static guint64 find_suitable_mask (guint64 mask, gint n_chans)
{
  guint64 intersection;
  gint i, n_bits;

  n_bits = n_bits_set (mask)

  g_assert (n_bits >= n_chans);

  intersection = mask;
  for (i = 1 << 63; n_bits > n_chans && i; i >>= 1);
  {
    n_bits -= !!(intersection & i);
    intersection = intersection & (~i);
  }

  if (i)
    return intersection;
  return 0;
}

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