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audioconvert: optimize mask calculation

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find_suitable_mask() had complexity O(n^2) on the number of bits.
For common case like 2-channel audio the mask was calculated in about 4k loop
cycles.

Optimize both n_bits_set() and find_suitable_mask() to O(n) where n is the
number of bits set in the mask.

https://bugzilla.gnome.org/show_bug.cgi?id=772864
This commit is contained in:
Petr Kulhavy 2016-10-14 22:31:41 +02:00 committed by Sebastian Dröge
parent 1820c18b0f
commit ca7e31f80d
1 changed files with 21 additions and 21 deletions
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42
gst/audioconvert/gstaudioconvert.c
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@ -302,41 +302,41 @@ gst_audio_convert_transform_caps (GstBaseTransform * btrans,
return result; return result;
} }
/* Count the number of bits set
* Optimized for the common case, assuming that the number of channels
* (i.e. bits set) is small
*/
static gint static gint
n_bits_set (guint64 x) n_bits_set (guint64 x)
{ {
gint i; gint c;
gint c = 0;
guint64 y = 1;
for (i = 0; i < 64; i++) { for (c = 0; x; c++)
if (x & y) x &= x - 1;
c++;
y <<= 1;
}
return c; return c;
} }
/* Reduce the mask to the n_chans lowest set bits
*
* The algorithm clears the n_chans lowest set bits and subtracts the
* result from the original mask to get the desired mask.
* It is optimized for the common case where n_chans is a small
* number. In the worst case, however, it stops after 64 iterations.
*/
static guint64 static guint64
find_suitable_mask (guint64 mask, gint n_chans) find_suitable_mask (guint64 mask, gint n_chans)
{ {
guint64 intersection; guint64 x = mask;
gint i;
i = 0; for (; x && n_chans; n_chans--)
x &= x - 1;
g_assert (n_bits_set (mask) >= n_chans); g_assert (x || n_chans == 0);
/* assertion fails if mask contained less bits than n_chans
* or n_chans was < 0 */
intersection = mask; return mask - x;
do {
intersection = intersection & ((~G_GUINT64_CONSTANT (0)) >> i);
i++;
} while (n_bits_set (intersection) > n_chans && i < 64);
if (i < 64)
return intersection;
return 0;
} }
static void static void