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avutil/intmath: use de Bruijn based ff_ctz

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It has already been demonstrated that the de Bruijn method has benefits
over the current implementation: commit 971d12b7f9.
That commit implemented it for long long, this extends it to the int version.

Tested with FATE.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
This commit is contained in:
Ganesh Ajjanagadde 2015-10-14 10:26:59 -04:00 committed by Ronald S. Bultje
parent 2d0a10e369
commit 55d3e97970
1 changed files with 7 additions and 25 deletions
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32
libavutil/intmath.h
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@ -129,33 +129,15 @@ static av_always_inline av_const int ff_log2_16bit_c(unsigned int v)
* @return the number of trailing 0-bits * @return the number of trailing 0-bits
*/ */
#if !defined( _MSC_VER ) #if !defined( _MSC_VER )
/* We use the De-Bruijn method outlined in:
* http://supertech.csail.mit.edu/papers/debruijn.pdf. */
static av_always_inline av_const int ff_ctz_c(int v) static av_always_inline av_const int ff_ctz_c(int v)
{ {
int c; static const uint8_t debruijn_ctz32[32] = {
0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
if (v & 0x1) 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
return 0; };
return debruijn_ctz32[(uint32_t)((v & -v) * 0x077CB531U) >> 27];
c = 1;
if (!(v & 0xffff)) {
v >>= 16;
c += 16;
}
if (!(v & 0xff)) {
v >>= 8;
c += 8;
}
if (!(v & 0xf)) {
v >>= 4;
c += 4;
}
if (!(v & 0x3)) {
v >>= 2;
c += 2;
}
c -= v & 0x1;
return c;
} }
#else #else
static av_always_inline av_const int ff_ctz_c( int v ) static av_always_inline av_const int ff_ctz_c( int v )