This does some miscellaneous stuff mainly avoiding the usage of pow to achieve significant speedups. This is not speed critical, but is unnecessary latency and cycles wasted for a user.
All tables tested and are identical to the old ones (bit-exact even in floating point case). Sample benchmark (x86-64, Haswell, GNU/Linux): old: 102329530 decicycles in mpegaudio_tableinit, 1 runs, 0 skips new: 34111900 decicycles in mpegaudio_tableinit, 1 runs, 0 skips Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> --- libavcodec/mpegaudio_tablegen.h | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) diff --git a/libavcodec/mpegaudio_tablegen.h b/libavcodec/mpegaudio_tablegen.h index 86b2cd3..cea466b 100644 --- a/libavcodec/mpegaudio_tablegen.h +++ b/libavcodec/mpegaudio_tablegen.h @@ -26,6 +26,7 @@ #include <stdint.h> #include <math.h> #include "libavutil/attributes.h" +#include "libavutil/libm.h" #define TABLE_4_3_SIZE (8191 + 16)*4 #if CONFIG_HARDCODED_TABLES @@ -45,12 +46,21 @@ static float expval_table_float[512][16]; static av_cold void mpegaudio_tableinit(void) { int i, value, exponent; + double pow2_lut[4] = { + 1.00000000000000000000, /* 2 ^ (0 * 0.25) */ + 1.18920711500272106672, /* 2 ^ (1 * 0.25) */ + M_SQRT2 , /* 2 ^ (2 * 0.25) */ + 1.68179283050742908606, /* 2 ^ (3 * 0.25) */ + }; + double cbrt_lut[16]; + for (i = 0; i < 16; ++i) + cbrt_lut[i] = cbrt(i); + for (i = 1; i < TABLE_4_3_SIZE; i++) { double value = i / 4; double f, fm; int e, m; - /* cbrtf() isn't available on all systems, so we use powf(). */ - f = value / IMDCT_SCALAR * pow(value, 1.0 / 3.0) * pow(2, (i & 3) * 0.25); + f = value / IMDCT_SCALAR * cbrt(value) * pow2_lut[i & 3]; fm = frexp(f, &e); m = (uint32_t)(fm * (1LL << 31) + 0.5); e += FRAC_BITS - 31 + 5 - 100; @@ -61,10 +71,8 @@ static av_cold void mpegaudio_tableinit(void) } for (exponent = 0; exponent < 512; exponent++) { for (value = 0; value < 16; value++) { - /* cbrtf() isn't available on all systems, so we use powf(). */ - double f = (double)value * pow(value, 1.0 / 3.0) * pow(2, (exponent - 400) * 0.25 + FRAC_BITS + 5) / IMDCT_SCALAR; - /* llrint() isn't always available, so round and cast manually. */ - expval_table_fixed[exponent][value] = (long long int) (f < 0xFFFFFFFF ? floor(f + 0.5) : 0xFFFFFFFF); + double f = value * cbrt_lut[value] * pow(2, (exponent - 400) * 0.25 + FRAC_BITS + 5) / IMDCT_SCALAR; + expval_table_fixed[exponent][value] = (f < 0xFFFFFFFF ? llrint(f) : 0xFFFFFFFF); expval_table_float[exponent][value] = f; } exp_table_fixed[exponent] = expval_table_fixed[exponent][1]; -- 2.6.2 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel