[FFmpeg-devel] [PATCHv2] lavc/cbrt_tablegen: speed up tablegen

[Michael Niedermayer]

On Thu, Jan 07, 2016 at 05:20:55PM -0800, Ganesh Ajjanagadde wrote:
> On Thu, Jan 7, 2016 at 4:48 PM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> > On Mon, Jan 04, 2016 at 06:33:59PM -0800, Ganesh Ajjanagadde wrote:
> >> This exploits an approach based on the sieve of Eratosthenes, a popular
> >> method for generating prime numbers.
> >>
> >> Tables are identical to previous ones.
> >>
> >> Tested with FATE with/without --enable-hardcoded-tables.
> >>
> >> Sample benchmark (Haswell, GNU/Linux+gcc):
> >> prev:
> >> 7860100 decicycles in cbrt_tableinit,       1 runs,      0 skips
> >> 7777490 decicycles in cbrt_tableinit,       2 runs,      0 skips
> >> [...]
> >> 7582339 decicycles in cbrt_tableinit,     256 runs,      0 skips
> >> 7563556 decicycles in cbrt_tableinit,     512 runs,      0 skips
> >>
> >> new:
> >> 2099480 decicycles in cbrt_tableinit,       1 runs,      0 skips
> >> 2044470 decicycles in cbrt_tableinit,       2 runs,      0 skips
> >> [...]
> >> 1796544 decicycles in cbrt_tableinit,     256 runs,      0 skips
> >> 1791631 decicycles in cbrt_tableinit,     512 runs,      0 skips
> >>
> >> Both small and large run count given as this is called once so small run
> >> count may give a better picture, small numbers are fairly consistent,
> >> and there is a consistent downward trend from small to large runs,
> >> at which point it stabilizes to a new value.
> >>
> >> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> >> ---
> >>  libavcodec/aacdec_fixed.c           |  4 +--
> >>  libavcodec/aacdec_template.c        |  2 +-
> >>  libavcodec/cbrt_tablegen.h          | 53 ++++++++++++++++++++++++++-----------
> >>  libavcodec/cbrt_tablegen_template.c | 12 ++++++++-
> >>  4 files changed, 51 insertions(+), 20 deletions(-)
> >>
> >> diff --git a/libavcodec/aacdec_fixed.c b/libavcodec/aacdec_fixed.c
> >> index 396a874..f7b882b 100644
> >> --- a/libavcodec/aacdec_fixed.c
> >> +++ b/libavcodec/aacdec_fixed.c
> >> @@ -155,9 +155,9 @@ static void vector_pow43(int *coefs, int len)
> >>      for (i=0; i<len; i++) {
> >>          coef = coefs[i];
> >>          if (coef < 0)
> >> -            coef = -(int)cbrt_tab[-coef];
> >> +            coef = -(int)cbrt_tab[-coef].i;
> >>          else
> >> -            coef = (int)cbrt_tab[coef];
> >> +            coef = (int)cbrt_tab[coef].i;
> >>          coefs[i] = coef;
> >>      }
> >>  }
> >> diff --git a/libavcodec/aacdec_template.c b/libavcodec/aacdec_template.c
> >> index d819958..1380510 100644
> >> --- a/libavcodec/aacdec_template.c
> >> +++ b/libavcodec/aacdec_template.c
> >> @@ -1791,7 +1791,7 @@ static int decode_spectrum_and_dequant(AACContext *ac, INTFLOAT coef[1024],
> >>                                          v = -v;
> >>                                      *icf++ = v;
> >>  #else
> >> -                                    *icf++ = cbrt_tab[n] | (bits & 1U<<31);
> >> +                                    *icf++ = cbrt_tab[n].i | (bits & 1U<<31);
> >>  #endif /* USE_FIXED */
> >>                                      bits <<= 1;
> >>                                  } else {
> >> diff --git a/libavcodec/cbrt_tablegen.h b/libavcodec/cbrt_tablegen.h
> >> index 59b5a1d..e3d6634 100644
> >> --- a/libavcodec/cbrt_tablegen.h
> >> +++ b/libavcodec/cbrt_tablegen.h
> >> @@ -26,14 +26,13 @@
> >>  #include <stdint.h>
> >>  #include <math.h>
> >>  #include "libavutil/attributes.h"
> >> +#include "libavutil/intfloat.h"
> >>  #include "libavcodec/aac_defines.h"
> >>
> >> -#if USE_FIXED
> >> -#define CBRT(x) lrint((x).f * 8192)
> >> -#else
> >> -#define CBRT(x) x.i
> >> -#endif
> >> -
> >
> >> +union ff_int32float64 {
> >> +    uint32_t i;
> >> +    double   f;
> >> +};
> >>  #if CONFIG_HARDCODED_TABLES
> >>  #if USE_FIXED
> >>  #define cbrt_tableinit_fixed()
> >> @@ -43,20 +42,42 @@
> >>  #include "libavcodec/cbrt_tables.h"
> >>  #endif
> >>  #else
> >> -static uint32_t cbrt_tab[1 << 13];
> >> +static union ff_int32float64 cbrt_tab[1 << 13];
> >
> > this doubles the size of the cpu cache needed at runtime to store
> > the same number of elements
> 
> Yes, it does, and it was a tradeoff I made that I forgot to list. One
> can of course use floats; but this loses accuracy at significant
> levels.
> 
> So one could malloc and free a double precision array (for temporary
> storage) at costs of some code complexity, possible heap
> fragmentation, and the problem of possible failure (may be ok since
> anyway aac_decode_init is not guaranteed to succeed; it allocates
> memory for the dsp context). Malloc/free is AFAIK ~ 100's of cycles,
> dwarfed by the table generation cost.
> 
> The problem is that it is impossible to give an answer as to precisely
> what impact that will have on decoding/encoding performance, and
> results of course vary based on hardware. This is the same problem
> that plagues static/dynamic table performance analysis.
> 
> I don't have a measurable performance regression on my machine for aac
> decoding because of this. But then, my Haswell setup is not exactly
> representative.

you can use 2 seperate arrays without union or maybe make the arrays
part of the union instead of the array elements

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

He who knows, does not speak. He who speaks, does not know. -- Lao Tsu
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