[FFmpeg-devel] [PATCH] AAC decoder

[Michael Niedermayer]

On Sat, May 24, 2008 at 06:35:37PM +0100, Robert Swain wrote:
> 2008/5/23 Michael Niedermayer <michaelni at gmx.at>:
> > On Fri, May 23, 2008 at 01:59:41PM +0100, Robert Swain wrote:
> >> Index: aac.c
> >> ===================================================================
> >> --- aac.c     (revision 2185)
> >> +++ aac.c     (working copy)
> >> @@ -366,7 +366,7 @@
> >>      DECLARE_ALIGNED_16(float, sine_short_128[128]);
> >>      DECLARE_ALIGNED_16(float, pow2sf_tab[256]);
> >>      DECLARE_ALIGNED_16(float, intensity_tab[256]);
> >> -    DECLARE_ALIGNED_16(float, ivquant_tab[256]);
> >> +    DECLARE_ALIGNED_16(float, ivquant_tab[128]);
> >>      MDCTContext mdct;
> >>      MDCTContext mdct_small;
> >>      MDCTContext *mdct_ltp;
> >> @@ -890,8 +890,11 @@
> >>      // BIAS method instead needs values -1<x<1
> >>      for (i = 0; i < 256; i++)
> >>          ac->intensity_tab[i] = pow(0.5, (i - 100) / 4.);
> >> -    for (i = 0; i < sizeof(ac->ivquant_tab)/sizeof(ac->ivquant_tab[0]); i++)
> >> -        ac->ivquant_tab[i] = pow(i, 4./3);
> >> +    for (i = 0; i < sizeof(ac->ivquant_tab)/(sizeof(ac->ivquant_tab[0])<<1); i++) {
> >> +        int idx = i<<1;
> >> +        ac->ivquant_tab[idx]     =  pow(i, 4./3);
> >> +        ac->ivquant_tab[idx + 1] = -ac->ivquant_tab[idx];
> >> +    }
> >>
> >>      if(ac->dsp.float_to_int16 == ff_float_to_int16_c) {
> >>          ac->add_bias = 385.0f;
> >
> >> @@ -1035,13 +1038,12 @@
> >>  }
> >>
> >>  static inline float ivquant(AACContext * ac, int a) {
> >
> >> -    static const float sign[2] = { -1., 1. };
> >>      int tmp = (a>>31);
> >>      int abs_a = (a^tmp)-tmp;
> >> -    if (abs_a < sizeof(ac->ivquant_tab)/sizeof(ac->ivquant_tab[0]))
> >> -        return sign[tmp+1] * ac->ivquant_tab[abs_a];
> >> +    if (abs_a < sizeof(ac->ivquant_tab)/(sizeof(ac->ivquant_tab[0])<<1))
> >> +        return ac->ivquant_tab[(abs_a<<1) + !!tmp];
> >
> > ehh... this should be:
> >
> > if(a + 127U < 255U)
> >    return ivquant_tab[a + 127U];
> >
> > (or other constants depending on what table size is best ...)
> >
> >
> >>      else
> >> -        return sign[tmp+1] * pow(abs_a, 4./3);
> >> +        return (2 * tmp + 1) * pow(abs_a, 4./3);
> >
> > pow(fabs(a), 1./3) * a;
> 
> With those suggestions it is much faster. The alternating sign
> construction for the table wasn't my idea, but I won't name names. :)
> Anyway, see attached. Benchmarks on the same FAAC encoded South Park
> episode:
> 
> old size 256
[...]
> 3956 dezicycles in ivquant, 2096816 runs, 336 skipsup=0 drop=0
> 
> new size 8
[...]
> 4840 dezicycles in ivquant, 2066668 runs, 30484 skips=0 drop=0
> 
> new size 16
[...]
> 3650 dezicycles in ivquant, 2093424 runs, 3728 skipsp=0 drop=0
> 
> new size 32
[...]
> 3438 dezicycles in ivquant, 2096888 runs, 264 skipsup=0 drop=0
> 
> new size 64
[...]
> 3447 dezicycles in ivquant, 2096915 runs, 237 skipsup=0 drop=0
> 
> new size 128
[...]
> 3431 dezicycles in ivquant, 2096918 runs, 234 skipsup=0 drop=0
> 
> new size 256
[...]
> 3431 dezicycles in ivquant, 2096953 runs, 199 skipsup=0 drop=0
> 
> new size 512
[...]
> 3438 dezicycles in ivquant, 2097093 runs, 59 skipsdup=0 drop=0
> 
> It looks to me like there's little difference in performance when the
> table is of size 32 or larger. Should I use size 32?

>From the numbers i see, yes 32 seems the best choice.

What bitrate did your test file have? High bitrate files might be faster
with larger tables, so if it was low bitrate then it might be worth retrying
with some higher bitrate.

[...]
> +    for (i = 1; i < IVQUANT_SIZE/2; i++) {
> +        ac->ivquant_tab[IVQUANT_SIZE/2 - 1 + i] =  pow(i, 4./3);
> +        ac->ivquant_tab[IVQUANT_SIZE/2 - 1 - i] = -ac->ivquant_tab[IVQUANT_SIZE/2 - 1 + i];
> +    }

cant that be simplified with pow(fabs(i), 1./3) * i as well?

besides these, patch ok

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many that live deserve death. And some that die deserve life. Can you give
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even the very wise cannot see all ends. -- Gandalf
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