[FFmpeg-devel] [PATCH] Another optimization for mp3 decoder

[Måns Rullgård]

Vitor Sessak <vitor1001 at gmail.com> writes:

> $subj, it replaces writing two consecutive identical 16-bit values by
> a single 32-bit one (note that the table band_size_long[] has only
> even values).
>
> -Vitor
>
> Index: libavcodec/mpegaudiodec.c
> ===================================================================
> --- libavcodec/mpegaudiodec.c	(revision 23840)
> +++ libavcodec/mpegaudiodec.c	(working copy)
> @@ -1410,6 +1410,11 @@
>      slen[0] = sf;
>  }
>  
> +static av_always_inline uint32_t dup16to32(int a)
> +{
> +    return (a&0xFFFF) + (a<<16);
> +}

This is PACK_2S16(a, a).

>  static void exponents_from_scale_factors(MPADecodeContext *s,
>                                           GranuleDef *g,
>                                           int16_t *exponents)
> @@ -1425,10 +1430,14 @@
>      bstab = band_size_long[s->sample_rate_index];
>      pretab = mpa_pretab[g->preflag];
>      for(i=0;i<g->long_end;i++) {
> +        int v1;
>          v0 = gain - ((g->scale_factors[i] + pretab[i]) << shift) + 400;
> -        len = bstab[i];
> -        for(j=len;j>0;j--)
> -            *exp_ptr++ = v0;
> +        v1 = dup16to32(v0);
> +        len = bstab[i] >> 1;
> +        for(j=len;j>0;j--) {
> +            AV_WN32A(exp_ptr, v1);
> +            exp_ptr += 2;
> +        }
>      }
>  
>      if (g->short_start < 13) {
> @@ -1979,7 +1988,7 @@
>      int nb_granules, main_data_begin, private_bits;
>      int gr, ch, blocksplit_flag, i, j, k, n, bits_pos;
>      GranuleDef *g;
> -    int16_t exponents[576]; //FIXME try INTFLOAT
> +    LOCAL_ALIGNED(4, int16_t, exponents, [576]); //FIXME try INTFLOAT

LOCAL_ALIGNED() is meant for alignments greater than what can be
achieved by normal means.

I also wonder if it wouldn't be better still to halve the size of this
array and adjust the indexing.

-- 
M?ns Rullg?rd
mans at mansr.com