[FFmpeg-devel] [PATCH 4/4] avcodec/cbrt_tablegen: speed up dynamic table creation

[Ganesh Ajjanagadde]

On Sat, Nov 28, 2015 at 12:03 PM, Ganesh Ajjanagadde
<gajjanagadde at gmail.com> wrote:
> On systems having cbrt, there is no reason to use the slow pow function.
>
> Sample benchmark (x86-64, Haswell, GNU/Linux):
> new:
> 5124920 decicycles in cbrt_tableinit,       1 runs,      0 skips
>
> old:
> 12321680 decicycles in cbrt_tableinit,       1 runs,      0 skips
>
> Reviewed-by: Ronald S. Bultje <rsbultje at gmail.com>
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libavcodec/cbrt_tablegen.h          | 5 ++---
>  libavcodec/cbrt_tablegen_template.c | 1 +
>  2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/libavcodec/cbrt_tablegen.h b/libavcodec/cbrt_tablegen.h
> index 27a3e3a..59b5a1d 100644
> --- a/libavcodec/cbrt_tablegen.h
> +++ b/libavcodec/cbrt_tablegen.h
> @@ -29,7 +29,7 @@
>  #include "libavcodec/aac_defines.h"
>
>  #if USE_FIXED
> -#define CBRT(x) (int)floor((x).f * 8192 + 0.5)
> +#define CBRT(x) lrint((x).f * 8192)
>  #else
>  #define CBRT(x) x.i
>  #endif
> @@ -49,13 +49,12 @@ static av_cold void AAC_RENAME(cbrt_tableinit)(void)
>  {
>      if (!cbrt_tab[(1<<13) - 1]) {
>          int i;
> -        /* cbrtf() isn't available on all systems, so we use powf(). */
>          for (i = 0; i < 1<<13; i++) {
>              union {
>                  float f;
>                  uint32_t i;
>              } f;
> -            f.f = pow(i, 1.0 / 3.0) * i;
> +            f.f = cbrt(i) * i;
>              cbrt_tab[i] = CBRT(f);
>          }
>      }
> diff --git a/libavcodec/cbrt_tablegen_template.c b/libavcodec/cbrt_tablegen_template.c
> index 9dd2cf5..1d71d34 100644
> --- a/libavcodec/cbrt_tablegen_template.c
> +++ b/libavcodec/cbrt_tablegen_template.c
> @@ -23,6 +23,7 @@
>  #include <stdlib.h>
>  #define CONFIG_HARDCODED_TABLES 0
>  #include "cbrt_tablegen.h"
> +#include "libavutil/tablegen.h"
>  #include "tableprint.h"
>
>  int main(void)
> --
> 2.6.2
>

pushed, identical stand-alone patch reviewed by Ronald.