[FFmpeg-devel] [RFC] use av_get_cpu_flags for real runtime CPU detection in swscale

[Michael Niedermayer]

On Wed, Sep 08, 2010 at 06:17:27PM +0200, Janne Grunau wrote:
> Hi,
> 
> attached patch implements runtime CPU detection in libswscale. One minor
> problem is that it changes behaviour on existing code which sets
> individual flags but obviously not the new SWS_CPU_CAPS_FORCE. I think
> it's acceptable since the flags have no effect with
> !CONFIG_RUNTIME_CPUDETECT.
> 
> Janne

>  swscale.h |    3 ++-
>  utils.c   |   22 +++++++++++++++++++++-
>  2 files changed, 23 insertions(+), 2 deletions(-)
> c54cc19db9288e0cce8f8de94ec509442ef9b369  swscale_runtime_cpu_detection.diff
> diff --git a/swscale.h b/swscale.h
> index 4e11c9a..ca63796 100644
> --- a/swscale.h
> +++ b/swscale.h
> @@ -30,7 +30,7 @@
>  #include "libavutil/avutil.h"
>  
>  #define LIBSWSCALE_VERSION_MAJOR 0
> -#define LIBSWSCALE_VERSION_MINOR 11
> +#define LIBSWSCALE_VERSION_MINOR 12
>  #define LIBSWSCALE_VERSION_MICRO 0
>  
>  #define LIBSWSCALE_VERSION_INT  AV_VERSION_INT(LIBSWSCALE_VERSION_MAJOR, \
> @@ -93,6 +93,7 @@ const char *swscale_license(void);
>  #define SWS_CPU_CAPS_ALTIVEC  0x10000000
>  #define SWS_CPU_CAPS_BFIN     0x01000000
>  #define SWS_CPU_CAPS_SSE2     0x02000000
> +#define SWS_CPU_CAPS_FORCE    0x00100000
>  
>  #define SWS_MAX_REDUCE_CUTOFF 0.002
>  
> diff --git a/utils.c b/utils.c
> index e9400f8..9375489 100644
> --- a/utils.c
> +++ b/utils.c
> @@ -44,6 +44,7 @@
>  #include "libavutil/avutil.h"
>  #include "libavutil/bswap.h"
>  #include "libavutil/pixdesc.h"
> +#include "libavutil/cpu.h"
>  
>  unsigned swscale_version(void)
>  {
> @@ -722,7 +723,26 @@ static int handle_jpeg(enum PixelFormat *format)
>  
>  static int update_flags_cpu(int flags)
>  {
> -#if !CONFIG_RUNTIME_CPUDETECT //ensure that the flags match the compiled variant if cpudetect is off
> +#if CONFIG_RUNTIME_CPUDETECT
> +    int cpuflags;
> +
> +    if (!(flags & SWS_CPU_CAPS_FORCE)) {
> +        flags &= ~(SWS_CPU_CAPS_MMX|SWS_CPU_CAPS_MMX2|SWS_CPU_CAPS_3DNOW|SWS_CPU_CAPS_ALTIVEC|SWS_CPU_CAPS_BFIN);
> +
> +        cpuflags = av_get_cpu_flags();
> +
> +        if (ARCH_X86 && cpuflags & AV_CPU_FLAG_MMX)
> +            flags |= SWS_CPU_CAPS_MMX;
> +        if (ARCH_X86 && cpuflags & AV_CPU_FLAG_MMX2)
> +            flags |= SWS_CPU_CAPS_MMX2;
> +        if (ARCH_X86 && cpuflags & AV_CPU_FLAG_3DNOW)
> +            flags |= SWS_CPU_CAPS_3DNOW;;
> +        if (ARCH_X86 && cpuflags & AV_CPU_FLAG_SSE2)
> +            flags |= SWS_CPU_CAPS_SSE2;
> +        if (ARCH_PPC && cpuflags & AV_CPU_FLAG_ALTIVEC)
> +            flags |= SWS_CPU_CAPS_ALTIVEC;
> +    }

update_flags_cpu() is called on each sws_getCachedContext() and redoing cpu
detect on each such calls seems hackish
besides a AUTODETECT instead of FORCE would not break API

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

Rewriting code that is poorly written but fully understood is good.
Rewriting code that one doesnt understand is a sign that one is less smart
then the original author, trying to rewrite it will not make it better.
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