[FFmpeg-devel] [PATCH] dsputil: add bswap16_buf()

[Michael Niedermayer]

On Mon, Jun 14, 2010 at 01:15:47PM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Sun, Jun 13, 2010 at 05:59:20PM +0100, Mans Rullgard wrote:
> >> ---
> >>  libavcodec/dsputil.c |    7 +++++++
> >>  libavcodec/dsputil.h |    1 +
> >>  2 files changed, 8 insertions(+), 0 deletions(-)
> >> 
> >> diff --git a/libavcodec/dsputil.c b/libavcodec/dsputil.c
> >> index 0701324..1ecd73f 100644
> >> --- a/libavcodec/dsputil.c
> >> +++ b/libavcodec/dsputil.c
> >> @@ -260,6 +260,12 @@ static void bswap_buf(uint32_t *dst, const uint32_t *src, int w){
> >>      }
> >>  }
> >>  
> >> +static void bswap16_buf(uint16_t *dst, const uint16_t *src, int len)
> >> +{
> >> +    while (len--)
> >> +        *dst++ = bswap_16(*src++);
> >> +}
> >
> > on 64bit arch this is likely faster:
> >
> > uint64_t u= ((uint64_t*)src)[i];
> > ((uint64_t*)dst)[i]= ((u>>8)&0x...) + ((u<<8)&0x...)
> 
> That depends entirely on the specifics of the machine, but point taken.
> It would also in general require 8-byte alignment.  Not sure if that
> can be guaranteed.
> 
> The purpose of having this in dsputil is to allow optimised
> implementations for specific machines.  A 32-bit block bswap function
> already exists, and some patch posted was doing a 16-bit block swap.
> If one makes sense, so does the other.
> 
> >>  static int sse4_c(void *v, uint8_t * pix1, uint8_t * pix2, int line_size, int h)
> >>  {
> >>      int s, i;
> >> @@ -4455,6 +4461,7 @@ av_cold void dsputil_init(DSPContext* c, AVCodecContext *avctx)
> >>      c->add_hfyu_left_prediction  = add_hfyu_left_prediction_c;
> >>      c->add_hfyu_left_prediction_bgr32 = add_hfyu_left_prediction_bgr32_c;
> >>      c->bswap_buf= bswap_buf;
> >> +    c->bswap16_buf = bswap16_buf;
> >>  #if CONFIG_PNG_DECODER
> >>      c->add_png_paeth_prediction= ff_add_png_paeth_prediction;
> >>  #endif
> >> diff --git a/libavcodec/dsputil.h b/libavcodec/dsputil.h
> >> index fd2d07f..f3926cd 100644
> >> --- a/libavcodec/dsputil.h
> >> +++ b/libavcodec/dsputil.h
> >> @@ -365,6 +365,7 @@ typedef struct DSPContext {
> >>      /* this might write to dst[w] */
> >>      void (*add_png_paeth_prediction)(uint8_t *dst, uint8_t *src, uint8_t *top, int w, int bpp);
> >>      void (*bswap_buf)(uint32_t *dst, const uint32_t *src, int w);
> >
> >> +    void (*bswap16_buf)(uint16_t *dst, const uint16_t *src, int len);
> >
> > how much alignment can we assume to have for dst/src? and len?
> 
> I don't know as I haven't located every place that could use this
> function.

the alignment requirements are prerequesit to writing optimized
implementations. And would be part of the API of bswap16_buf()

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

While the State exists there can be no freedom; when there is freedom there
will be no State. -- Vladimir Lenin
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