Re: [FFmpeg-devel] [PATCH v2 12/15] avfilter/vf_bwdif: Add a filter_line3 method for optimisation

["Martin Storsjö" <martin@martin.st>]

On Sun, 2 Jul 2023, Thomas Mundt wrote:

> Am So., 2. Juli 2023 um 14:34 Uhr schrieb John Cox <jc@kynesim.co.uk>:
>       Add an optional filter_line3 to the available optimisations.
>
>       filter_line3 is equivalent to filter_line, memcpy, filter_line
>
>       filter_line shares quite a number of loads and some calculations
>       in
>       common with its next iteration and testing shows that using
>       aarch64
>       neon filter_line3s performance is 30% better than two
>       filter_lines
>       and a memcpy.
>
>       Signed-off-by: John Cox <jc@kynesim.co.uk>
>       ---
>        libavfilter/bwdif.h    |  7 +++++++
>        libavfilter/vf_bwdif.c | 31 +++++++++++++++++++++++++++++++
>        2 files changed, 38 insertions(+)
>
>       diff --git a/libavfilter/bwdif.h b/libavfilter/bwdif.h
>       index cce99953f3..496cec72ef 100644
>       --- a/libavfilter/bwdif.h
>       +++ b/libavfilter/bwdif.h
>       @@ -35,6 +35,9 @@ typedef struct BWDIFContext {
>            void (*filter_edge)(void *dst, void *prev, void *cur, void
>       *next,
>                                int w, int prefs, int mrefs, int
>       prefs2, int mrefs2,
>                                int parity, int clip_max, int spat);
>       +    void (*filter_line3)(void *dst, int dstride,
>       +                         const void *prev, const void *cur,
>       const void *next, int prefs,
>       +                         int w, int parity, int clip_max);
>        } BWDIFContext;
>
>        void ff_bwdif_init_filter_line(BWDIFContext *bwdif, int
>       bit_depth);
>       @@ -53,4 +56,8 @@ void ff_bwdif_filter_line_c(void *dst1, void
>       *prev1, void *cur1, void *next1,
>                                    int prefs3, int mrefs3, int prefs4,
>       int mrefs4,
>                                    int parity, int clip_max);
>
>       +void ff_bwdif_filter_line3_c(void * dst1, int d_stride,
>       +                             const void * prev1, const void *
>       cur1, const void * next1, int s_stride,
>       +                             int w, int parity, int clip_max);
>       +
>        #endif /* AVFILTER_BWDIF_H */
>       diff --git a/libavfilter/vf_bwdif.c b/libavfilter/vf_bwdif.c
>       index 26349da1fd..52bc676cf8 100644
>       --- a/libavfilter/vf_bwdif.c
>       +++ b/libavfilter/vf_bwdif.c
>       @@ -150,6 +150,31 @@ void ff_bwdif_filter_line_c(void *dst1,
>       void *prev1, void *cur1, void *next1,
>            FILTER2()
>        }
>
>       +#define NEXT_LINE()\
>       +    dst += d_stride; \
>       +    prev += prefs; \
>       +    cur  += prefs; \
>       +    next += prefs;
>       +
>       +void ff_bwdif_filter_line3_c(void * dst1, int d_stride,
>       +                             const void * prev1, const void *
>       cur1, const void * next1, int s_stride,
>       +                             int w, int parity, int clip_max)
>       +{
>       +    const int prefs = s_stride;
>       +    uint8_t * dst  = dst1;
>       +    const uint8_t * prev = prev1;
>       +    const uint8_t * cur  = cur1;
>       +    const uint8_t * next = next1;
>       +
>       +    ff_bwdif_filter_line_c(dst, (void*)prev, (void*)cur,
>       (void*)next, w,
>       +                           prefs, -prefs, prefs * 2, - prefs *
>       2, prefs * 3, -prefs * 3, prefs * 4, -prefs * 4, parity,
>       clip_max);
>       +    NEXT_LINE();
>       +    memcpy(dst, cur, w);
>       +    NEXT_LINE();
>       +    ff_bwdif_filter_line_c(dst, (void*)prev, (void*)cur,
>       (void*)next, w,
>       +                           prefs, -prefs, prefs * 2, - prefs *
>       2, prefs * 3, -prefs * 3, prefs * 4, -prefs * 4, parity,
>       clip_max);
>       +}
>       +
>        void ff_bwdif_filter_edge_c(void *dst1, void *prev1, void
>       *cur1, void *next1,
>                                    int w, int prefs, int mrefs, int
>       prefs2, int mrefs2,
>                                    int parity, int clip_max, int spat)
>       @@ -244,6 +269,11 @@ static int filter_slice(AVFilterContext
>       *ctx, void *arg, int jobnr, int nb_jobs)
>                                       refs << 1, -(refs << 1),
>                                       td->parity ^ td->tff, clip_max,
>                                       (y < 2) || ((y + 3) > td->h) ? 0
>       : 1);
>       +            } else if (s->filter_line3 && y + 2 < slice_end &&
>       y + 6 < td->h) {
>       +                s->filter_line3(dst,
>       td->frame->linesize[td->plane],
>       +                                prev, cur, next, linesize,
>       td->w,
>       +                                td->parity ^ td->tff,
>       clip_max);
>       +                y += 2;
>                    } else {
>                        s->filter_line(dst, prev, cur, next, td->w,
>                                       refs, -refs, refs << 1, -(refs
>       << 1),
> 
> 
> Maybe I'm missing something, but doesn't this kick out most of the x86 SIMD
> optimization because there is no filter_line3?

It looks to me like it doesn't; it adds a new optional function pointer, 
but the reference C function isn't set. So by default (and on x86) 
filter_line3 is null, and the old assembly optimized codepaths are used. 
But if an architecture does implement filter_line3, that's used instead of 
filter_line.

// Martin
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