Re: [FFmpeg-devel] [PATCH] checkasm: h264dsp: test luma_dc_dequant

[Tristan Matthews <tmatth@videolan.org>]

On Fri, Jun 13, 2025 at 2:08 AM Martin Storsjö <martin@martin.st> wrote:
>
> On Fri, 13 Jun 2025, Tristan Matthews wrote:
>
> > On Thu, Jun 12, 2025 at 4:14 PM Martin Storsjö <martin@martin.st> wrote:
> >>
> >> On Thu, 12 Jun 2025, Tristan Matthews wrote:
> >>
> >>> ---
> >>> tests/checkasm/h264dsp.c | 37 +++++++++++++++++++++++++++++++++++++
> >>> 1 file changed, 37 insertions(+)
> >>>
> >>> diff --git a/tests/checkasm/h264dsp.c b/tests/checkasm/h264dsp.c
> >>> index d1228ed985..5fba31cf69 100644
> >>> --- a/tests/checkasm/h264dsp.c
> >>> +++ b/tests/checkasm/h264dsp.c
> >>> @@ -22,6 +22,7 @@
> >>> #include "checkasm.h"
> >>> #include "libavcodec/h264dsp.h"
> >>> #include "libavcodec/h264data.h"
> >>> +#include "libavcodec/h264idct.h"
> >>> #include "libavcodec/h264_parse.h"
> >>> #include "libavutil/common.h"
> >>> #include "libavutil/intreadwrite.h"
> >>> @@ -324,6 +325,41 @@ static void check_idct_multiple(void)
> >>>     }
> >>> }
> >>>
> >>> +static void check_idct_dequant(void)
> >>> +{
> >>> +    static const int depths[5] = { 8, 9, 10, 12, 14 };
> >>> +    LOCAL_ALIGNED_16(int16_t, src, [16]);
> >>> +    LOCAL_ALIGNED_16(int16_t, dst0, [16 * 16]);
> >>> +    LOCAL_ALIGNED_16(int16_t, dst1, [16 * 16]);
> >>> +    H264DSPContext h;
> >>> +    int bit_depth, i, qmul;
> >>> +    declare_func_emms(AV_CPU_FLAG_MMX | AV_CPU_FLAG_SSE2, void, int16_t *output, int16_t *input, int qmul);
> >>> +
> >>> +    for (int j = 0; j < 16; j++)
> >>> +        src[j] = (rnd() % 512) - 256;
> >>> +
> >>> +    qmul = rnd() % 4096;
> >>> +
> >>> +    memset(dst0, 0, 16 * 16 * sizeof(dst0[0]));
> >>> +    memset(dst1, 0, 16 * 16 * sizeof(dst1[0]));
> >>> +
> >>> +    for (i = 0; i < FF_ARRAY_ELEMS(depths); i++) {
> >>> +        bit_depth = depths[i];
> >>> +        ff_h264dsp_init(&h, bit_depth, 1);
> >>> +
> >>> +        if (check_func(h.h264_luma_dc_dequant_idct, "h264_luma_dc_dequant_idct_%d", bit_depth)) {
> >>> +
> >>> +            call_ref(dst0, src, qmul);
> >>> +            call_new(dst1, src, qmul);
> >>> +
> >>> +            if (memcmp(dst0, dst1, 16 * 16 * sizeof(*dst0)))
> >>> +                fail();
> >>
> >> If possible, use the checkasm_check_*() helpers for validation for new
> >> code; this gives you printout of the differing values if you run "checkasm
> >> -v" and more. In this case, I think checkasm_check(int16_t, dst0,
> >> 16*sizeof(int16_t), dst1, 16*sizeof(int16_t), 16, 16, "dst") would be
> >> suitable one.
> >
> > Good catch, also I realized that the output buffers were too small,
> > will be fixed in the next version.
>
> Why was that too small? If we write (and check) 16x16 int16_t elements,
> the previous allocation of LOCAL_ALIGNED_16(int16_t, dst0, [16 * 16])
> sounds just right? Or does the function use the [16*16,2*16*16) area of
> the destination as scratch space?

That's what I thought too until I noticed the FATE failures (e.g.
https://patchwork.ffmpeg.org/check/124147/), and on further digging
realized that dctcoef (used for dst here:
https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/fb65ecbc9b805571e5ff707b935c343803137e54:/libavcodec/h264idct_template.c#l256
) will be either 2 or 4 bytes depending on bit-depth IIUC (see
https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/fb65ecbc9b805571e5ff707b935c343803137e54:/libavcodec/bit_depth_template.c#l54
)

Best,
Tristan
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