[FFmpeg-devel] [PATCH v2] avcodec/fft_template: libavcodec/fft_template: improve performance of the ff_fft_init in fft_template

[Paul B Mahol]

On 12/25/18, Liu Steven <lq at chinaffmpeg.org> wrote:
>
>
>> 在 2018年12月25日，上午1:28，Paul B Mahol <onemda at gmail.com> 写道：
>>
>> On 12/24/18, Michael Niedermayer <michael at niedermayer.cc> wrote:
>>> On Fri, Dec 21, 2018 at 06:09:50PM +0800, Steven Liu wrote:
>>>> Before patch:
>>>> init nbits = 17, get 10000 samples, average cost: 16105 us
>>>> After patch:
>>>> init nbits = 17, get 10000 samples, average cost: 15221 us
>>>>
>>>> Signed-off-by: Steven Liu <lq at chinaffmpeg.org>
>>>> ---
>>>> libavcodec/fft_template.c | 26 +++++++++++++++-----------
>>>> 1 file changed, 15 insertions(+), 11 deletions(-)
>>>>
>>>> diff --git a/libavcodec/fft_template.c b/libavcodec/fft_template.c
>>>> index 762c014bc8..2b528be882 100644
>>>> --- a/libavcodec/fft_template.c
>>>> +++ b/libavcodec/fft_template.c
>>>> @@ -261,17 +261,21 @@ av_cold int ff_fft_init(FFTContext *s, int nbits,
>>>> int inverse)
>>>>     if (s->fft_permutation == FF_FFT_PERM_AVX) {
>>>>         fft_perm_avx(s);
>>>>     } else {
>>>> -        for(i=0; i<n; i++) {
>>>> -            int k;
>>>> -            j = i;
>>>> -            if (s->fft_permutation == FF_FFT_PERM_SWAP_LSBS)
>>>> -                j = (j&~3) | ((j>>1)&1) | ((j<<1)&2);
>>>> -            k = -split_radix_permutation(i, n, s->inverse) & (n-1);
>>>> -            if (s->revtab)
>>>> -                s->revtab[k] = j;
>>>> -            if (s->revtab32)
>>>> -                s->revtab32[k] = j;
>>>> -        }
>>>> +#define SPLIT_RADIX_PERMUTATION(num) do { \
>>>> +    for(i=0; i<n; i++) {\
>>>> +        int k;\
>>>> +        j = i;\
>>>
>>>> +        if (s->fft_permutation == FF_FFT_PERM_SWAP_LSBS)\
>>>
>>> maybe this if() should be factored out too ?
>>>
>>> the change looks good though, and iam not sure this is speed relevant
>>> enough
>>
>> Well, it helps. But this code is partially going to be rewritten
>> anyway in future.
>
> Hi Paul,
>
>     So what i should do? Go on update this patch with Michael’s comment  or
> waiting for your rewritten?

Update patch.