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

[Liu Steven]

> 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?

Thanks

Steven
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