[FFmpeg-devel] [PATCH] avcodec/vaapi_encode_vp8: Use av_clip_uintp2()

[James Almer]

On 2/16/2017 5:15 PM, Michael Niedermayer wrote:
> On Thu, Feb 16, 2017 at 05:11:54PM +0000, Mark Thompson wrote:
>> On 16/02/17 16:20, Michael Niedermayer wrote:
>>> Its used elsewhere for 2^p-1 cliping
>>>
>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>> ---
>>>  libavcodec/vaapi_encode_vp8.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/libavcodec/vaapi_encode_vp8.c b/libavcodec/vaapi_encode_vp8.c
>>> index 4a1c85e66c..3d3831c46d 100644
>>> --- a/libavcodec/vaapi_encode_vp8.c
>>> +++ b/libavcodec/vaapi_encode_vp8.c
>>> @@ -161,12 +161,12 @@ static av_cold int vaapi_encode_vp8_configure(AVCodecContext *avctx)
>>>      VAAPIEncodeContext     *ctx = avctx->priv_data;
>>>      VAAPIEncodeVP8Context *priv = ctx->priv_data;
>>>  
>>> -    priv->q_index_p = av_clip(avctx->global_quality, 0, 127);
>>> +    priv->q_index_p = av_clip_uintp2(avctx->global_quality, 7);
>>>      if (avctx->i_quant_factor > 0.0)
>>> -        priv->q_index_i = av_clip((avctx->global_quality *
>>> +        priv->q_index_i = av_clip_uintp2((avctx->global_quality *
>>>                                     avctx->i_quant_factor +
>>>                                     avctx->i_quant_offset) + 0.5,
>>> -                                  0, 127);
>>> +                                  7);
>>>      else
>>>          priv->q_index_i = priv->q_index_p;
>>
>> IMO this makes the code less readable, not more.  It doesn't really matter to anything, though, so commit it if you really want to.
>>
>> (If this is mainly objecting to the magic number being visible there then please do introduce a constant to hide it rather than making the constant smaller - VP8_QINDEX_RANGE, say, to match <https://tools.ietf.org/html/rfc6386#section-14.1>.)
> 
> I just suggested it because its the only case we have in the codebase
> that matches this:
> git grep -E 'av_clip *\(.*, *0 *, *(3|7|15|31|63|127|255|511|1023|2047|4095|8191|16383|32767|65535|131071|262143|524287|1048575|2097151|4194303|8388607|16777215|33554431|67108863|134217727|268435455|536870911|1073741823) *\)'
> 
> If we dont use the more optimized code everywhere then finding
> cases where it makes a difference and isnt used is harder and not
> something i would attempt.

These specialized integer av_clip* functions are optimized only for
arm.
On x86 i don't think anyone ever did benchmarks to see if av_clip(),
when the compiler properly uses cmov, isn't faster than all the
arithmetical instructions from the alternatives. Especially intp2.

> 
> i can also move this into fate-source
> (there it might even be possible to keep track of exceptions)
> but previous additions to fate-source had opposition, so iam not
> doing that unless theres some positive feedback in that direction
> first ...
> 
> [...]
> 
> 
> 
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