[FFmpeg-devel] [PATCH 1/3] avutil/imgutils: Optimize writing 4 bytes in memset_bytes()

[Marton Balint]

On Fri, 28 Dec 2018, Michael Niedermayer wrote:

> On Wed, Dec 26, 2018 at 10:16:47PM +0100, Marton Balint wrote:
>>
>>
>> On Wed, 26 Dec 2018, Paul B Mahol wrote:
>>
>>> On 12/26/18, Michael Niedermayer <michael at niedermayer.cc> wrote:
>>>> On Wed, Dec 26, 2018 at 04:32:17PM +0100, Paul B Mahol wrote:
>>>>> On 12/25/18, Michael Niedermayer <michael at niedermayer.cc> wrote:
>>>>>> Fixes: Timeout
>>>>>> Fixes:
>>>>>> 11502/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WCMV_fuzzer-5664893810769920
>>>>>> Before: Executed
>>>>>> clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WCMV_fuzzer-5664893810769920
>>>>>> in 11294 ms
>>>>>> After : Executed
>>>>>> clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WCMV_fuzzer-5664893810769920
>>>>>> in 4249 ms
>>>>>>
>>>>>> Found-by: continuous fuzzing process
>>>>>> https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>>>>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>>>>> ---
>>>>>>  libavutil/imgutils.c | 6 ++++++
>>>>>>  1 file changed, 6 insertions(+)
>>>>>>
>>>>>> diff --git a/libavutil/imgutils.c b/libavutil/imgutils.c
>>>>>> index 4938a7ef67..cc38f1e878 100644
>>>>>> --- a/libavutil/imgutils.c
>>>>>> +++ b/libavutil/imgutils.c
>>>>>> @@ -529,6 +529,12 @@ static void memset_bytes(uint8_t *dst, size_t
>>>>>> dst_size,
>>>>>> uint8_t *clear,
>>>>>>          }
>>>>>>      } else if (clear_size == 4) {
>>>>>>          uint32_t val = AV_RN32(clear);
>>>>>> +        uint64_t val8 = val * 0x100000001ULL;
>>>>>> +        for (; dst_size >= 32; dst_size -= 32) {
>>>>>> +            AV_WN64(dst   , val8); AV_WN64(dst+ 8, val8);
>>>>>> +            AV_WN64(dst+16, val8); AV_WN64(dst+24, val8);
>>>>>> +            dst += 32;
>>>>>> +        }
>>>>>>          for (; dst_size >= 4; dst_size -= 4) {
>>>>>>              AV_WN32(dst, val);
>>>>>>              dst += 4;
>>>>>> --
>>>>>> 2.20.1
>>>>>>
>>>>>
>>>>> NAK, implement special memset function instead.
>>>>
>>>> I can move the added loop into a seperate function, if thats what you
>>>> suggest ?
>>>
>>> No, don't do that.
>>>
>>>> All the code is already in a "special" memset though, this is
>>>> memset_bytes()
>>>>
>>>
>>> I guess function is less useful if its static. So any duplicate should
>>> be avoided in codebase.
>>
>> Isn't av_memcpy_backptr does almost exactly what is needed here? That can
>> also be optimized further if needed.
>
> av_memcpy_backptr() copies data with overlap, its more like a recursive
> memmove().

So? As far as I see the memset_bytes function in imgutils.c can be 
replaced with this:

     if (clear_size > dst_size)
         clear_size = dst_size;
     memcpy(dst, clear, clear_size);
     av_memcpy_backptr(dst + clear_size, clear_size, dst_size - clear_size);

I am not against an av_memset_bytes API addition, but I believe it should 
share code with av_memcpy_backptr to avoid duplication.

Regards,
Marton