Re: [FFmpeg-devel] [PATCH] avcodec/xbmenc: Avoid snprintf() for data->hex conversion

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Marton Balint:
> 
> 
> On Fri, 8 Mar 2024, Andreas Rheinhardt wrote:
> 
>> Andreas Rheinhardt:
>>> Use a small LUT instead. Improves performance.
>>>
>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>>> ---
>>>  libavcodec/xbmenc.c | 21 +++++++++++++++------
>>>  1 file changed, 15 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/libavcodec/xbmenc.c b/libavcodec/xbmenc.c
>>> index cd8b73afa3..5231d4691d 100644
>>> --- a/libavcodec/xbmenc.c
>>> +++ b/libavcodec/xbmenc.c
>>> @@ -20,11 +20,9 @@
>>>   * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
>>> 02110-1301 USA
>>>   */
>>>
>>> -#include "libavutil/reverse.h"
>>>  #include "avcodec.h"
>>>  #include "codec_internal.h"
>>>  #include "encode.h"
>>> -#include "mathops.h"
>>>
>>>  #define ANSI_MIN_READLINE 509
>>>
>>> @@ -57,14 +55,25 @@ static int xbm_encode_frame(AVCodecContext
>>> *avctx, AVPacket *pkt,
>>>      buf += snprintf(buf, 39, "static unsigned char image_bits[] =
>>> {\n");
>>>      for (i = 0, l = lineout; i < avctx->height; i++) {
>>>          for (j = 0; j < linesize; j++) {
>>> -            buf += snprintf(buf, 6, " 0x%02X", ff_reverse[*ptr++]);
>>> +            // 0..15 bitreversed as chars
>>> +            static const char lut[] = {
>>> +                '0', '8', '4', 'C', '2', 'A', '6', 'E',
>>> +                '1', '9', '5', 'D', '3', 'B', '7', 'F'
>>> +            };
>>> +            buf[0] = ' ';
>>> +            buf[1] = '0';
>>> +            buf[2] = 'x';
>>> +            buf[3] = lut[*ptr & 0xF];
>>> +            buf[4] = lut[*ptr >> 4];
> 
> Maybe you could use *buf++ = ... here as well, to avoid the next line.
> But fine either way I guess.
> 

You mean *ptr++ to avoid the line after the next line? That would make
the two lut accesses unsymmetric. And actually I prefer that both
pointers are incremented side-by-side.

>>> +            buf += 5;
>>> +            ptr++;

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