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

[Marton Balint <cus@passwd.hu>]

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.

Thanks,
Marton

>> +            buf += 5;
>> +            ptr++;
>>              if (--commas <= 0) {
>> -                buf += snprintf(buf, 2, "\n");
>> +                *buf++ = '\n';
>>                  break;
>>              }
>> -            buf += snprintf(buf, 2, ",");
>> +            *buf++ = ',';
>>              if (--l <= 0) {
>> -                buf += snprintf(buf, 2, "\n");
>> +                *buf++ = '\n';
>>                  l = lineout;
>>              }
>>          }
>
> Will apply this patch tomorrow unless there are objections.
>
> - Andreas
>
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