[Ffmpeg-devel] [PATCH] h264 - loopify some get_cabac calls

[Alexander Strange]

This changes two series of straight-line get_cabac calls to loops.

Before (skips are intentional):
45552 dezicycles in decode_mb_cabac, 8188 runs, 4 skips
39043 dezicycles in decode_mb_cabac, 13456 runs, 2928 skips
34729 dezicycles in decode_mb_cabac, 23559 runs, 9209 skips
32233 dezicycles in decode_mb_cabac, 44016 runs, 21520 skips
31117 dezicycles in decode_mb_cabac, 86431 runs, 44641 skips
30542 dezicycles in decode_mb_cabac, 171150 runs, 90994 skips
30203 dezicycles in decode_mb_cabac, 339742 runs, 184546 skips
30131 dezicycles in decode_mb_cabac, 710510 runs, 338066 skips
33107 dezicycles in decode_mb_cabac, 1411827 runs, 685325 skips
34922 dezicycles in decode_mb_cabac, 2822050 runs, 1372254 skips

After:
45467 dezicycles in decode_mb_cabac, 8188 runs, 4 skips
38793 dezicycles in decode_mb_cabac, 13453 runs, 2931 skips
34406 dezicycles in decode_mb_cabac, 23555 runs, 9213 skips
31879 dezicycles in decode_mb_cabac, 44011 runs, 21525 skips
30729 dezicycles in decode_mb_cabac, 86434 runs, 44638 skips
30168 dezicycles in decode_mb_cabac, 171152 runs, 90992 skips
29826 dezicycles in decode_mb_cabac, 339746 runs, 184542 skips
29746 dezicycles in decode_mb_cabac, 710509 runs, 338067 skips
32694 dezicycles in decode_mb_cabac, 1411793 runs, 685359 skips
34513 dezicycles in decode_mb_cabac, 2822036 runs, 1372268 skips

Not a big win, but there is a definite one.
Since get_cabac is in asm, there's obviously some load/store hoisting  
opportunities that aren't being taken. Not much to do about it though.

BTW, I haven't abandoned those other two patches yet, but haven't got  
them quite as good on x86 as they should be.
There's some more AltiVec code here we'll probably send soon: http:// 
trac.perian.org/ticket/113

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