[FFmpeg-devel] [PATCH] Some IWMMXT functions for libavcodec #2

[Dmitry Antipov]

Hello again,

here are some more efforts on IWMMXT stuff for libavcodec - all inner
loops are rewritten in assembly and a few more functions added.

The performance measurement is not so simple because:
  1) gprof doesn't provide reliable results for small (20-30 instructions)
     functions;
  2) the hardware provides the fast, low-overhead clock source (similar
     to x86 TSC), but it may be accessed from the privileged mode (i.e. kernel)
     only;
  3) although 2) may be done via oprofile, there is no oprofile support for
     my hardware yet :-(.

So, the only benchmark I'm using for now is the simple 'synthetic' benchmark
which measures 'all C' vs. 'all IWMMXT' stuff with plain gettimeofday() (see
speedrun() functions within http://78.153.153.8/tmp/dspwmmx.c; it's 24K, so
not attached here). There are some results from it:

MAX_SIZE MAX_LINE C     IWMMXT Speedup
--------------------------------------
64       8        100   40     2.5
128      16       560   190    2.95
256      32       2120  660    3.21
512      64       6670  1970   3.39
1024     128      19090 5360   3.56

(GCC 3.4.3, '-fomit-frame-pointer -O3', XScale Core3 at 312 MHz and 622 BogoMIPS).

According to these results, I suppose that IWMMXT functions are ~2-3 times faster
in general, but the mileage may vary from function to function - I didn't perform
per-function measurements yet.

It would be nice if someone proposes a real video processing task which loads these
functions heavily.

Dmitry
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