[FFmpeg-devel] [PATCH] Demuxer for Leitch/Harris' VR native stream format (LXF)
Michael Niedermayer
michaelni
Mon Sep 13 23:20:07 CEST 2010
On Mon, Sep 13, 2010 at 09:51:22PM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
>
> > On Mon, Sep 13, 2010 at 08:47:37PM +0100, M?ns Rullg?rd wrote:
> >> Michael Niedermayer <michaelni at gmx.at> writes:
> >>
> >> > On Mon, Sep 13, 2010 at 04:56:40PM +0100, M?ns Rullg?rd wrote:
> >> >> Daniel Verkamp <daniel at drv.nu> writes:
> >> >>
> >> >> > 2010/9/13 M?ns Rullg?rd <mans at mansr.com>:
> >> >> >> Tomas H?rdin <tomas.hardin at codemill.se> writes:
> >> >> >>
> >> >> >>>> > > > +//returns number of bits set in value
> >> >> >>>> > > > +static int num_set_bits(uint32_t value) {
> >> >> >>>> > > > + ? ?int ret;
> >> >> >>>> > > > +
> >> >> >>>> > > > + ? ?for(ret = 0; value; ret += (value & 1), value >>= 1);
> >> >> >>>> > > > +
> >> >> >>>> > > > + ? ?return ret;
> >> >> >>>> > > > +}
> >> >> >>>> > >
> >> >> >>>> > > if we dont have a population count function yet, than one should be added
> >> >> >>>> > > to some header in libavutil
> >> >> >>>> >
> >> >> >>>> > I couldn't find one. That probably belongs in its own thread though.
> >> >> >>>> >
> >> >> >>>> > Which files would such a function belong in - intmath.h/c, common.h or
> >> >> >>>> > somewhere else? Also, which name would be best: ff_count_bits(),
> >> >> >>>> > av_count_bits() or something else?
> >> >> >>>>
> >> >> >>>> av_popcount()
> >> >> >>>> would be similar to gccs __builtin_popcount()
> >> >> >>>
> >> >> >>> OK. I attached popcount.patch which adds such a function to common.h.
> >> >> >>> Also bumped minor of lavu. The implementation uses a 16-byte LUT and
> >> >> >>> therefore counts four bits at a time. I suspect there are better
> >> >> >>> solutions though. I did verify that it returns exactly the same number
> >> >> >>> the other implementation does for all 2^32 possible input values.
> >> >> >>
> >> >> >> I can't think of a better generic solution off the top of my head.
> >> >> >
> >> >> > There is at least one algorithm to do this without loops or lookup
> >> >> > tables using SWAR tricks, but I haven't benchmarked it:
> >> >> > http://aggregate.org/MAGIC/#Population Count (Ones Count)
> >> >>
> >> >> That method will be several times slower on any modern hardware.
> >> >
> >> > hardly
> >> > the patch needs 32 operations (i assume its unrolled) 8 of which are
> >> > table lookups which might be less than fast on some hw
> >> > the aggregate.org code needs 15 operations the suggested modification for
> >> > athlons (aka modernb cpus with fast multipler) would reduce that to 12
> >>
> >> Did you count the operations needed to construct the constants? I
> >> think not.
> >
> > i used simple mathematical counting of operations as you surely see
> > and as you know constants dont count there. a counting of cpu cycles
> > of your specific embeded cpu on which constant construction is
> > expensice i could not do, iam sorry.
>
> Naive operation counting is useless. Even division counts a only 1
> there, while everybody knows it takes many times longer than addition
> on real hardware. Even on modern hardware Multiplication also takes
> significantly longer than addition.
theres neither multiplication nor divission in the 32 and 15 operations
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Everything should be made as simple as possible, but not simpler.
-- Albert Einstein
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