Re: [LAU] OT(ish): Strange coding problem (audio related)

Excerpts from torbenh's message of 2011-02-01 14:00:17 +0100:
> On Sun, Jan 30, 2011 at 02:29:13PM +0100, Philipp Überbacher wrote:
> > Excerpts from Peter Nelson's message of 2011-01-29 13:47:07 +0100:
> > > On Sat, 2011-01-29 at 12:07 +0100, Philipp Überbacher wrote:
> > > > Excerpts from fons's message of 2011-01-28 16:11:52 +0100:
> > > > > On Fri, Jan 28, 2011 at 02:02:36PM +0100, Philipp Überbacher wrote:
> > > > >
> > > > > > rant_begin
> > > > > > Why can't log mean the same thing everywhere? Why does it need to be
> > > > > > base e here and base 10 there? Why is there no consistency?
> > > > > > And why is there no proper logarithmus dualis function? Because you
> > > > > > can simply do log(n)/log(2)? We've just seen how well this works.
> > > > > > How about:
> > > > > > log() - base 10
> > > > > > ln() - base e - logarithmus naturalis
> > > > > > ld() - base 2 - logarithmus dualis
> > > > > > rant_end
> > > > >
> > > > > Libm has log(), log10, and log2().
> > > >
> > > > Took me a while to figure out that libm is part of glibc :)
> > > > Good to know that those functions are available on probably pretty much
> > > > all linux systems.
> > > >
> > > > > > The next obvious question is: Does the inaccuracy reliably result in
> > > > > > values bigger than 11?
> > > > >
> > > > > No.
> > > > >
> > > > > If the input is a power of two, and you expect an integer as
> > > > > a result, just do
> > > > >
> > > > > k = (int)(log2(x) + 1e-6)
> > > >
> > > > log2() suffers from the same problem? I somewhat dislike the idea of
> > > > adding a constant.
> > > >
> > > > > or
> > > > >
> > > > > k = (int)(log(x)/log(2) + 1e-6)
> > > > >
> > > > > or
> > > > >
> > > > > int m, k;
> > > > > for (k = 0, m = 1; m < x; k++, m <<= 1);
> > > > >
> > > > > which will round up if x is not a power of 2.
> > > >
> > > > Neat. I thought about it myself yesterday but my ideas weren't exactly
> > > > brilliant. One idea was to divide by 2, the other to use a small
> > > > lookup table for powers of 2. I don't really know about efficiency, but
> > > > I guess bit shifting is as efficient as it gets?
> > > > Anyway, it's a neat way to avoid the problem and the rounding properties
> > > > of mult/div in case of not power of 2 could be useful as well.
> > >
> > > Well, now I'm just being pedantic :-), but as a quick test using rdtsc
> > > (i.e. profiling to be taken with a grain of salt):
> > >
> > > 1: log(x)/log(2)
> > > 2: (1 << k) < x
> > > 3: m < x & m <<= 1
> > >
> > > This is 1000 iterations; cycling through x of 512, 1024, 2048, 4096 (to
> > > prevent the compiler optimizing log(x)/log(2) to a single call
> > > throughout the whole test). The fourth line is the sum of the iterator
> > > in each loop.
> > >
> > > ./a.out (unoptimized)
> > > 1 3132000 cycles
> > > 2 261468 cycles
> > > 3 285273 cycles
> > > 1 50500, 2 50500, 3 50500
> > >
> > > ./a.out (-O3)
> > > 1 2598345 cycles
> > > 2 170055 cycles
> > > 3 173673 cycles
> > > 1 50500, 2 50500, 3 50500
> > >
> > > I must admit, I'm surprised that fons way shows slightly slower in my
> > > test!
> >
> > Is it really surprising (for the non-optimised variant)? Your variant
> > uses one less variable and assignment. That this isn't optimised away
> > completely with -O3 does surprise me too.
> >
> > > As an aside, here's the result with (-O3 -ffast-math)
> > >
> > > ./a.out
> > > 1 150894 cycles,
> > > 2 169983 cycles,
> > > 3 158850 cycles,
> > > 1 47750, 2 50500, 3 50500
> > >
> > > Yeah, faster, but wrong :-)
> > >
> > > Peter.
> >
> > After a brief look at man gcc -ffast-math doesn't seem like a very good
> > idea to use it in general, so no surprise it's not part of -O3 or
> > something.
>
> i like -ffast-math... this whole thread is about float not being exact.
> sure its worse with -ffast-math ... but if you dont assume correctness,
> and do appropriate workarounds in case where you need correct results,
> its the way to go.

Since --ffast-math is a collection of other options I would have thought
that you'd pick the ones that make sense for your use case instead of
--ffast-math.

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Received on Sun Feb 13 20:15:02 2011