On Freitag, 28. Juni 2019 16:56:15 CEST Christian Schoenebeck via Qemu-devel
> > > + */
> > > +#define EXP_GOLOMB_K 0
> > > +
> > > +# if !EXP_GOLOMB_K
> > > +
> > > +/** @brief Exponential Golomb algorithm limited to the case k=0.
> > > + *
> >
> > This doesn't really help to have a special implementation for k=0. The
> > resulting function is nearly identical to the general case. It is likely
> > that the compiler can optimize it and generate the same code.
>
> I don't think the compiler's optimizer would really drop all the
> instructions automatically of the generalized variant of that particular
> function. Does it matter in practice? No, I actually just provided that
> optimized variant for the special case k=0 because I think k=0 will be the
> default value for that parameter and because you were already picky about a
> simple
>
> if (pdu->s->dev_id == 0)
>
> to be optimized. In practice users won't feel the runtime difference either
> one of the two optimization scenarios.
I just checked my assmption made here with a simple C test unit:
// Use manually optimized function for case k=0.
VariLenAffix implK0(uint64_t n) {
return expGolombEncodeK0(n);
}
// Rely on compiler to optimize generalized function for k=0
VariLenAffix implGenK(uint64_t n) {
return expGolombEncode(n, 0);
}
And : gcc -S -O3 -ffast-math k.c
Like expected the generated 2 functions are almost identical, except that the
manually optimized variant saves the following 3 instructions:
movl $0, %eax
...
testl %edx, %edx
cmovns %edx, %eax
But like I said, it is a tiny difference of course. Not really relevant in
practice.
Best regards,
Christian Schoenebeck
