Re: [Qemu-ppc] [PATCH 2/6] target-ppc: Implement darn instruction

[David Gibson]

On Mon, Aug 08, 2016 at 07:33:37AM +1000, Benjamin Herrenschmidt wrote:
> On Sun, 2016-08-07 at 23:06 +0530, Nikunj A Dadhania wrote:
> > +target_ulong helper_darn(uint32_t l)
> > +{
> > +    target_ulong r = UINT64_MAX;
> > +
> > +    if (l <= 2) {
> > +        do {
> > +            r = random() * random();
> > +            r &= l ? UINT64_MAX : UINT32_MAX;
> > +        } while (r == UINT64_MAX);
> > +    }
> > +
> > +    return r;
> > +}
> >  #endif
> 
> Isn't this a bit week ? Look at the implementation of H_RANDOM...

Indeed, you should be using the rng backend that H_RANDOM, virtio-rng
and the Intel random number instruction all use.

But, worse than that: even if random() was a good RNG, I'm pretty sure
than although random() * random() will give you a random number with
twice as many bits as random() alone, it won't be uniformly
distributed.  That's probably not what you want.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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