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Date: Wed, 1 Jul 2020 23:23:09 +0200
From: Valentin Ochs <a@....de>
To: musl@...ts.openwall.com
Subject: Re: Superfluous shift in qsort()?
On Wed, Jul 01, 2020 at 04:44:56PM -0400, Rich Felker wrote:
> On Wed, Jul 01, 2020 at 08:50:26PM +0200, Markus Wichmann wrote:
> > Hi all,
> >
> > I noticed something while reading code today: Near the end of qsort(),
> > we have this gem:
> >
> > shl(p, 2);
> > pshift -= 2;
> > p[0] ^= 7;
> > shr(p, 1);
> >
> > Now, I don't know if I am missing something, but don't the shl and the
> > shr partially cancel out? Isn't this the same as
> >
> > shl(p, 1);
> > p[0] ^= 3;
> >
> > As it is, it isn't wrong, just weird.
>
> Assuming non-overflow, I think they're equivalent (also assuming you
> keep the pshift-=2).
Yes, it looks that way. I'm afraid I don't have any further insight -
it's been quite a while since I thought about the qsort code, and I've
not been doing much work on algorithms over the last couple of years.
The only thing I can think of is that one could figure out which
behaviour with regard to overflow in shl() should be the valid one. I
suspect that replacing it would be valid and this is some transformation
I did while implementing smoothsort without realizing that this can be
simplified.
Cheers,
Valentin
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