From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Cc: Valentin Ochs <a@0au.de>
Subject: Re: [musl] Superfluous shift in qsort()?
Date: Wed, 1 Jul 2020 16:44:56 -0400 [thread overview]
Message-ID: <20200701204456.GI6430@brightrain.aerifal.cx> (raw)
In-Reply-To: <20200701185026.GA6635@voyager>
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). I've CC'd the original author but we've not been
in touch for a long time so I don't know whether to expect a response.
I don't have any insight on why it was done this way.
Rich
next prev parent reply other threads:[~2020-07-01 20:45 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-01 18:50 Markus Wichmann
2020-07-01 20:44 ` Rich Felker [this message]
2020-07-01 21:23 ` Valentin Ochs
2020-07-02 14:44 ` Markus Wichmann
2020-07-06 22:01 ` Rich Felker
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