From: Muli Ben-Yehuda <mulix@mulix.org>
To: Jamie Lokier <jamie@shareable.org>
Cc: Linus Torvalds <torvalds@osdl.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Stop mprotect() changing MAP_SHARED and other cleanup
Date: Fri, 5 Sep 2003 09:34:22 +0300 [thread overview]
Message-ID: <20030905063422.GA1145@actcom.co.il> (raw)
In-Reply-To: <20030904220435.GI31590@mail.jlokier.co.uk>
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On Thu, Sep 04, 2003 at 11:04:35PM +0100, Jamie Lokier wrote:
> Muli Ben-Yehuda wrote:
> > > +/* Optimisation macro. */
> > > +#define _calc_vm_trans(x,bit1,bit2) \
> > > + ((bit1) <= (bit2) ? ((x) & (bit1)) * ((bit2) / (bit1)) \
> > > + : ((x) & (bit1)) / ((bit1) / (bit2))
> >
> > Why is this necessary? the original version of the macro was much
> > simpler. If this isn't just for shaving a couple of optimization,
I meant "shaving a couple of instructions", of course.
> > please document it. If it is, I urge you to reconsider ;-)
>
> When the bits don't match, mine reduces to a mask-and-shift. The
> original reduces to a mask-and-conditional, which is usually slower.
Ok. Your version is also incomprehensible (to me, at least) without
working it out using a pen and paper, whereas the original is clear
and concise. Are the saved CPU cycles worth the wasted programmer
cycles in this case? I doubt it.
--
Muli Ben-Yehuda
http://www.mulix.org
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next prev parent reply other threads:[~2003-09-05 6:34 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-09-04 19:34 [PATCH] Stop mprotect() changing MAP_SHARED and other cleanup Jamie Lokier
2003-09-04 20:18 ` Muli Ben-Yehuda
2003-09-04 22:04 ` Jamie Lokier
2003-09-05 6:34 ` Muli Ben-Yehuda [this message]
2003-09-04 23:46 ` Jamie Lokier
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