Date: Thu, 4 Sep 2003 23:04:35 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Muli Ben-Yehuda <mulix@mulix.org>
Cc: Linus Torvalds <torvalds@osdl.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Stop mprotect() changing MAP_SHARED and other cleanup
Message-ID: <20030904220435.GI31590@mail.jlokier.co.uk>
References: <20030904193454.GA31590@mail.jlokier.co.uk> <20030904201851.GK13947@actcom.co.il>
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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,
> 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.

Hopefully GCC optimises the latter to the former these days, but there
is no harm in helping.

I vaguely recall GCC being able to optimise (x&mask1) | (x&mask2) to
x&(mask1|mask2), not 100% sure though.  If so, the PROT_ bits
translation will reduce to prot & 7.

-- Jamie
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