On Thu, 23 Apr 2009, Julia Lawall wrote:
> On Thu, 23 Apr 2009, Robert P. J. Day wrote:
>
> > On Thu, 23 Apr 2009, Julia Lawall wrote:
> >
> > > I found 8 occurrences with the following Coccinelle semantic match:
> > >
> > > @@ expression n; @@
> > >
> > > * (n & (n-1)) == 0
> > >
> > >
> > > The most unpleasant was:
> > >
> > > (!(((fp)->ipend & ~0x30) & (((fp)->ipend & ~0x30) - 1)))
> > >
> > > in the file arch/blackfin/kernel/traps.c.
> > >
> > > At least some of them seemed to have comments nearby that suggested
> > > that searching for a single 1-bit was indeed the goal. I haven't
> > > looked at all of them, though.
> >
> > behold, the power of shell and REs, which searches for four
> > variations of that test:
>
> Actually, it seems that it was specifying == 0 that was not really
> necessary. Perhaps the possibility of parentheses is useful too.
>
> With the following semantic patch, I find 93 occurrences.
>
> @@ expression n; @@
>
> * (n) & ((n)-1)
>
> That could indeed be worth doing something about, if they are indeed all
> representing a check for a single bit.
they're not. we're actually now discussing this on the main LKML,
but that construct -- n & (n - 1) -- has two semantic meanings:
a) power of two?
b) single bit set?
mathematically, those two things have the same form but
*semantically* they're read very differently, which is why i'm
proposing to add a simple function or two for single-bit testing that
would make a lot of the code easier to read.
rday
--
========================================================================
Robert P. J. Day Waterloo, Ontario, CANADA
Linux Consulting, Training and Annoying Kernel Pedantry.
Web page: http://crashcourse.ca
Linked In: http://www.linkedin.com/in/rpjday
Twitter: http://twitter.com/rpjday
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