On Tue, 11 Oct 2005, linux-os (Dick Johnson) wrote:
> On Tue, 11 Oct 2005, Paul Jackson wrote:
>
> > Alan asked:
> >> But why do people go to the
> >> effort of confusing readers by using "^" instead of "!="?
> >
> > My guess - eor (^) was quicker than not-equal (!=) on a PDP-11.
> >
> > That code fragment for checking uid's has been around a -long-
> > time, if my memory serves me.
> >
> > It's gotten to be like the infamous "!!" boolean conversion
> > operator, a bit of vernacular that would be harder to read if
> > recoded using modern coding style.
Surely Linux uses entirely original code, with no hangovers from the
original AT&T Unix... Besides, to the best of my recollection, the two
operations are equal in speed on a PDP-11.
"!!" makes sense as an idiom. But "^" for "!=" doesn't, at least not in
this context.
> Also, at one time, people used to spend a lot of time
> minimizing the number of CPU cycles used in the code.
>
> For instance, when it's appropriate, using XOR makes the
> resulting generated code simpler and usually faster:
...
Yes, sometimes XOR can yield simpler object code. But not in cases like
this, where it's part of a Boolean test:
if (... && (a1^b1) && (a2^b2) && (a3^b3)) ...
On any architecture I know of, "^" and "!=" would be equally efficient
here.
Alan Stern
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