--- "Hansen, Knud" <[EMAIL PROTECTED]> wrote: > Roger, > Thanks for your input. I really appreciate the time you took to write this > response. I have done a lot of bit masking. I have only used the logical > operators(&, |, ^,...) for masking. I can see in special cases a shift > operator maybe more efficient.
the shift is most often needed when you need to pack a bit or byte into an already populated int or long in a specific position. > I was intrigued by your small integer arithmetic. I do not see how this > would be faster. It seems to me, the trips to the CPU needed to pack and > unpack, would balance out against arithmetic operations. I have no expertise > at low level programming so any input on your part would be appreciated. If you just do pack, add, unpack, that is true. But if you can leave the bytes packed through many operations then you can get a sizable payoff. Think of the packing and unpacking as a fixed cost, a latency. All you need is enough work to overcome the cost of the latency. -- Roger > -----Original Message----- > From: Roger Glover [mailto:[EMAIL PROTECTED]] > Sent: Thursday, September 05, 2002 3:34 PM > To: JDJList > Subject: [jdjlist] Re: Shift Operator: What is it good for? > > > There is no doubt that in C-based languages the shift operators, like > "bitwise > logic" operators, fall deeply into that twilight zone of program "bit > twiddling". Remember that the first "application" written in the C language > was the Unix kernel, so the developers of the language were deeply attached > to > such things. However, the shift operators have found *some* usage outside > of > low level systems programming. > > David's mentioned of bit masking operations, which is, of course the > original > motivating reason for creating the operators in the first place. In most > application programming, especially in Java, there tends to be very little > need > for this. However, there are some special purpose usages that are fairly > interesting to application programmers. > > For example, there is also a little known usage for compressed small integer > arithmetic. It turns out that you can fit 5 8-bit numbers into a 64-bit > long > (the other space is used for sign correction and overflow control), and do > add > and subtract arithmetic on them. On 64-bit architectures, you then get 5 > arithemetic operations for the price of one. In this very esoteric usage, > the > shift operations are only needed for packing and unpacking the 5 8-bit > numbers. > > However, probably the most common application programming usage of shift > operators in C-based languages (outside C++) is for special case > multiplication > and division. Left shift is a fast way to multiply an integer by a power of > 2, > while right shift is an equally fast way to divide an integer by a power of > 2. > > As for C++, far and away the most common usage of the shift operators is > for, > believe it or not, I/O. :-P :-P :-P :-P :-P :-P :-P :-P :-P :-P :-P :-P :-P > > Left and right shift also have many, many other fascinating uses in the > field > of cryptography, but very few of us have opportunity to work on such apps. > > -- Roger Glover > > --- David Rosenstrauch <[EMAIL PROTECTED]> wrote: > > At 02:19 PM 9/5/2002 -0400, you wrote: > > > > >During the discussion about 'Getting digits from an integer' there was a > > >reference to the binary shift operator >>. In my 15 years of programming > > > >I have never once used the shift operator. I am sure it must be used for > > > >something, but I can not think of anything useful. Anybody out there > > >every use it? If so, what for? > > > > > >Thanks > > >Knud Hansen > > > > > > I think it's most commonly used in applying bit-masks (e.g., when you want > > > to strip out the "red" value out of RGB color integer values). > > > > > > DR > > > ===== > -- Roger Glover > > __________________________________________________ > Do You Yahoo!? > Yahoo! Finance - Get real-time stock quotes > http://finance.yahoo.com > > To change your JDJList options, please visit: > http://www.sys-con.com/java/list.cfm > > > To change your JDJList options, please visit: > http://www.sys-con.com/java/list.cfm > ===== -- Roger Glover __________________________________________________ Do You Yahoo!? Yahoo! Finance - Get real-time stock quotes http://finance.yahoo.com To change your JDJList options, please visit: http://www.sys-con.com/java/list.cfm
