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Imagine a symphony orchestra where each musician plays without regard to the conductor or the other musicians. At the first tap of the conductor’s baton, each musician goes through all of his or her sheet music. Some finish far ahead of others, leave the stage, and go home. The cacophony wouldn’t resemble music (come to think of it, it would resemble experimental jazz) because it would be totally uncoordinated. Of course this isn’t how music is played. A computer program, like a musical piece, is woven on a fabric that unfolds in time (though perhaps woven more loosely). Certain things must happen before or along with others, and there is a rate to the whole process.

With computer programs, whenever event A must occur before event B can, we say that B is dependent on A. We call the relationship between them a dependency. Sometimes dependencies exist because of calculations or memory operations; we call these data dependencies . Other times, we are waiting for a branch or do-loop exit to take place; this is called a control dependency . Each is present in every program to varying degrees. The goal is to eliminate as many dependencies as possible. Rearranging a program so that two chunks of the computation are less dependent exposes parallelism , or opportunities to do several things at once.

Control dependencies

Just as variable assignments can depend on other assignments, a variable’s value can also depend on the flow of control within the program. For instance, an assignment within an if-statement can occur only if the conditional evaluates to true. The same can be said of an assignment within a loop. If the loop is never entered, no statements inside the loop are executed.

When calculations occur as a consequence of the flow of control, we say there is a control dependency , as in the code below and shown graphically in [link] . The assignment located inside the block-if may or may not be executed, depending on the outcome of the test X .NE. 0. In other words, the value of Y depends on the flow of control in the code around it. Again, this may sound to you like a concern for compiler designers, not programmers, and that’s mostly true. But there are times when you might want to move control-dependent instructions around to get expensive calculations out of the way (provided your compiler isn’t smart enough to do it for you). For example, say that [link] represents a little section of your program. Flow of control enters at the top and goes through two branch decisions. Furthermore, say that there is a square root operation at the entry point, and that the flow of control almost always goes from the top, down to the leg containing the statement A=0.0 . This means that the results of the calculation A=SQRT(B) are almost always discarded because A gets a new value of 0.0 each time through. A square root operation is always “expensive” because it takes a lot of time to execute. The trouble is that you can’t just get rid of it; occasionally it’s needed. However, you could move it out of the way and continue to observe the control dependencies by making two copies of the square root operation along the less traveled branches, as shown in [link] . This way the SQRT would execute only along those paths where it was actually needed.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
what is titration
John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
hello friend how are you
Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
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