Loop unrolling is a loop transformation technique that helps to optimize the execution time of a program. We basically remove or reduce iterations. Loop unrolling increases the program’s speed by eliminating loop control instruction and loop test instructions.
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Program 2 is more efficient than program 1 because in program 1 there is a need to check the value of i and increment the value of i every time round the loop. So small loops like this or loops where there is fixed number of iterations are involved can be unrolled completely to reduce the loop overhead.
- Increases program efficiency.
- Reduces loop overhead.
- If statements in loop are not dependent on each other, they can be executed in parallel.
- Increased program code size, which can be undesirable.
- Possible increased usage of register in a single iteration to store temporary variables which may reduce performance.
- Apart from very small and simple codes, unrolled loops that contain branches are even slower than recursions.
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