4.8. Moving invariant expressions to outside of the loop

4.8.1. Target for this tuning

The target for tuning in this section is the function “calc_function_2”, which is in the measurement region “Solving the system of equations”. In the initial version of the Application, the cost of this function was 1.9% of that of the entire Application. This function was the second highest cost functions in “Solving the system of equations” in the initial version.

4.8.2. Analysis

The following nested loop was selected as a target after the analysis of the function “calc_function_2”. The key points of this source code are as follows:

  • The loop (1) was unrolled successfully by the compiler.

  • The quotients of the division operations (2) did not depend on the counter variable of the loop (1). Therefore, it was possible to move the division operations (2) to the outside of the loop (1).

[Some lines from function “calc_function_2” before this tuning was performed]

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4.8.3. Tuning

The following tuning was performed.

  • In order to reduce the operational amount in the loop (1), the division operations, which did not depend on the loop counter “I”, were moved to the outside of the loop (1).

[Some lines from function “calc_function_2” after this tuning was performed]

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4.8.4. Evaluation of the performance

To evaluate the effect of this tuning, “cycle accounting” (output by fapp) in the measurement region “Solving the system of equations” before performing this tuning was compared with that after performing this tuning.

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As seen in the graphs above, the execution time measured before performing this tuning (depends on the longest process, Process 388) was 24.48 seconds. On the other hand, the execution time measured after performing this tuning (similarly, Process 392) was 24.38 seconds. This means about 0.4% improvement. This indicates the effect of this tuning.