Intel® High Level Synthesis Compiler Pro Edition: Reference Manual

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Date 12/04/2023
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6.6. Loop Iteration Speculation (speculated_iterations Pragma)

With speculated_iterations pragma control, you can adjust the number of speculated iterations for a loop. Speculated iterations are loop iterations that are initiated while the loop exit condition is being calculated. Adjusting the number of speculated iterations can help enable more efficient loop pipelining in your component.

The Intel® HLS Compiler determines the number of speculated iterations on a per-loop basis. You can see the number of speculated iterations for a loop in the Loop Analysis Report in the High Level Design Report (report.html).

For loops where the exit condition calculation is a bottleneck (as shown in the Loop Analysis Report), consider increasing the number of speculated iterations with the speculated_iterations pragma. Increasing the number of speculated iterations might not improve the loop II if other bottlenecks in the loop are found.

For frequently invoked loops with a low latency loop body (for example, an inner loop with a short trip count), you might want to use the speculated_iterations pragma to reduce the number of speculated iterations to reduce the overhead of your design. However, setting the number of speculated iterations too low might increase the loop II because there is not enough time to evaluate the exit condition.

For examples of the effects of using the speculated_iterations pragma, review the following tutorials:
<quartus_installdir>/hls/examples/tutorials/loop_controls/speculated_iterations
<quartus_installdir>/hls/examples/tutorials/loop_controls/small_speculated_iterations