FPGA Optimization Guide for Intel® oneAPI Toolkits

Developer Guide

Contents

speculated_iterations
 Attribute

Use the
speculated_iterations
 attribute to direct the
Intel® oneAPI
DPC++/C++
 Compiler
 to improve the performance of pipelined loops. The
speculated_iterations
 attribute is applied to loops and hence, it must appear directly before the loop (the same place as other loop attributes). For more information, refer to Optimize Loops With Loop Speculation.
Syntax
[[intel::speculated_iterations(N)]]
Where, the integer argument
N
 specifies the permissible number of iterations to speculate.
The
Intel® oneAPI
DPC++/C++
 Compiler
 generates hardware to run
N
 extra iterations of the loop while ensuring the extra iterations do not affect anything. This allows either reducing the II of the loop or increasing the f
max
. The deciding factor is how quickly the exit condition of the loop is calculated. If the calculation takes many cycles, it is better to have larger
speculated_iterations
.
Extra iterations increase the time before the next invocation of the loop can begin. This may be a factor if the actual number of iterations of the loop is very small (less than 5 to 10 or similar). In this case, specify the
N
 value as 0 to allow subsequent loop iterations to start immediately but at the cost of a larger II to allow more time to evaluate the exit condition. Refer to the Loop Analysis report to identify whether the exit condition is a bottleneck for II.
Example
[[intel::speculated_iterations(1)]]
  while (m*m*m < N) {
    m += 1;
  }
  dst[0] = m;
The loop in this example will have one speculated iteration.
For additional information, refer to the FPGA tutorial sample "Speculated Iterations" listed in the Intel® oneAPI Samples Browser on Linux* or Windows*, or access the code sample in GitHub.

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.