Developer Guide
FPGA Optimization Guide for Intel® oneAPI Toolkits
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Visible to Intel only — GUID: GUID-ABD7982E-6528-48E0-9245-F39B3C523990
Loop Analysis
The report.html file contains information about all loops in your design and their unroll statuses. The Loop Analysis report helps you examine whether the Intel® oneAPI DPC++/C++ Compiler can maximize your kernel's throughput.
To view the Loop Analysis report, click Throughput Analysis > Loop Analysis. The purpose of this view is to show estimates of performance indicators (such as II) and potential performance bottlenecks. For each loop, you can identify the following using the report:
- Whether the loop is pipelined
- Whether the loop uses a hyper-optimized loop structure
- Any pragma or attribute applied to the loop
- II of the loop
- Loop Analysis report does not report anything about loops in NDRange kernels.
- FPGA optimization reports support user-defined loop labels and replace the system-generated loop labels.
The left-hand Loops List pane of the Loop Analysis report displays the following types of loops:
- Fused loops (see Fuse Loops to Reduce Overhead and Improve Performance)
- Fused subloops
- Coalesced loops
- Fully unrolled loops
- Partial unrolled loops
- Regular loops
Loop Pragma and Attributes
You can use the Loop Analysis report to help determine where to deploy one or more of the following pragmas or attributes on your loops:
Key Performance Metrics
The Loop Analysis report captures the following key performance metrics on all blocks:
- Source Location: Indicates the loop location in the source code.
- Pipelined: Indicates whether a loop is pipelined. Pipelining allows for many data items to be processed concurrently (in the same clock cycle) while efficiently using of the hardware in the datapath by keeping it occupied.
- II: Shows the sustainable initiation interval (II) of the loop. Processing data in loops is an additional source of pipeline parallelism. When you pipeline a loop, the next iteration of the loop begins before previous iterations complete. You can determine the number of clock cycles between iterations by the number of clock cycles you require to resolve any dependencies between iterations. You can refer to this number as the initiation interval (II) of the loop. The Intel® oneAPI DPC++/C++ Compiler automatically identifies these dependencies and builds hardware to resolve these dependencies while minimizing the II.
- Scheduled fMAX: Shows the scheduled maximum clock frequency at which the loop operates. The fMAX is the maximum rate at which the outputs of registers are updated. If the scheduled fMAX is below the target frequency, then the scheduled fMAX appears in red color and a question mark with a tooltip displaying the target frequency displays.
The physical propagation delay of the signal between two consecutive registers limits the clock speed. This propagation delay is a function of the complexity of the Boolean logic in the path. The path with the most logic (and the highest delay) limits the speed of the entire circuit, and you can refer to this path as the critical path.
The fMAX is calculated as the inverse of the critical path delay. High fMAX is desirable because it correlates directly with high performance in the absence of other bottlenecks. The compiler attempts to optimize for different objectives for the scheduled fMAX depending on whether the fMAX target is set and whether the #pragma II is set for each of the loops. The fMAX target is a strong suggestion, and the compiler does not error out if it cannot achieve this fMAX, whereas the #pragma II triggers an error if the compiler is not able to achieve the requested II. The fMAX achieved for each block of code is shown in the Loop Analysis report. This behavior is outlined in the following table:
Explicitly specify fMAX?
Explicitly specify II?
Compiler's Scheduler Behavior
No
No
Use heuristic to achieve best fMAX/II trade-off.
No
Yes
Best effort to achieve the II for the corresponding loop (may not achieve the best possible fMAX).
Yes
No
Best effort to achieve fMAX specified (may not achieve the best possible II).
Yes
Yes
Best effort to achieve the fMAX specified at the given II. The compiler errors out if it cannot achieve the requested II.
NOTE:Intel® recommends that if you are using an fMAX target in the command line or for a kernel, use #pragma II = <N> for performance-critical loops in your design.
- Latency: Shows the number of clock cycles a loop takes to complete one or more instructions. Typically, you want to have low latency. However, lowering latency often results in decreased fMAX.
- Speculated Iterations: Shows the loop speculation. Loop speculation is an optimization technique that enables more efficient loop pipelining by allowing future iterations to be initiated before determining whether the loop was exited already.
- Max Interleaving Iterations: Indicates the number of interleaved invocations of an inner loop that can be executed simultaneously. For more information, refer to max_interleaving Attribute.
Example
The following is a SYCL* kernel example that includes three loops:
cgh.single_task<class example>([=]() { #pragma unroll for (int i = 0; i < 10; i++) { acc_data[i] += i; } #pragma unroll 1 for (int k = 0; k < N; k++) { #pragma unroll 5 for (int j = 0; j < N; j++) { acc_data[j] = j + k; } } });
The Loop Analysis report of this design example highlights the unrolling strategy for the different kinds of loops defined in the code.
The Intel® oneAPI DPC++/C++ Compiler implements the following loop unrolling strategies based on the source code:
- Fully unrolls the first inner loop (line 3) because of the #pragma unroll specification.
- Does not unroll the second loop (line 7), which is an outer loop because of the #pragma unroll 1 specification.
- Unrolls the third loop (line 9, an inner loop of the second loop) five times because of the #pragma unroll 5 specification.
For more examples, refer to Loops section.