Function Call Graph - 2024.1 English

Vitis High-Level Synthesis User Guide (UG1399)

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2024.1 English

The Function Call Graph illustrates your full design after C Synthesis or C/RTL Co-simulation. The report shows the throughput of the design in terms of latency and II. It helps identify the critical path in your design and helps you identify bottlenecks in the design to improve throughput. It can also show the paths through the design where throughput might be imbalanced leading to FIFO stalls and/or deadlock.

Figure 1. Performance Metrics Synthesis

In some cases, the displayed hierarchy of the design might not be the same as your source code as a result of HLS optimizations that convert loops into function pipelines, etc. Functions that are in-lined will no longer be visible in the call graph, as they are no longer separate functions in the synthesized code. If multiple instances of a function are created, each unique instance of the function is shown in the call graph. This lets you see what functions contribute to a calling function's latency and II.

The graph as shown above displays functions as rectangular boxes, and loops as oval boxes. The graph displays hierarchy and nested functions or loops. A table under the graph displays each function or loop with II, latency, and resource or timing data depending on the specific view.

The Function Call Graph report is available under both C SYNTHESIS and C/RTL COSIMULATION headings in the Flow Navigator. The performance and resource metrics that are shown in the graph from the C SYNTHESIS phase are estimates from the HLS tool. The Function Call Graph report available after C/RTL COSIMULATION is more detailed, providing actual II and latency numbers reported along with stalling percentages.

Tip: For more accurate resource and timing estimates, logic synthesis or implementation can be performed as part of Running Implementation.

You can also use the Heat Map feature to highlight several metrics of interest:

  • II (min, max, avg)
  • Latency (min, max, avg)
  • Resource Utilization
  • Stalling Time Percentage
Figure 2. Performance Metrics

The heat map uses color coding to highlight problematic modules. Using a color scale of red to green where red indicates the high value of the metric (highest II or highest latency) while green indicates a low value of the metric in question. The colors that are neither red nor green represent the range of values that are in between the highest and lowest values. As shown above, this helps in quickly identifying the modules that need attention. In the example shown above, a heat map for LATENCY is shown and the red module indicates where high latency is observed.

As mentioned before, the Function Call Graph illustrates at a high level, the throughput numbers of your design. The user can view the Function Call Graph as a cockpit from which further investigations can be carried out. Right-click on any of the displayed modules to display a menu of options that you can use to display additional information. This lets you see the overall design and then jump into specific parts of the design which need extra attention. Additional reports include the Schedule Viewer, Synthesis Summary report, Dataflow Viewer, and source files. The Function Call Graph lets you see the full picture of your design and have the latency and II information of each module available for analysis.

Tip: On the right hand side of the report there is a Settings command and a Legend command to let you display additional details of the report.