Function Call Graph Viewer - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

The new Function Call Graph Viewer, which can be opened from the Flow Navigator, illustrates your full design after C Synthesis or C/RTL Co-simulation. The goal of this viewer is to show 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 focus on to improve throughput. It can also show the paths through the design where throughput may 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, each with II, latency, and resource or timing data depending on the specific view. Before C/RTL co-simulation is completed the performance and resource metrics that are shown in the graph are from the C Synthesis phase, and are therefore estimates from the HLS tool.

Note: For more accurate resource and timing estimates, logic synthesis or implementation can be performed as part of Exporting the RTL Design.

After co-simulation, actual II and latency numbers are reported along with stalling percentages, and this information is back annotated from data collected during co-simulation. You can toggle between the synthesis performance metrics and co-simulation metrics using the drop-down menu at the upper-left of the Function Call Graph viewer.

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

  • II (min, max, avg)
  • Latency (min, max, avg)
  • 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 (i.e. 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, we are showing a heat map for LATENCY MAX and the path of red modules indicates where the high latency values are 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 is the one viewer in Vitis HLS where you can see the full picture of your design and have the latency and II information of each module available for analysis - this includes the dataflow modules for whom the performance information can only be obtained after co-simulation.

Tip: Additional performance and resource metrics are displayed for each function/loop in the Modules/Loops table under the report.