Pre-Synthesis Control Flow - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English
Important: This feature is only available on Linux platforms, and is not supported on Windows systems.

You can generate the Pre-Synthesis Control Flow Graph (CFG) as an option from the Run C Simulation dialog box. Select the Enable Pre-Synthesis Control Flow Viewer check box on the dialog box to generate the report. After generating the report you can open it by selecting it from the C Simulation > Reports & Viewers section of the Flow Navigator.

The Pre-Synthesis Control Flow viewer helps you to identify the hot spots in your function, the compute-intensive control structures, and to apply pragmas or directives to improve or optimize the results. The CFG shows the control flow through your C code, as shown in the following figure, to help you visualize the top-level function. The CFG also provides static profiling, such as the trip-count of loops, and dynamic profiling information to analyze the design.

Figure 1. Pre-Synthesis Control Flow Viewer

As shown in the figure above, the Pre-Synthesis Control Flow viewer has multiple elements:

  • Function Call Tree on the upper left.
  • Control Flow Graph (CFG) in the middle.
  • Source Code viewer on the upper right.
  • Loops view in the lower Console area that is associated with, and provides cross-probing with the CFG viewer.

Selecting a sub-function or loop in one view, also selects it in other views. This lets you quickly navigate the hierarchy of your code. Every function call can be further expanded to see the control structure of the loops and condition statements. Click on the control structure to view the source code.

Double-clicking on a function in the Function Tree opens that level of the code hierarchy in the CFG, single clicking in the Function Tree simply selects the function. The CFG can be expanded to show the hierarchy of the design using the Expand function call command display the function levels of hierarchy specified.

You can also type in the Search field of the Function Call Tree to highlight the first matching occurrence of the typed text. You can use this to quickly navigate through your code.

The CFG can help you analyze the dynamic behavior of your function, reporting the number of times the function or sub-function executed on different control paths. Loops are often a source of computing intensity, and the Loops window provides statistics such as access time, total and average loop iterations (tripcount). This information regarding the respective loops can be found in the Loops view, which has cross-linking capabilities. Clicking on a loop will highlight both the source code and the control structure.

Memory operations can also be annotated in the CFG viewer, identifying another area for possible performance optimization.