Verification in the Vitis HLS flow can be separated into two distinct processes.
- Pre-synthesis validation that the C program correctly implements the required functionality.
- Post-synthesis verification that the generated RTL code performs as expected.
Both processes are referred to as simulation: C simulation and C/RTL co-simulation.
Before synthesis, the function to be synthesized should be validated with
a C or C++ test bench using C simulation. A C/C++ test bench includes a main() top-level function, that calls the function to be
synthesized by the Vitis HLS project. The test bench
can also include other functions. An ideal test bench has the following features:
- The test bench is self-checking, and validates that the results from the function to be synthesized are correct.
- If the results are correct the test bench returns a value of 0 to
main(). Otherwise, the test bench should return any non-zero value.
In the Vitis HLS GUI, clicking the
Run C Simulation toolbar button 
opens the C Simulation
Dialog box, as shown in the following figure:
The options for the C Simulation Dialog box include the following:
- Launch Debugger
- This compiles the C code and automatically opens the Debug perspective. From within the Debug perspective, the Synthesis perspective button (top left) can be used to return the windows to the Synthesis perspective.
- Build Only
- Compiles the source code and test bench, but does not run simulation. This option can be used to test the compilation process and resolve any issues with the build prior to running simulation. It generates a csim.exe file that can be used to launch simulation from a command shell.
- Clean Build
- Remove any existing executable and object files from the project before compiling the code.
- Optimizing Compile
- By default the design is compiled with debug information
enabled, allowing the compilation to be analyzed and debugged. The
Optimizing Compile
option uses a higher level of optimization effort when compiling the design, but
does not add information required by the debugger. This increases the compile
time but should reduce the simulation runtime.Tip: The Launch Debugger and Optimizing Compile options are mutually exclusive. Selecting one in the C Simulation Dialog box disables the other.
- Enable Pre-Synthesis Control Flow Viewer
- Generates the Pre-synthesis Control Flow report as described in Pre-Synthesis Control Flow.
- Input Arguments
- Specify any inputs required by your test bench
main()function. - Do not show this dialog box again
- Lets you disable the display of the C Simulation Dialog box.Tip: You can re-enable the display of the C Simulation Dialog box by selecting and selecting the Simulation settings.
printf statements from the test
bench, or hls::print statements from the kernel or IP.
When the simulation completes successfully, the following message is also returned to
the console:INFO: [SIM 211-1] CSim done with 0 errors.
INFO: [SIM 211-3] *************** CSIM finish ***************
Finished C simulation.
@E Simulation failed: Function 'main' returns nonzero value '1'.
ERROR: [SIM 211-100] 'csim_design' failed: nonzero return value.
INFO: [SIM 211-3] *************** CSIM finish ***************
If you select the Launch Debugger option, the tool automatically switches to the Debug layout view as shown in the following figure. The simulation is started, but lets you step through the code to observe and debug the function. This is a full featured debug environment: you can step into and over code, specify breakpoints, and observe and set the value of variables in the code.
