Introduction - mbits-mirafra/verilator GitHub Wiki

VERILATOR

Verilator is a free,high performance and open-source software tool which converts a Hardware description language to a cycle-accurate behavioral model in C++ or System C that after compiling can be executed.

• Verilator is not a traditional simulator but is like a compiler which compiles the code.

• It can handle all versions of Verilog and also some SystemVerilog and achieves that speed by generating highly optimized C++ code from a given hardware design and is closer to synthesis than event-driven simulation.

• Verilog was the new Synthesis Language and C++ or System verilog is the Test-bench Language

History Of Verilator

• Verilator was born in 1994 by by Paul Wasson

• In 2001, Wilson Snyder took the kit, added a SystemC mode, and called it Verilator2. This was the first packaged public release.

• In 2002, Wilson Snyder created Verilator 3.000 by rewriting Verilator from scratch in C++. This added many optimizations, yielding about a 2-5x performance gain.

• In 2009, major SystemVerilog and DPI language support was added.

• In 2018, Verilator 4.000 was released with multithreaded support.

• In 2019, Verilator joined the CHIPS Alliance.

• In 2022, Verilator 5.000 was released with IEEE scheduling semantics, fork/join, delay handling, DFG performance optimizations, and other improvements.

Why Verilator?

• Outperforms many commercial simulators as Verilator promotes Faster RTL Development and verilog is a standard language where the waveform looks the same across RTL or SystemC.

• Supports automatically partitioning designs into single and multi threaded output models improving the performance.

• Community driven, Professionally supported and Openly Licensed that is its guided by the CHIPS Alliance and Linux Foundations.

• Also one of the key tools in the ASIC and FPGA ecosystem, which Antmicro is actively using and developing.

• Widely used by industries and academic deployment like Arm and RISC-V vendor IP

Note:

Single threaded processes contain the execution of instructions in a single sequence. In other words, one command is processes at a time.

Multi threaded processes allow the execution of multiple parts of a program at the same time.

How a Verilator works?

• The verilator executable is invoked with parameters similar to GCC (GNU Compiler collection is a compiler supporting various programming languages) or other simulators such as Cadence Verilog-XL/NC-Verilog, or Synopsys VCS.

• So the Verilator reads the specified SystemVerilog or RTL code, lints it (statically analysing the designs for issues or errors),optionally adds coverage and waveform tracing support, and compiles the design into a source-level multi threaded C++ or SystemC model.

• Then the resulting model’s code is output as .cpp and .hfiles.

Note: This is referred to as “Verilating”, and the process is “to Verilate” where as the output is a “Verilated” model.

• For simulation, a small-user written C++ wrapper file is required, the“wrapper”. This wrapper defines the C++ main function and instantiates the Verilated model as a C++/SystemC object.

• The C++ wrapper thus creates a runtime library provided by Verilator and are compiled to create a simulation executable performing the actual simulation during simulation runtime.

• The executable may also generate waveform traces of the design that may be viewed. It may also create coverage analysis data for post-analysis.

Verilator example with detailed steps

Let us consider a simple example and run it using Verilator. Steps are as follows:

Step 1

SV design code and testbench code is written in Verilator

Simple addition is taken as an example\

Step 2

To run the above testbench follow the two below commands

Command 1:

--binary: telling Verilator to do everything needed to create a simulation executable. It creates a new directory called obj_dir which contains executable file.

Command 2

Upon running the executable file using the above command we get the display statement containing message “hello”.

Step 3

To open gtkwave follow the below commands

Install gtkwave using sudo apt-get install gtkwave

Step 4

Waveform window opens which looks as shown below: