3. Approach

Yosys is a tool for synthesising (behavioural) Verilog HDL code to target architecture netlists. Yosys aims at a wide range of application domains and thus must be flexible and easy to adapt to new tasks. This chapter covers the general approach followed in the effort to implement this tool.

3.1. Data- and control-flow

The data- and control-flow of a typical synthesis tool is very similar to the data- and control-flow of a typical compiler: different subsystems are called in a predetermined order, each consuming the data generated by the last subsystem and generating the data for the next subsystem (see Fig. 3.1).


Fig. 3.1 General data- and control-flow of a synthesis tool

The first subsystem to be called is usually called a frontend. It does not process the data generated by another subsystem but instead reads the user input—in the case of a HDL synthesis tool, the behavioural HDL code.

The subsystems that consume data from previous subsystems and produce data for the next subsystems (usually in the same or a similar format) are called passes.

The last subsystem that is executed transforms the data generated by the last pass into a suitable output format and writes it to a disk file. This subsystem is usually called the backend.

In Yosys all frontends, passes and backends are directly available as commands in the synthesis script. Thus the user can easily create a custom synthesis flow just by calling passes in the right order in a synthesis script.

3.2. Internal formats in Yosys

Yosys uses two different internal formats. The first is used to store an abstract syntax tree (AST) of a Verilog input file. This format is simply called AST and is generated by the Verilog Frontend. This data structure is consumed by a subsystem called AST Frontend [1]. This AST Frontend then generates a design in Yosys’ main internal format, the Register-Transfer-Level-Intermediate-Language (RTLIL) representation. It does that by first performing a number of simplifications within the AST representation and then generating RTLIL from the simplified AST data structure.

The RTLIL representation is used by all passes as input and outputs. This has the following advantages over using different representational formats between different passes:

  • The passes can be rearranged in a different order and passes can be removed or inserted.

  • Passes can simply pass-thru the parts of the design they don’t change without the need to convert between formats. In fact Yosys passes output the same data structure they received as input and performs all changes in place.

  • All passes use the same interface, thus reducing the effort required to understand a pass when reading the Yosys source code, e.g. when adding additional features.

The RTLIL representation is basically a netlist representation with the following additional features:

  • An internal cell library with fixed-function cells to represent RTL datapath and register cells as well as logical gate-level cells (single-bit gates and registers).

  • Support for multi-bit values that can use individual bits from wires as well as constant bits to represent coarse-grain netlists.

  • Support for basic behavioural constructs (if-then-else structures and multi-case switches with a sensitivity list for updating the outputs).

  • Support for multi-port memories.

The use of RTLIL also has the disadvantage of having a very powerful format between all passes, even when doing gate-level synthesis where the more advanced features are not needed. In order to reduce complexity for passes that operate on a low-level representation, these passes check the features used in the input RTLIL and fail to run when unsupported high-level constructs are used. In such cases a pass that transforms the higher-level constructs to lower-level constructs must be called from the synthesis script first.

3.3. Typical use case

The following example script may be used in a synthesis flow to convert the behavioural Verilog code from the input file design.v to a gate-level netlist synth.v using the cell library described by the Liberty file :

 1# read input file to internal representation
 2read_verilog design.v
 4# convert high-level behavioral parts ("processes") to d-type flip-flops and muxes
 7# perform some simple optimizations
10# convert high-level memory constructs to d-type flip-flops and multiplexers
13# perform some simple optimizations
16# convert design to (logical) gate-level netlists
19# perform some simple optimizations
22# map internal register types to the ones from the cell library
23dfflibmap -liberty cells.lib
25# use ABC to map remaining logic to cells from the cell library
26abc -liberty cells.lib
28# cleanup
31# write results to output file
32write_verilog synth.v

A detailed description of the commands available in Yosys can be found in Command line reference.