yosys/README

 /-----------------------------------------------------------------------------\
 |                                                                             |
 |  yosys -- Yosys Open SYnthesis Suite                                        |
 |                                                                             |
 |  Copyright (C) 2012  Clifford Wolf <clifford@clifford.at>                   |
 |                                                                             |
 |  Permission to use, copy, modify, and/or distribute this software for any   |
 |  purpose with or without fee is hereby granted, provided that the above     |
 |  copyright notice and this permission notice appear in all copies.          |
 |                                                                             |
 |  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES   |
 |  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF           |
 |  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR    |
 |  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES     |
 |  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN      |
 |  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF    |
 |  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.             |
 |                                                                             |
 \-----------------------------------------------------------------------------/


yosys -- Yosys Open SYnthesis Suite
===================================

This is a framework for RTL synthesis tools. It currently has
extensive Verilog-2005 support and provides a basic set of
synthesis algorithms for various application domains.

Yosys can be adapted to perform any synthesis job by combining
the existing passes (algorithms) using synthesis scripts and
adding additional passes as needed by extending the yosys C++
code base.

Yosys is free software licensed under the ISC license (a GPL
compatible license that is similar in terms to the MIT license
or the 2-clause BSD license).


Web Site
========

More information and documentation can be found on the Yosys web site:

	http://www.clifford.at/yosys/


Getting Started
===============

You need a C++ compiler with C++11 support (up-to-date CLANG or GCC is
recommended) and some standard tools such as GNU Flex, GNU Bison, and GNU Make.
The Qt4 library is needed for the yosys SVG viewer, that is used to display
schematics, the minisat library is required for the SAT features in yosys
and TCL for the scripting functionality. The extensive test suite requires
Icarus Verilog. For example on Ubuntu Linux 12.04 LTS the following commands
will install all prerequisites for building yosys:

	$ yosys_deps="git g++ clang make bison flex libreadline-dev
	              tcl8.5-dev zlib1g-dev libqt4-dev mercurial
	              iverilog graphviz"
	$ sudo apt-get install $yosys_deps

There are also pre-compiled packages for Yosys on Ubuntu. Visit the Yosys
download page to learn more about this:

	http://www.clifford.at/yosys/download.html 

To configure the build system to use a specific set of compiler and
build configuration, use one of

	$ make config-clang-debug
	$ make config-gcc-debug
	$ make config-release

For other compilers and build configurations it might be
necessary to make some changes to the config section of the
Makefile.

	$ vi Makefile             ..or..
	$ vi Makefile.conf

To build Yosys simply type 'make' in this directory.

	$ make
	$ make test
	$ sudo make install

Note that this also downloads, builds and installs ABC (using yosys-abc
as executeable name).

Yosys can be used with the interactive command shell, with
synthesis scripts or with command line arguments. Let's perform
a simple synthesis job using the interactive command shell:

	$ ./yosys
	yosys>

the command "help" can be used to print a list of all available
commands and "help <command>" to print details on the specified command:

	yosys> help help

reading the design using the verilog frontend:

	yosys> read_verilog tests/simple/fiedler-cooley.v

writing the design to the console in yosys's internal format:

	yosys> write_ilang

convert processes ("always" blocks) to netlist elements and perform
some simple optimizations:

	yosys> proc; opt

display design netlist using the yosys svg viewer:

	yosys> show

the same thing using 'gv' as postscript viewer:

	yosys> show -format ps -viewer gv

translating netlist to gate logic and perform some simple optimizations:

	yosys> techmap; opt

write design netlist to a new verilog file:

	yosys> write_verilog synth.v

a similar synthesis can be performed using yosys command line options only:

	$ ./yosys -o synth.v -p proc -p opt -p techmap -p opt tests/simple/fiedler-cooley.v

or using a simple synthesis script:

	$ cat synth.ys
	read_verilog tests/simple/fiedler-cooley.v
	proc; opt; techmap; opt
	write_verilog synth.v

	$ ./yosys synth.ys

It is also possible to only have the synthesis commands but not the read/write
commands in the synthesis script:

	$ cat synth.ys
	proc; opt; techmap; opt

	$ ./yosys -o synth.v tests/simple/fiedler-cooley.v synth.ys

The following synthesis script works reasonable for all designs:

	# check design hierarchy
	hierarchy

	# translate processes (always blocks) and memories (arrays)
	proc; memory; opt

	# detect and optimize FSM encodings
	fsm; opt

	# convert to gate logic
	techmap; opt

If ABC is enabled in the Yosys build configuration and a cell library is given
in the liberty file mycells.lib, the following synthesis script will synthesize
for the given cell library:

	# the high-level stuff
	hierarchy; proc; memory; opt; fsm; opt

	# mapping to internal cell library
	techmap; opt

	# mapping flip-flops to mycells.lib
	dfflibmap -liberty mycells.lib

	# mapping logic to mycells.lib
	abc -liberty mycells.lib

	# cleanup
	clean

If you do not have a liberty file but want to test this synthesis script,
you can use the file techlibs/cmos/cmos_cells.lib from the yosys sources.

Yosys is under construction. A more detailed documentation will follow.


Unsupported Verilog-2005 Features
=================================

The following Verilog-2005 features are not supported by
yosys and there are currently no plans to add support
for them:

- Non-sythesizable language features as defined in
	IEC 62142(E):2005 / IEEE Std. 1364.1(E):2002

- The "tri", "triand", "trior", "wand" and "wor" net types

- The "config" keyword and library map files

- The "disable", "primitive" and "specify" statements

- Latched logic (is synthesized as logic with feedback loops)


Verilog Attributes and non-standard features
============================================

- The 'full_case' attribute on case statements is supported
  (also the non-standard "// synopsys full_case" directive)

- The 'parallel_case' attribute on case statements is supported
  (also the non-standard "// synopsys parallel_case" directive)

- The "// synopsys translate_off" and "// synopsys translate_on"
  directives are also supported (but the use of `ifdef .. `endif
  is strongly recommended instead).

- The "nomem2reg" attribute on modules or arrays prohibits the
  automatic early conversion of arrays to separate registers.

- The "mem2reg" attribute on modules or arrays forces the early
  conversion of arrays to separate registers.

- The "nolatches" attribute on modules or always-blocks
  prohibits the generation of logic-loops for latches. Instead
  all not explicitly assigned values default to x-bits. This does
  not affect clocked storage elements such as flip-flops.

- The "nosync" attribute on registers prohibits the generation of a
  storage element. The register itself will always have all bits set
  to 'x' (undefined). The variable may only be used as blocking assigned
  temporary variable within an always block. This is mostly used internally
  by yosys to synthesize verilog functions and access arrays.

- The "blackbox" attribute on modules is used to mark empty stub modules
  that have the same ports as the real thing but do not contain information
  on the internal configuration. This modules are only used by the synthesis
  passes to identify input and output ports of cells. The verilog backend
  also does not output blackbox modules on default.

- The "keep" attribute on cells and wires is used to mark objects that should
  never be removed by the optimizer. This is used for example for cells that
  have hidden connections that are not part of the netlist, such as IO pads.

- The "init" attribute on wires is set by the frontend when a register is
  initialized "FPGA-style" with 'reg foo = val'. It can be used during synthesis
  to add the necessary reset logic.

- The "top" attribute on a module marks this module as the top of the
  design hierarchy. The "hierarchy" command sets this attribute when called
  with "-top". Other commands, such as "flatten" and various backends
  use this attribute to determine the top module.

- In addition to the (* ... *) attribute syntax, yosys supports
  the non-standard {* ... *} attribute syntax to set default attributes
  for everything that comes after the {* ... *} statement. (Reset
  by adding an empty {* *} statement.)

- Sized constants (the syntax <size>'s?[bodh]<value>) support constant
  expressions as <size>. If the expresion is not a simple identifier, it
  must be put in parentheses. Examples: WIDTH'd42, (4+2)'b101010


Supported features from SystemVerilog
=====================================

When read_verilog is called with -sv, it accepts some language features
from SystemVerilog:

- The "assert" statement from SystemVerilog is supported in its most basic
  form. In module context: "assert property (<expression>);" and within an
  always block: "assert(<expression>);". It is transformed to a $assert cell
  that is supported by the "sat" and "write_btor" commands.

- The keywords "always_comb", "always_ff" and "always_latch", "logic" and
  "bit" are supported.


Roadmap / Large-scale TODOs
===========================

- Verification and Regression Tests
   - VlogHammer: http://www.clifford.at/yosys/vloghammer.html
   - yosys-bigsim: https://github.com/cliffordwolf/yosys-bigsim

- Technology mapping for real-world applications
   - Add bit-wise const-folding via cell parameters to techmap pass
   - Rewrite current stdcells.v techmap rules (modular and clean)
   - Improve Xilinx FGPA synthesis (RAMB, CARRY4, SLR, etc.)

- Implement SAT-based formal equivialence checker
   - Write equiv pass based on hint-based register mapping

- Re-implement Verilog frontend (far future)
   - cleaner (easier to use, harder to use wrong) AST format
   - pipeline of well structured AST transformations
   - true contextual name lookup


Other Unsorted TODOs
====================

- Implement missing Verilog 2005 features:

  - Multi-dimensional arrays
  - Support for real (float) const. expressions and parameters
  - ROM modeling using $readmemh/$readmemb in "initial" blocks
  - Ignore what needs to be ignored (e.g. drive and charge strengths)
  - Check standard vs. implementation to identify missing features

- Miscellaneous TODO items: 

  - Add brief source code documentation to most passes and kernel code
  - Implement mux-to-tribuf pass and rebalance mixed mux/tribuf trees
  - Add more commands for changing the design (delete, add, modify objects)
  - Add full support for $lut cell type (const evaluation, sat solving, etc.)
  - Smarter resource sharing pass (add MUXes and get rid of duplicated cells)