OpenFPGA/yosys/CodingReadme

This file contains some very brief documentation on things like programming APIs.
Also consult the Yosys manual and the section about programming in the presentation.
(Both can be downloaded as PDF from the yosys webpage.)


--snip-- only the lines below this mark are included in the yosys manual --snip--
Getting Started
===============


Outline of a Yosys command
--------------------------

Here is a the C++ code for a "hello_world" Yosys command (hello.cc):

	#include "kernel/yosys.h"

	USING_YOSYS_NAMESPACE
	PRIVATE_NAMESPACE_BEGIN

	struct HelloWorldPass : public Pass {
		HelloWorldPass() : Pass("hello_world") { }
		virtual void execute(vector<string>, Design*) {
			log("Hello World!\n");
		}
	} HelloWorldPass;

	PRIVATE_NAMESPACE_END

This can be built into a Yosys module using the following command:

	yosys-config --exec --cxx --cxxflags --ldflags -o hello.so -shared hello.cc --ldlibs

Or short:

	yosys-config --build hello.so hello.cc

And then executed using the following command:

	yosys -m hello.so -p hello_world


Yosys Data Structures
---------------------

Here is a short list of data structures that you should make yourself familiar
with before you write C++ code for Yosys. The following data structures are all
defined when "kernel/yosys.h" is included and USING_YOSYS_NAMESPACE is used.

  1. Yosys Container Classes

Yosys uses dict<K, T> and pool<T> as main container classes. dict<K, T> is
essentially a replacement for std::unordered_map<K, T> and pool<T> is a
replacement for std::unordered_set<T>. The main characteristics are:

	- dict<K, T> and pool<T> are about 2x faster than the std containers

	- references to elements in a dict<K, T> or pool<T> are invalidated by
	  insert and remove operations (similar to std::vector<T> on push_back()).

	- some iterators are invalidated by erase(). specifically, iterators
	  that have not passed the erased element yet are invalidated. (erase()
	  itself returns valid iterator to the next element.)

	- no iterators are invalidated by insert(). elements are inserted at
	  begin(). i.e. only a new iterator that starts at begin() will see the
	  inserted elements.

	- the method .count(key, iterator) is like .count(key) but only
	  considers elements that can be reached via the iterator.

	- iterators can be compared. it1 < it2 means that the position of t2
	  can be reached via t1 but not vice versa.

	- the method .sort() can be used to sort the elements in the container
	  the container stays sorted until elements are added or removed.

	- dict<K, T> and pool<T> will have the same order of iteration across
	  all compilers, standard libraries and architectures.

In addition to dict<K, T> and pool<T> there is also an idict<K> that
creates a bijective map from K to the integers. For example:

	idict<string, 42> si;
	log("%d\n", si("hello"));      // will print 42
	log("%d\n", si("world"));      // will print 43
	log("%d\n", si.at("world"));   // will print 43
	log("%d\n", si.at("dummy"));   // will throw exception
	log("%s\n", si[42].c_str()));  // will print hello
	log("%s\n", si[43].c_str()));  // will print world
	log("%s\n", si[44].c_str()));  // will throw exception

It is not possible to remove elements from an idict.

Finally mfp<K> implements a merge-find set data structure (aka. disjoint-set or
union-find) over the type K ("mfp" = merge-find-promote).

  2. Standard STL data types

In Yosys we use std::vector<T> and std::string whenever applicable. When
dict<K, T> and pool<T> are not suitable then std::map<K, T> and std::set<T>
are used instead.

The types std::vector<T> and std::string are also available as vector<T>
and string in the Yosys namespace.

  3. RTLIL objects

The current design (essentially a collection of modules, each defined by a
netlist) is stored in memory using RTLIL object (declared in kernel/rtlil.h,
automatically included by kernel/yosys.h). You should glance over at least
the declarations for the following types in kernel/rtlil.h:

	RTLIL::IdString
		This is a handle for an identifier (e.g. cell or wire name).
		It feels a lot like a std::string, but is only a single int
		in size. (The actual string is stored in a global lookup
		table.)

	RTLIL::SigBit
		A single signal bit. I.e. either a constant state (0, 1,
		x, z) or a single bit from a wire.

	RTLIL::SigSpec
		Essentially a vector of SigBits.

	RTLIL::Wire
	RTLIL::Cell
		The building blocks of the netlist in a module.

	RTLIL::Module
	RTLIL::Design
		The module is a container with connected cells and wires
		in it. The design is a container with modules in it.

All this types are also available without the RTLIL:: prefix in the Yosys
namespace.

  4. SigMap and other Helper Classes

There are a couple of additional helper classes that are in wide use
in Yosys. Most importantly there is SigMap (declared in kernel/sigtools.h).

When a design has many wires in it that are connected to each other, then a
single signal bit can have multiple valid names. The SigMap object can be used
to map SigSpecs or SigBits to unique SigSpecs and SigBits that consistently
only use one wire from such a group of connected wires. For example:

	SigBit a = module->addWire(NEW_ID);
	SigBit b = module->addWire(NEW_ID);
	module->connect(a, b);

	log("%d\n", a == b); // will print 0

	SigMap sigmap(module);
	log("%d\n", sigmap(a) == sigmap(b)); // will print 1


Using the RTLIL Netlist Format
------------------------------

In the RTLIL netlist format the cell ports contain SigSpecs that point to the
Wires. There are no references in the other direction. This has two direct
consequences:

(1) It is very easy to go from cells to wires but hard to go in the other way.

(2) There is no danger in removing cells from the netlists, but removing wires
can break the netlist format when there are still references to the wire
somewhere in the netlist.

The solution to (1) is easy: Create custom indexes that allow you to make fast
lookups for the wire-to-cell direction. You can either use existing generic
index structures to do that (such as the ModIndex class) or write your own
index. For many application it is simplest to construct a custom index. For
example:

	SigMap sigmap(module);
	dict<SigBit, Cell*> sigbit_to_driver_index;

	for (auto cell : module->cells())
		for (auto &conn : cell->connections())
			if (cell->output(conn.first))
				for (auto bit : sigmap(conn.second))
					sigbit_to_driver_index[bit] = cell;

Regarding (2): There is a general theme in Yosys that you don't remove wires
from the design. You can rename them, unconnect them, but you do not actually remove
the Wire object from the module. Instead you let the "clean" command take care
of the dangling wires. On the other hand it is safe to remove cells (as long as
you make sure this does not invalidate a custom index you are using in your code).


Example Code
------------

The following yosys commands are a good starting point if you are looking for examples
of how to use the Yosys API:

	manual/CHAPTER_Prog/stubnets.cc
	manual/PRESENTATION_Prog/my_cmd.cc


Notes on the existing codebase
------------------------------

For historical reasons not all parts of Yosys adhere to the current coding
style. When adding code to existing parts of the system, adhere to this guide
for the new code instead of trying to mimic the style of the surrounding code.



Coding Style
============


Formatting of code
------------------

- Yosys code is using tabs for indentation. Tabs are 8 characters.

- A continuation of a statement in the following line is indented by
  two additional tabs.

- Lines are as long as you want them to be. A good rule of thumb is
  to break lines at about column 150.

- Opening braces can be put on the same or next line as the statement
  opening the block (if, switch, for, while, do). Put the opening brace
  on its own line for larger blocks, especially blocks that contains
  blank lines.

- Otherwise stick to the Linux Kernel Coding Style:
    https://www.kernel.org/doc/Documentation/CodingStyle


C++ Language
-------------

Yosys is written in C++11. At the moment only constructs supported by
gcc 4.8 are allowed in Yosys code. This will change in future releases.

In general Yosys uses "int" instead of "size_t". To avoid compiler
warnings for implicit type casts, always use "GetSize(foobar)" instead
of "foobar.size()". (GetSize() is defined in kernel/yosys.h)

Use range-based for loops whenever applicable.


--snap-- only the lines above this mark are included in the yosys manual --snap--


Creating the Visual Studio Template Project
===========================================

1. Create an empty Visual C++ Win32 Console App project

	Microsoft Visual Studio Express 2013 for Windows Desktop
	Open New Project Wizard (File -> New Project..)

	Project Name: YosysVS
	Solution Name: YosysVS
	[X] Create directory for solution
	[ ] Add to source control

	[X] Console applications
	[X] Empty Project
	[ ] SDL checks

2. Open YosysVS Project Properties

	Select Configuration: All Configurations

	C/C++ -> General -> Additional Include Directories
		Add: ..\yosys

	C/C++ -> Preprocessor -> Preprocessor Definitions
		Add: _YOSYS_;_CRT_SECURE_NO_WARNINGS

3. Resulting file system tree:

	YosysVS/
	YosysVS/YosysVS
	YosysVS/YosysVS/YosysVS.vcxproj
	YosysVS/YosysVS/YosysVS.vcxproj.filters
	YosysVS/YosysVS.sdf
	YosysVS/YosysVS.sln
	YosysVS/YosysVS.v12.suo

4. Zip YosysVS as YosysVS-Tpl-v1.zip



Checklist for adding internal cell types
========================================

Things to do right away:

	- Add to kernel/celltypes.h (incl. eval() handling for non-mem cells)
	- Add to InternalCellChecker::check() in kernel/rtlil.cc
	- Add to techlibs/common/simlib.v
	- Add to techlibs/common/techmap.v

Things to do after finalizing the cell interface:

	- Add support to kernel/satgen.h for the new cell type
	- Add to manual/CHAPTER_CellLib.tex (or just add a fixme to the bottom)
	- Maybe add support to the Verilog backend for dumping such cells as expression



Checklist for creating Yosys releases
=====================================

Update the CHANGELOG file:

	cd ~yosys
	gitk &
	vi CHANGELOG


Update and check documentation:

	cd ~yosys
	make update-manual
	make manual
	- sanity check the figures in the appnotes and presentation
	    - if there are any odd things -> investigate
	    - make cosmetic changes to the .tex files if necessary

	cd ~yosys
	vi README CodingReadme
	- is the information provided in those file still up to date


Then with default config setting:

	cd ~yosys
	make vgtest

	cd ~yosys
	./yosys -p 'proc; show' tests/simple/fiedler-cooley.v
	./yosys -p 'proc; opt; show' tests/simple/fiedler-cooley.v
	./yosys -p 'synth; show' tests/simple/fiedler-cooley.v
	./yosys -p 'synth_xilinx -top up3down5; show' tests/simple/fiedler-cooley.v

	cd ~yosys/examples/cmos
	bash testbench.sh

	cd ~yosys/examples/basys3
	bash run.sh


Test building plugins with various of the standard passes:

	yosys-config --build test.so equiv_simple.cc
	- also check the code examples in CodingReadme


And if a version of the verific library is currently available:

	cd ~yosys
	cat frontends/verific/build_amd64.txt
	- follow instructions

	cd frontends/verific
	../../yosys test_navre.ys


Finally run all tests with "make config-{clang,gcc,gcc-4.8}":

	cd ~yosys
	make clean
	make test
	make vloghtb
	make install

	cd ~yosys-bigsim
	make clean
	make full

	cd ~vloghammer
	make purge gen_issues gen_samples
	make SYN_LIST="yosys" SIM_LIST="icarus yosim verilator" REPORT_FULL=1 world
	chromium-browser report.html


Release:

	- set YOSYS_VER to x.y.z in Makefile
	- update version string in CHANGELOG
	git commit -am "Yosys x.y.z"

	- push tag to github
	- post changelog on github
	- post short release note on reddit


Updating the website:

	cd ~yosys
	make manual
	make install

	- update pdf files on the website

	cd ~yosys-web
	make update_cmd
	make update_show
	git commit -am update
	make push