yosys/docs/source/appendix/APPNOTE_011_Design_Investigation.rst

==========================================
011: Interactive design investigation page
==========================================

Installation and prerequisites
==============================

This Application Note is based on the `Yosys GIT`_ `Rev. 2b90ba1`_ from
2013-12-08. The README file covers how to install Yosys. The ``show`` command
requires a working installation of `GraphViz`_ and `xdot` for generating the
actual circuit diagrams.

.. _Yosys GIT: https://github.com/YosysHQ/yosys

.. _Rev. 2b90ba1: https://github.com/YosysHQ/yosys/tree/2b90ba1

.. _GraphViz: http://www.graphviz.org/

.. _xdot: https://github.com/jrfonseca/xdot.py

Overview
========

This application note is structured as follows:

:ref:`intro_show` introduces the ``show`` command and explains the symbols used
in the circuit diagrams generated by it.

:ref:`navigate` introduces additional commands used to navigate in the design,
select portions of the design, and print additional information on the elements
in the design that are not contained in the circuit diagrams.

:ref:`poke` introduces commands to evaluate the design and solve SAT problems
within the design.

:ref:`conclusion` concludes the document and summarizes the key points.

.. _intro_show:

Introduction to the show command
================================

.. code-block:: console
   :caption: Yosys script with ``show`` commands and example design
   :name: example_src

   $ cat example.ys
   read_verilog example.v
   show -pause
   proc
   show -pause
   opt
   show -pause

   $ cat example.v
   module example(input clk, a, b, c,
                  output reg [1:0] y);
       always @(posedge clk)
           if (c)
               y <= c ? a + b : 2'd0;
   endmodule

.. figure:: ../../images/011/example_out.*
   :class: width-helper
   :name: example_out
   
   Output of the three ``show`` commands from :numref:`example_src`

The ``show`` command generates a circuit diagram for the design in its current
state. Various options can be used to change the appearance of the circuit
diagram, set the name and format for the output file, and so forth. When called
without any special options, it saves the circuit diagram in a temporary file
and launches ``xdot`` to display the diagram. Subsequent calls to show re-use the
``xdot`` instance (if still running).

A simple circuit
----------------

:numref:`example_src` shows a simple synthesis script and a Verilog file that
demonstrate the usage of show in a simple setting. Note that ``show`` is called with
the ``-pause`` option, that halts execution of the Yosys script until the user
presses the Enter key. The ``show -pause`` command also allows the user to enter
an interactive shell to further investigate the circuit before continuing
synthesis.

So this script, when executed, will show the design after each of the three
synthesis commands. The generated circuit diagrams are shown in
:numref:`example_out`.

The first diagram (from top to bottom) shows the design directly after being
read by the Verilog front-end. Input and output ports are displayed as octagonal
shapes. Cells are displayed as rectangles with inputs on the left and outputs on
the right side. The cell labels are two lines long: The first line contains a
unique identifier for the cell and the second line contains the cell type.
Internal cell types are prefixed with a dollar sign. The Yosys manual contains a
chapter on the internal cell library used in Yosys.

Constants are shown as ellipses with the constant value as label. The syntax
``<bit_width>'<bits>`` is used for for constants that are not 32-bit wide and/or
contain bits that are not 0 or 1 (i.e. ``x`` or ``z``). Ordinary 32-bit
constants are written using decimal numbers.

Single-bit signals are shown as thin arrows pointing from the driver to the
load. Signals that are multiple bits wide are shown as think arrows.

Finally *processes* are shown in boxes with round corners. Processes are Yosys'
internal representation of the decision-trees and synchronization events
modelled in a Verilog ``always``-block. The label reads ``PROC`` followed by a
unique identifier in the first line and contains the source code location of the
original ``always``-block in the 2nd line. Note how the multiplexer from the
``?:``-expression is represented as a ``$mux`` cell but the multiplexer from the
``if``-statement is yet still hidden within the process.

The ``proc`` command transforms the process from the first diagram into a
multiplexer and a d-type flip-flip, which brings us to the 2nd diagram.

The Rhombus shape to the right is a dangling wire. (Wire nodes are only shown if
they are dangling or have "public" names, for example names assigned from the
Verilog input.) Also note that the design now contains two instances of a
``BUF``-node. This are artefacts left behind by the ``proc``-command. It is
quite usual to see such artefacts after calling commands that perform changes in
the design, as most commands only care about doing the transformation in the
least complicated way, not about cleaning up after them. The next call to
``clean`` (or ``opt``, which includes ``clean`` as one of its operations) will
clean up this artefacts. This operation is so common in Yosys scripts that it
can simply be abbreviated with the ``;;`` token, which doubles as separator for
commands. Unless one wants to specifically analyze this artefacts left behind
some operations, it is therefore recommended to always call ``clean`` before
calling ``show``.

In this script we directly call ``opt`` as next step, which finally leads us to
the 3rd diagram in :numref:`example_out`. Here we see that the ``opt`` command
not only has removed the artifacts left behind by ``proc``, but also determined
correctly that it can remove the first ``$mux`` cell without changing the
behavior of the circuit.

.. figure:: ../../images/011/splice.*
   :class: width-helper
   :name: splice_dia

   Output of ``yosys -p 'proc; opt; show' splice.v``

.. literalinclude:: ../APPNOTE_011_Design_Investigation/splice.v
   :caption: ``splice.v``
   :name: splice_src

.. figure:: ../../images/011/splitnets_libfile.*
   :class: width-helper
   :name: splitnets_libfile

   Effects of ``splitnets`` command and of providing a cell library. (The
   circuit is a half-adder built from simple CMOS gates.)

Break-out boxes for signal vectors
----------------------------------

As has been indicated by the last example, Yosys is can manage signal vectors
(aka. multi-bit wires or buses) as native objects. This provides great
advantages when analyzing circuits that operate on wide integers. But it also
introduces some additional complexity when the individual bits of of a signal
vector are accessed. The example ``show`` in :numref:`splice_src` demonstrates
how such circuits are visualized by the ``show`` command.

The key elements in understanding this circuit diagram are of course the boxes
with round corners and rows labeled ``<MSB_LEFT>:<LSB_LEFT> -
<MSB_RIGHT>:<LSB_RIGHT>``. Each of this boxes has one signal per row on one side
and a common signal for all rows on the other side. The ``<MSB>:<LSB>`` tuples
specify which bits of the signals are broken out and connected. So the top row
of the box connecting the signals ``a`` and ``x`` indicates that the bit 0 (i.e.
the range 0:0) from signal ``a`` is connected to bit 1 (i.e. the range 1:1) of
signal ``x``.

Lines connecting such boxes together and lines connecting such boxes to
cell ports have a slightly different look to emphasise that they are not
actual signal wires but a necessity of the graphical representation.
This distinction seems like a technicality, until one wants to debug a
problem related to the way Yosys internally represents signal vectors,
for example when writing custom Yosys commands.

Gate level netlists
-------------------

Finally :numref:`splitnets_libfile` shows two common pitfalls when working with
designs mapped to a cell library. The top figure has two problems: First Yosys
did not have access to the cell library when this diagram was generated,
resulting in all cell ports defaulting to being inputs. This is why all ports
are drawn on the left side the cells are awkwardly arranged in a large column.
Secondly the two-bit vector ``y`` requires breakout-boxes for its individual
bits, resulting in an unnecessary complex diagram.

For the 2nd diagram Yosys has been given a description of the cell library as
Verilog file containing blackbox modules. There are two ways to load cell
descriptions into Yosys: First the Verilog file for the cell library can be
passed directly to the ``show`` command using the ``-lib <filename>`` option.
Secondly it is possible to load cell libraries into the design with the
``read_verilog -lib <filename>`` command. The 2nd method has the great advantage
that the library only needs to be loaded once and can then be used in all
subsequent calls to the ``show`` command.

In addition to that, the 2nd diagram was generated after ``splitnet -ports`` was
run on the design. This command splits all signal vectors into individual signal
bits, which is often desirable when looking at gate-level circuits. The
``-ports`` option is required to also split module ports. Per default the
command only operates on interior signals.

Miscellaneous notes
-------------------

Per default the ``show`` command outputs a temporary dot file and launches
``xdot`` to display it. The options ``-format``, ``-viewer`` and ``-prefix`` can
be used to change format, viewer and filename prefix. Note that the ``pdf`` and
``ps`` format are the only formats that support plotting multiple modules in one
run.

In densely connected circuits it is sometimes hard to keep track of the
individual signal wires. For this cases it can be useful to call ``show`` with
the ``-colors <integer>`` argument, which randomly assigns colors to the nets.
The integer (> 0) is used as seed value for the random color assignments.
Sometimes it is necessary it try some values to find an assignment of colors
that looks good.

The command ``help show`` prints a complete listing of all options supported by
the ``show`` command.

.. _navigate:

Navigating the design
=====================

Plotting circuit diagrams for entire modules in the design brings us
only helps in simple cases. For complex modules the generated circuit
diagrams are just stupidly big and are no help at all. In such cases one
first has to select the relevant portions of the circuit.

In addition to *what* to display one also needs to carefully decide *when*
to display it, with respect to the synthesis flow. In general it is a
good idea to troubleshoot a circuit in the earliest state in which a
problem can be reproduced. So if, for example, the internal state before
calling the ``techmap`` command already fails to verify, it is better to
troubleshoot the coarse-grain version of the circuit before ``techmap`` than
the gate-level circuit after ``techmap``.

.. Note:: It is generally recommended to verify the internal state of a
   design by writing it to a Verilog file using ``write_verilog -noexpr``
   and using the simulation models from ``simlib.v`` and ``simcells.v`` 
   from the Yosys data directory (as printed by ``yosys-config --datdir``).

Interactive navigation
----------------------

.. code-block:: none
   :caption: Demonstration of ``ls`` and ``cd`` using ``example.v`` from :numref:`example_src`
   :name: lscd

   yosys> ls

   1 modules:
     example

   yosys> cd example

   yosys [example]> ls

   7 wires:
     $0\y[1:0]
     $add$example.v:5$2_Y
     a
     b
     c
     clk
     y

   3 cells:
     $add$example.v:5$2
     $procdff$7
     $procmux$5

.. code-block:: RTLIL
   :caption: Output of ``dump $2`` using the design from :numref:`example_src` 
             and :numref:`example_out`
   :name: dump2

     attribute \src "example.v:5"
     cell $add $add$example.v:5$2
       parameter \A_SIGNED 0
       parameter \A_WIDTH 1
       parameter \B_SIGNED 0
       parameter \B_WIDTH 1
       parameter \Y_WIDTH 2
       connect \A \a
       connect \B \b
       connect \Y $add$example.v:5$2_Y
     end

Once the right state within the synthesis flow for debugging the circuit has
been identified, it is recommended to simply add the ``shell`` command to the
matching place in the synthesis script. This command will stop the synthesis at
the specified moment and go to shell mode, where the user can interactively
enter commands.

For most cases, the shell will start with the whole design selected (i.e. when
the synthesis script does not already narrow the selection). The command ``ls``
can now be used to create a list of all modules. The command ``cd`` can be used
to switch to one of the modules (type ``cd ..`` to switch back). Now the `ls`
command lists the objects within that module. :numref:`lscd` demonstrates this
using the design from :numref:`example_src`.

There is a thing to note in :numref:`lscd`: We can see that the cell names from
:numref:`example_out` are just abbreviations of the actual cell names, namely
the part after the last dollar-sign. Most auto-generated names (the ones
starting with a dollar sign) are rather long and contains some additional
information on the origin of the named object. But in most cases those names can
simply be abbreviated using the last part.

Usually all interactive work is done with one module selected using the ``cd``
command. But it is also possible to work from the design-context (``cd ..``). In
this case all object names must be prefixed with ``<module_name>/``. For example
``a*/b*`` would refer to all objects whose names start with ``b`` from all
modules whose names start with ``a``.

The ``dump`` command can be used to print all information about an object. For
example ``dump $2`` will print :numref:`dump2`. This can for example be useful
to determine the names of nets connected to cells, as the net-names are usually
suppressed in the circuit diagram if they are auto-generated.

For the remainder of this document we will assume that the commands are
run from module-context and not design-context.

Working with selections
-----------------------

.. figure:: ../../images/011/example_03.*
   :class: width-helper
   :name: seladd

   Output of ``show`` after ``select $2`` or ``select t:$add`` (see also
   :numref:`example_out`)

When a module is selected using the ``cd`` command, all commands (with a few
exceptions, such as the ``read_`` and ``write_`` commands) operate only on the
selected module. This can also be useful for synthesis scripts where different
synthesis strategies should be applied to different modules in the design.

But for most interactive work we want to further narrow the set of
selected objects. This can be done using the ``select`` command.

For example, if the command ``select $2`` is executed, a subsequent ``show``
command will yield the diagram shown in :numref:`seladd`. Note that the nets are
now displayed in ellipses. This indicates that they are not selected, but only
shown because the diagram contains a cell that is connected to the net. This of
course makes no difference for the circuit that is shown, but it can be a useful
information when manipulating selections.

Objects can not only be selected by their name but also by other properties. For
example ``select t:$add`` will select all cells of type ``$add``. In this case
this is also yields the diagram shown in :numref:`seladd`.

.. literalinclude:: ../APPNOTE_011_Design_Investigation/foobaraddsub.v
   :caption: Test module for operations on selections
   :name: foobaraddsub
   :language: verilog

The output of ``help select`` contains a complete syntax reference for
matching different properties.

Many commands can operate on explicit selections. For example the command ``dump
t:$add`` will print information on all ``$add`` cells in the active module.
Whenever a command has ``[selection]`` as last argument in its usage help, this
means that it will use the engine behind the ``select`` command to evaluate
additional arguments and use the resulting selection instead of the selection
created by the last ``select`` command.

Normally the ``select`` command overwrites a previous selection. The commands
``select -add`` and ``select -del`` can be used to add or remove objects from
the current selection.

The command ``select -clear`` can be used to reset the selection to the default,
which is a complete selection of everything in the current module.

Operations on selections
------------------------

.. literalinclude:: ../APPNOTE_011_Design_Investigation/sumprod.v
   :caption: Another test module for operations on selections
   :name: sumprod
   :language: verilog

.. figure:: ../../images/011/sumprod_00.*
   :class: width-helper
   :name: sumprod_00

   Output of ``show a:sumstuff`` on :numref:`sumprod`

The ``select`` command is actually much more powerful than it might seem on the
first glimpse. When it is called with multiple arguments, each argument is
evaluated and pushed separately on a stack. After all arguments have been
processed it simply creates the union of all elements on the stack. So the
following command will select all ``$add`` cells and all objects with the
``foo`` attribute set:

.. code-block:: yoscrypt

   select t:$add a:foo

(Try this with the design shown in :numref:`foobaraddsub`. Use the ``select
-list`` command to list the current selection.)

In many cases simply adding more and more stuff to the selection is an
ineffective way of selecting the interesting part of the design. Special
arguments can be used to combine the elements on the stack. For example
the ``%i`` arguments pops the last two elements from the stack, intersects
them, and pushes the result back on the stack. So the following command
will select all ``$add ``cells that have the ``foo`` attribute set:

.. code-block:: yoscrypt

   select t:$add a:foo %i

The listing in :numref:`sumprod` uses the Yosys non-standard ``{... *}`` syntax
to set the attribute ``sumstuff`` on all cells generated by the first assign
statement. (This works on arbitrary large blocks of Verilog code an can be used
to mark portions of code for analysis.)

Selecting ``a:sumstuff`` in this module will yield the circuit diagram shown in
:numref:`sumprod_00`. As only the cells themselves are selected, but not the
temporary wire ``$1_Y``, the two adders are shown as two disjunct parts. This
can be very useful for global signals like clock and reset signals: just
unselect them using a command such as ``select -del clk rst`` and each cell
using them will get its own net label.

In this case however we would like to see the cells connected properly. This can
be achieved using the ``%x`` action, that broadens the selection, i.e. for each
selected wire it selects all cells connected to the wire and vice versa. So
``show a:sumstuff %x`` yields the diagram shown in :numref:`sumprod_01`.

.. figure:: ../../images/011/sumprod_01.*
   :class: width-helper
   :name: sumprod_01

   Output of ``show a:sumstuff %x`` on :numref:`sumprod`

Selecting logic cones
---------------------

:numref:`sumprod_01` shows what is called the ``input cone`` of ``sum``, i.e.
all cells and signals that are used to generate the signal ``sum``. The ``%ci``
action can be used to select the input cones of all object in the top selection
in the stack maintained by the ``select`` command.

As the ``%x`` action, this commands broadens the selection by one "step".
But this time the operation only works against the direction of data
flow. That means, wires only select cells via output ports and cells
only select wires via input ports.

:numref:`select_prod` show the sequence of diagrams generated by the following
commands:

.. code-block:: yoscrypt

   show prod
   show prod %ci
   show prod %ci %ci
   show prod %ci %ci %ci

When selecting many levels of logic, repeating ``%ci`` over and over again can
be a bit dull. So there is a shortcut for that: the number of iterations can be
appended to the action. So for example the action ``%ci3`` is identical to
performing the ``%ci`` action three times.

The action ``%ci*`` performs the ``%ci`` action over and over again until it
has no effect anymore.

.. figure:: ../../images/011/select_prod.*
   :class: width-helper
   :name: select_prod
   
   Objects selected by ``select prod %ci...``

In most cases there are certain cell types and/or ports that should not be
considered for the ``%ci`` action, or we only want to follow certain cell types
and/or ports. This can be achieved using additional patterns that can be
appended to the ``%ci`` action.

Lets consider the design from :numref:`memdemo_src`. It serves no purpose other
than being a non-trivial circuit for demonstrating some of the advanced Yosys
features. We synthesize the circuit using ``proc; opt; memory; opt`` and change
to the ``memdemo`` module with ``cd memdemo``. If we type ``show`` now we see
the diagram shown in :numref:`memdemo_00`.

.. literalinclude:: ../APPNOTE_011_Design_Investigation/memdemo.v
   :caption: Demo circuit for demonstrating some advanced Yosys features
   :name: memdemo_src
   :language: verilog

.. figure:: ../../images/011/memdemo_00.*
   :class: width-helper
   :name: memdemo_00
   
   Complete circuit diagram for the design shown in :numref:`memdemo_src`

But maybe we are only interested in the tree of multiplexers that select the
output value. In order to get there, we would start by just showing the output
signal and its immediate predecessors:

.. code-block:: yoscrypt

   show y %ci2

From this we would learn that ``y`` is driven by a ``$dff cell``, that ``y`` is
connected to the output port ``Q``, that the ``clk`` signal goes into the
``CLK`` input port of the cell, and that the data comes from a auto-generated
wire into the input ``D`` of the flip-flop cell.

As we are not interested in the clock signal we add an additional pattern to the
``%ci`` action, that tells it to only follow ports ``Q`` and ``D`` of ``$dff``
cells:

.. code-block:: yoscrypt

   show y %ci2:+$dff[Q,D]

To add a pattern we add a colon followed by the pattern to the ``%ci`` action.
The pattern it self starts with ``-`` or ``+``, indicating if it is an include
or exclude pattern, followed by an optional comma separated list of cell types,
followed by an optional comma separated list of port names in square brackets.

Since we know that the only cell considered in this case is a ``$dff`` cell,
we could as well only specify the port names:

.. code-block:: yoscrypt

   show y %ci2:+[Q,D]

Or we could decide to tell the ``%ci`` action to not follow the ``CLK`` input:

.. code-block:: yoscrypt

   show y %ci2:-[CLK]

.. figure:: ../../images/011/memdemo_01.*
   :class: width-helper
   :name: memdemo_01
   
   Output of ``show y %ci2:+$dff[Q,D] %ci*:-$mux[S]:-$dff``

Next we would investigate the next logic level by adding another ``%ci2`` to
the command:

.. code-block:: yoscrypt

   show y %ci2:-[CLK] %ci2

From this we would learn that the next cell is a ``$mux`` cell and we would
add additional pattern to narrow the selection on the path we are
interested. In the end we would end up with a command such as

.. code-block:: yoscrypt

   show y %ci2:+$dff[Q,D] %ci*:-$mux[S]:-$dff

in which the first ``%ci`` jumps over the initial d-type flip-flop and the 2nd
action selects the entire input cone without going over multiplexer select
inputs and flip-flop cells. The diagram produces by this command is shown in
:numref:`memdemo_01`.

Similar to ``%ci`` exists an action ``%co`` to select output cones that accepts
the same syntax for pattern and repetition. The ``%x`` action mentioned
previously also accepts this advanced syntax.

This actions for traversing the circuit graph, combined with the actions for
boolean operations such as intersection (``%i``) and difference (``%d``) are
powerful tools for extracting the relevant portions of the circuit under
investigation.

See ``help select`` for a complete list of actions available in selections.

Storing and recalling selections
--------------------------------

The current selection can be stored in memory with the command ``select -set
<name>``. It can later be recalled using ``select @<name>``. In fact, the
``@<name>`` expression pushes the stored selection on the stack maintained by
the ``select`` command. So for example

.. code-block:: yoscrypt

   select @foo @bar %i

will select the intersection between the stored selections ``foo`` and ``bar``.

In larger investigation efforts it is highly recommended to maintain a
script that sets up relevant selections, so they can easily be recalled,
for example when Yosys needs to be re-run after a design or source code
change.

The ``history`` command can be used to list all recent interactive commands.
This feature can be useful for creating such a script from the commands
used in an interactive session.

.. _poke:

Advanced investigation techniques
=================================

When working with very large modules, it is often not enough to just select the
interesting part of the module. Instead it can be useful to extract the
interesting part of the circuit into a separate module. This can for example be
useful if one wants to run a series of synthesis commands on the critical part
of the module and wants to carefully read all the debug output created by the
commands in order to spot a problem. This kind of troubleshooting is much easier
if the circuit under investigation is encapsulated in a separate module.

:numref:`submod` shows how the ``submod`` command can be used to split the
circuit from :numref:`memdemo_src` and :numref:`memdemo_00` into its components.
The ``-name`` option is used to specify the name of the new module and also the
name of the new cell in the current module.

.. figure:: ../../images/011/submod_dots.*
   :class: width-helper
   :name: submod_dots

.. code-block:: yoscrypt
   :caption: The circuit from :numref:`memdemo_src` and :numref:`memdemo_00` 
             broken up using ``submod``
   :name: submod

   select -set outstage y %ci2:+$dff[Q,D] %ci*:-$mux[S]:-$dff
   select -set selstage y %ci2:+$dff[Q,D] %ci*:-$dff @outstage %d
   select -set scramble mem* %ci2 %ci*:-$dff mem* %d @selstage %d
   submod -name scramble @scramble
   submod -name outstage @outstage
   submod -name selstage @selstage

Evaluation of combinatorial circuits
------------------------------------

The ``eval`` command can be used to evaluate combinatorial circuits. For example
(see :numref:`submod` for the circuit diagram of ``selstage``):

::

      yosys [selstage]> eval -set s2,s1 4'b1001 -set d 4'hc -show n2 -show n1

      1. Executing EVAL pass (evaluate the circuit given an input).
      Full command line: eval -set s2,s1 4'b1001 -set d 4'hc -show n2 -show n1
      Eval result: \n2 = 2'10.
      Eval result: \n1 = 2'10.

So the ``-set`` option is used to set input values and the ``-show`` option is
used to specify the nets to evaluate. If no ``-show`` option is specified, all
selected output ports are used per default.

If a necessary input value is not given, an error is produced. The option
``-set-undef`` can be used to instead set all unspecified input nets to undef
(``x``).

The ``-table`` option can be used to create a truth table. For example:

::

      yosys [selstage]> eval -set-undef -set d[3:1] 0 -table s1,d[0]

      10. Executing EVAL pass (evaluate the circuit given an input).
      Full command line: eval -set-undef -set d[3:1] 0 -table s1,d[0]

        \s1 \d [0] |  \n1  \n2
       ---- ------ | ---- ----
       2'00    1'0 | 2'00 2'00
       2'00    1'1 | 2'xx 2'00
       2'01    1'0 | 2'00 2'00
       2'01    1'1 | 2'xx 2'01
       2'10    1'0 | 2'00 2'00
       2'10    1'1 | 2'xx 2'10
       2'11    1'0 | 2'00 2'00
       2'11    1'1 | 2'xx 2'11

      Assumed undef (x) value for the following signals: \s2

Note that the ``eval`` command (as well as the ``sat`` command discussed in the
next sections) does only operate on flattened modules. It can not analyze
signals that are passed through design hierarchy levels. So the ``flatten``
command must be used on modules that instantiate other modules before this
commands can be applied.

Solving combinatorial SAT problems
----------------------------------

.. literalinclude:: ../APPNOTE_011_Design_Investigation/primetest.v
   :language: verilog
   :caption: A simple miter circuit for testing if a number is prime. But it has
             a problem (see main text and :numref:`primesat`).
   :name: primetest

.. code-block::
   :caption: Experiments with the miter circuit from :numref:`primetest`. 
             The first attempt of proving that 31 is prime failed because the 
             SAT solver found a creative way of factorizing 31 using integer 
             overflow.
   :name: primesat

   yosys [primetest]> sat -prove ok 1 -set p 31

   8. Executing SAT pass (solving SAT problems in the circuit).
   Full command line: sat -prove ok 1 -set p 31

   Setting up SAT problem:
   Import set-constraint: \p = 16'0000000000011111
   Final constraint equation: \p = 16'0000000000011111
   Imported 6 cells to SAT database.
   Import proof-constraint: \ok = 1'1
   Final proof equation: \ok = 1'1

   Solving problem with 2790 variables and 8241 clauses..
   SAT proof finished - model found: FAIL!

      ______                   ___       ___       _ _            _ _
     (_____ \                 / __)     / __)     (_) |          | | |
      _____) )___ ___   ___ _| |__    _| |__ _____ _| | _____  __| | |
     |  ____/ ___) _ \ / _ (_   __)  (_   __|____ | | || ___ |/ _  |_|
     | |   | |  | |_| | |_| || |       | |  / ___ | | || ____( (_| |_
     |_|   |_|   \___/ \___/ |_|       |_|  \_____|_|\_)_____)\____|_|


     Signal Name                 Dec        Hex                   Bin
     -------------------- ---------- ---------- ---------------------
     \a                        15029       3ab5      0011101010110101
     \b                         4099       1003      0001000000000011
     \ok                           0          0                     0
     \p                           31         1f      0000000000011111

   yosys [primetest]> sat -prove ok 1 -set p 31 -set a[15:8],b[15:8] 0

   9. Executing SAT pass (solving SAT problems in the circuit).
   Full command line: sat -prove ok 1 -set p 31 -set a[15:8],b[15:8] 0

   Setting up SAT problem:
   Import set-constraint: \p = 16'0000000000011111
   Import set-constraint: { \a [15:8] \b [15:8] } = 16'0000000000000000
   Final constraint equation: { \a [15:8] \b [15:8] \p } = { 16'0000000000000000 16'0000000000011111 }
   Imported 6 cells to SAT database.
   Import proof-constraint: \ok = 1'1
   Final proof equation: \ok = 1'1

   Solving problem with 2790 variables and 8257 clauses..
   SAT proof finished - no model found: SUCCESS!

                     /$$$$$$      /$$$$$$$$     /$$$$$$$
                    /$$__  $$    | $$_____/    | $$__  $$
                   | $$  \ $$    | $$          | $$  \ $$
                   | $$  | $$    | $$$$$       | $$  | $$
                   | $$  | $$    | $$__/       | $$  | $$
                   | $$/$$ $$    | $$          | $$  | $$
                   |  $$$$$$/ /$$| $$$$$$$$ /$$| $$$$$$$//$$
                    \____ $$$|__/|________/|__/|_______/|__/
                          \__/

Often the opposite of the ``eval`` command is needed, i.e. the circuits output
is given and we want to find the matching input signals. For small circuits with
only a few input bits this can be accomplished by trying all possible input
combinations, as it is done by the ``eval -table`` command. For larger circuits
however, Yosys provides the ``sat`` command that uses a `SAT`_ solver,
`MiniSAT`_, to solve this kind of problems.

.. _SAT: http://en.wikipedia.org/wiki/Circuit_satisfiability

.. _MiniSAT: http://minisat.se/

The ``sat`` command works very similar to the ``eval`` command. The main
difference is that it is now also possible to set output values and find the
corresponding input values. For Example:

::

      yosys [selstage]> sat -show s1,s2,d -set s1 s2 -set n2,n1 4'b1001

      11. Executing SAT pass (solving SAT problems in the circuit).
      Full command line: sat -show s1,s2,d -set s1 s2 -set n2,n1 4'b1001

      Setting up SAT problem:
      Import set-constraint: \s1 = \s2
      Import set-constraint: { \n2 \n1 } = 4'1001
      Final constraint equation: { \n2 \n1 \s1 } = { 4'1001 \s2 }
      Imported 3 cells to SAT database.
      Import show expression: { \s1 \s2 \d }

      Solving problem with 81 variables and 207 clauses..
      SAT solving finished - model found:

        Signal Name                 Dec        Hex             Bin
        -------------------- ---------- ---------- ---------------
        \d                            9          9            1001
        \s1                           0          0              00
        \s2                           0          0              00

Note that the ``sat`` command supports signal names in both arguments to the
``-set`` option. In the above example we used ``-set s1 s2`` to constraint
``s1`` and ``s2`` to be equal. When more complex constraints are needed, a
wrapper circuit must be constructed that checks the constraints and signals if
the constraint was met using an extra output port, which then can be forced to a
value using the ``-set`` option. (Such a circuit that contains the circuit under
test plus additional constraint checking circuitry is called a ``miter``
circuit.)

:numref:`primetest` shows a miter circuit that is supposed to be used as a prime
number test. If ``ok`` is 1 for all input values ``a`` and ``b`` for a given
``p``, then ``p`` is prime, or at least that is the idea.

The Yosys shell session shown in :numref:`primesat` demonstrates that SAT
solvers can even find the unexpected solutions to a problem: Using integer
overflow there actually is a way of "factorizing" 31. The clean solution would
of course be to perform the test in 32 bits, for example by replacing ``p !=
a*b`` in the miter with ``p != {16'd0,a}b``, or by using a temporary variable
for the 32 bit product ``a*b``. But as 31 fits well into 8 bits (and as the
purpose of this document is to show off Yosys features) we can also simply force
the upper 8 bits of ``a`` and ``b`` to zero for the ``sat`` call, as is done in
the second command in :numref:`primesat` (line 31).

The ``-prove`` option used in this example works similar to ``-set``, but tries
to find a case in which the two arguments are not equal. If such a case is not
found, the property is proven to hold for all inputs that satisfy the other
constraints.

It might be worth noting, that SAT solvers are not particularly efficient at
factorizing large numbers. But if a small factorization problem occurs as part
of a larger circuit problem, the Yosys SAT solver is perfectly capable of
solving it.

Solving sequential SAT problems
-------------------------------

.. code-block::
   :caption: Solving a sequential SAT problem in the ``memdemo`` module from :numref:`memdemo_src`.
   :name: memdemo_sat

   yosys [memdemo]> sat -seq 6 -show y -show d -set-init-undef \
       -max_undef -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3

   6. Executing SAT pass (solving SAT problems in the circuit).
   Full command line: sat -seq 6 -show y -show d -set-init-undef
       -max_undef -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3

   Setting up time step 1:
   Final constraint equation: { } = { }
   Imported 29 cells to SAT database.

   Setting up time step 2:
   Final constraint equation: { } = { }
   Imported 29 cells to SAT database.

   Setting up time step 3:
   Final constraint equation: { } = { }
   Imported 29 cells to SAT database.

   Setting up time step 4:
   Import set-constraint for timestep: \y = 4'0001
   Final constraint equation: \y = 4'0001
   Imported 29 cells to SAT database.

   Setting up time step 5:
   Import set-constraint for timestep: \y = 4'0010
   Final constraint equation: \y = 4'0010
   Imported 29 cells to SAT database.

   Setting up time step 6:
   Import set-constraint for timestep: \y = 4'0011
   Final constraint equation: \y = 4'0011
   Imported 29 cells to SAT database.

   Setting up initial state:
   Final constraint equation: { \y \s2 \s1 \mem[3] \mem[2] \mem[1]
               \mem[0] } = 24'xxxxxxxxxxxxxxxxxxxxxxxx

   Import show expression: \y
   Import show expression: \d

   Solving problem with 10322 variables and 27881 clauses..
   SAT model found. maximizing number of undefs.
   SAT solving finished - model found:

     Time Signal Name                 Dec        Hex             Bin
     ---- -------------------- ---------- ---------- ---------------
     init \mem[0]                      --         --            xxxx
     init \mem[1]                      --         --            xxxx
     init \mem[2]                      --         --            xxxx
     init \mem[3]                      --         --            xxxx
     init \s1                          --         --              xx
     init \s2                          --         --              xx
     init \y                           --         --            xxxx
     ---- -------------------- ---------- ---------- ---------------
        1 \d                            0          0            0000
        1 \y                           --         --            xxxx
     ---- -------------------- ---------- ---------- ---------------
        2 \d                            1          1            0001
        2 \y                           --         --            xxxx
     ---- -------------------- ---------- ---------- ---------------
        3 \d                            2          2            0010
        3 \y                            0          0            0000
     ---- -------------------- ---------- ---------- ---------------
        4 \d                            3          3            0011
        4 \y                            1          1            0001
     ---- -------------------- ---------- ---------- ---------------
        5 \d                           --         --            001x
        5 \y                            2          2            0010
     ---- -------------------- ---------- ---------- ---------------
        6 \d                           --         --            xxxx
        6 \y                            3          3            0011

The SAT solver functionality in Yosys can not only be used to solve
combinatorial problems, but can also solve sequential problems. Let's consider
the entire memdemo module from :numref:`memdemo_src` and suppose we want to know
which sequence of input values for ``d`` will cause the output y to produce the
sequence 1, 2, 3 from any initial state. :numref:`memdemo_sat` show the solution
to this question, as produced by the following command:

.. code-block:: yoscrypt

      sat -seq 6 -show y -show d -set-init-undef \
        -max_undef -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3

The ``-seq 6`` option instructs the ``sat`` command to solve a sequential
problem in 6 time steps. (Experiments with lower number of steps have show that
at least 3 cycles are necessary to bring the circuit in a state from which the
sequence 1, 2, 3 can be produced.)

The ``-set-init-undef`` option tells the ``sat`` command to initialize all
registers to the undef (``x``) state. The way the ``x`` state is treated in
Verilog will ensure that the solution will work for any initial state.

The ``-max_undef`` option instructs the ``sat`` command to find a solution with
a maximum number of undefs. This way we can see clearly which inputs bits are
relevant to the solution.

Finally the three ``-set-at`` options add constraints for the ``y`` signal to
play the 1, 2, 3 sequence, starting with time step 4.

It is not surprising that the solution sets ``d = 0`` in the first step, as this
is the only way of setting the ``s1`` and ``s2`` registers to a known value. The
input values for the other steps are a bit harder to work out manually, but the
SAT solver finds the correct solution in an instant.

There is much more to write about the ``sat`` command. For example, there is a
set of options that can be used to performs sequential proofs using temporal
induction :cite:p:`een2003temporal`. The command ``help sat`` can be used to
print a list of all options with short descriptions of their functions.

.. _conclusion:

Conclusion
==========

Yosys provides a wide range of functions to analyze and investigate
designs. For many cases it is sufficient to simply display circuit
diagrams, maybe use some additional commands to narrow the scope of the
circuit diagrams to the interesting parts of the circuit. But some cases
require more than that. For this applications Yosys provides commands
that can be used to further inspect the behavior of the circuit, either
by evaluating which output values are generated from certain input
values (``eval``) or by evaluation which input values and initial conditions
can result in a certain behavior at the outputs (``sat``). The SAT command
can even be used to prove (or disprove) theorems regarding the circuit,
in more advanced cases with the additional help of a miter circuit.

This features can be powerful tools for the circuit designer using Yosys
as a utility for building circuits and the software developer using
Yosys as a framework for new algorithms alike.