yosys/docs/source/using_yosys/yosys_flows.rst

Flows, command types, and order
===============================

Command order
-------------

.. todo:: More surrounding text (esp as it relates to command order)

Intro to coarse-grain synthesis
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In coarse-grain synthesis the target architecture has cells of the same
complexity or larger complexity than the internal RTL representation.

For example:

.. code:: verilog

	wire [15:0] a, b;
	wire [31:0] c, y;
	assign y = a * b + c;

This circuit contains two cells in the RTL representation: one multiplier and
one adder. In some architectures this circuit can be implemented using
a single circuit element, for example an FPGA DSP core. Coarse grain synthesis
is this mapping of groups of circuit elements to larger components.

Fine-grain synthesis would be matching the circuit elements to smaller
components, such as LUTs, gates, or half- and full-adders.

The extract pass
~~~~~~~~~~~~~~~~

- Like the :cmd:ref:`techmap` pass, the :cmd:ref:`extract` pass is called with a
  map file. It compares the circuits inside the modules of the map file with the
  design and looks for sub-circuits in the design that match any of the modules
  in the map file.
- If a match is found, the :cmd:ref:`extract` pass will replace the matching
  subcircuit with an instance of the module from the map file.
- In a way the :cmd:ref:`extract` pass is the inverse of the techmap pass.

.. todo:: add/expand supporting text, also mention custom pattern matching and
   pmgen

Example code can be found in ``docs/source/code_examples/macc/``.

.. literalinclude:: /code_examples/macc/macc_simple_test.ys
    :language: yoscrypt
    :lines: 1-2

.. figure:: /_images/code_examples/macc/macc_simple_test_00a.*
    :class: width-helper
    
    before :cmd:ref:`extract`

.. literalinclude:: /code_examples/macc/macc_simple_test.ys
    :language: yoscrypt
    :lines: 6

.. figure:: /_images/code_examples/macc/macc_simple_test_00b.*
    :class: width-helper
    
    after :cmd:ref:`extract`

.. literalinclude:: /code_examples/macc/macc_simple_test.v
   :language: verilog
   :caption: ``macc_simple_test.v``

.. literalinclude:: /code_examples/macc/macc_simple_xmap.v
   :language: verilog
   :caption: ``macc_simple_xmap.v``

.. literalinclude:: /code_examples/macc/macc_simple_test_01.v
   :language: verilog
   :caption: ``macc_simple_test_01.v``

.. figure:: /_images/code_examples/macc/macc_simple_test_01a.*
    :class: width-helper

.. figure:: /_images/code_examples/macc/macc_simple_test_01b.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_simple_test_02.v
   :language: verilog
   :caption: ``macc_simple_test_02.v``

.. figure:: /_images/code_examples/macc/macc_simple_test_02a.*
    :class: width-helper

.. figure:: /_images/code_examples/macc/macc_simple_test_02b.*
    :class: width-helper

The wrap-extract-unwrap method
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Often a coarse-grain element has a constant bit-width, but can be used to
implement operations with a smaller bit-width. For example, a 18x25-bit multiplier
can also be used to implement 16x20-bit multiplication.

A way of mapping such elements in coarse grain synthesis is the
wrap-extract-unwrap method:

wrap
  Identify candidate-cells in the circuit and wrap them in a cell with a
  constant wider bit-width using :cmd:ref:`techmap`. The wrappers use the same
  parameters as the original cell, so the information about the original width
  of the ports is preserved. Then use the :cmd:ref:`connwrappers` command to
  connect up the bit-extended in- and outputs of the wrapper cells.

extract
  Now all operations are encoded using the same bit-width as the coarse grain
  element. The :cmd:ref:`extract` command can be used to replace circuits with
  cells of the target architecture.

unwrap
  The remaining wrapper cell can be unwrapped using :cmd:ref:`techmap`.

Example: DSP48_MACC
~~~~~~~~~~~~~~~~~~~

This section details an example that shows how to map MACC operations of
arbitrary size to MACC cells with a 18x25-bit multiplier and a 48-bit adder
(such as the Xilinx DSP48 cells).  Source code can be found in
``docs/source/code_examples/macc/``.

Preconditioning: ``macc_xilinx_swap_map.v``

Make sure ``A`` is the smaller port on all multipliers

.. todo:: add/expand supporting text

.. literalinclude:: /code_examples/macc/macc_xilinx_swap_map.v
   :language: verilog
   :caption: ``macc_xilinx_swap_map.v``

Wrapping multipliers: ``macc_xilinx_wrap_map.v``

.. literalinclude:: /code_examples/macc/macc_xilinx_wrap_map.v
   :language: verilog
   :lines: 1-46
   :caption: ``macc_xilinx_wrap_map.v``

Wrapping adders: ``macc_xilinx_wrap_map.v``

.. literalinclude:: /code_examples/macc/macc_xilinx_wrap_map.v
   :language: verilog
   :lines: 48-89
   :caption: ``macc_xilinx_wrap_map.v``

Extract: ``macc_xilinx_xmap.v``

.. literalinclude:: /code_examples/macc/macc_xilinx_xmap.v
   :language: verilog
   :caption: ``macc_xilinx_xmap.v``

... simply use the same wrapping commands on this module as on the design to
create a template for the :cmd:ref:`extract` command.

Unwrapping multipliers: ``macc_xilinx_unwrap_map.v``

.. literalinclude:: /code_examples/macc/macc_xilinx_unwrap_map.v
   :language: verilog
   :lines: 1-30
   :caption: ``$__mul_wrapper`` module in ``macc_xilinx_unwrap_map.v``

Unwrapping adders: ``macc_xilinx_unwrap_map.v``

.. literalinclude:: /code_examples/macc/macc_xilinx_unwrap_map.v
   :language: verilog
   :lines: 32-61
   :caption: ``$__add_wrapper`` module in ``macc_xilinx_unwrap_map.v``

.. literalinclude:: /code_examples/macc/macc_xilinx_test.v
   :language: verilog
   :lines: 1-6
   :caption: ``test1`` of ``macc_xilinx_test.v``

.. figure:: /_images/code_examples/macc/macc_xilinx_test1a.*
    :class: width-helper

.. figure:: /_images/code_examples/macc/macc_xilinx_test1b.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_xilinx_test.v
   :language: verilog
   :lines: 8-13
   :caption: ``test2`` of ``macc_xilinx_test.v``

.. figure:: /_images/code_examples/macc/macc_xilinx_test2a.*
    :class: width-helper

.. figure:: /_images/code_examples/macc/macc_xilinx_test2b.*
    :class: width-helper

Wrapping in ``test1``:

.. figure:: /_images/code_examples/macc/macc_xilinx_test1b.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_xilinx_test.ys
    :language: yoscrypt
    :start-after: part c
    :end-before: end part c

.. figure:: /_images/code_examples/macc/macc_xilinx_test1c.*
    :class: width-helper

Wrapping in ``test2``:

.. figure:: /_images/code_examples/macc/macc_xilinx_test2b.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_xilinx_test.ys
    :language: yoscrypt
    :start-after: part c
    :end-before: end part c

.. figure:: /_images/code_examples/macc/macc_xilinx_test2c.*
    :class: width-helper

Extract in ``test1``:

.. figure:: /_images/code_examples/macc/macc_xilinx_test1c.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_xilinx_test.ys
    :language: yoscrypt
    :start-after: part d
    :end-before: end part d

.. figure:: /_images/code_examples/macc/macc_xilinx_test1d.*
    :class: width-helper

Extract in ``test2``:

.. figure:: /_images/code_examples/macc/macc_xilinx_test2c.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_xilinx_test.ys
    :language: yoscrypt
    :start-after: part d
    :end-before: end part d

.. figure:: /_images/code_examples/macc/macc_xilinx_test2d.*
    :class: width-helper

Unwrap in ``test2``:

.. figure:: /_images/code_examples/macc/macc_xilinx_test2d.*
    :class: width-helper

.. literalinclude:: /code_examples/macc/macc_xilinx_test.ys
    :language: yoscrypt
    :start-after: part e
    :end-before: end part e

.. figure:: /_images/code_examples/macc/macc_xilinx_test2e.*
    :class: width-helper

Symbolic model checking
-----------------------

.. todo:: check text context

.. note:: 
    
    While it is possible to perform model checking directly in Yosys, it 
    is highly recommended to use SBY or EQY for formal hardware verification.

Symbolic Model Checking (SMC) is used to formally prove that a circuit has (or
has not) a given property.

One application is Formal Equivalence Checking: Proving that two circuits are
identical. For example this is a very useful feature when debugging custom
passes in Yosys.

Other applications include checking if a module conforms to interface standards.

The :cmd:ref:`sat` command in Yosys can be used to perform Symbolic Model
Checking.

Checking techmap
~~~~~~~~~~~~~~~~

.. todo:: add/expand supporting text

Let's look at the following example:

.. literalinclude:: /code_examples/synth_flow/techmap_01_map.v
   :language: verilog
   :caption: ``docs/source/code_examples/synth_flow/techmap_01_map.v``

.. literalinclude:: /code_examples/synth_flow/techmap_01.v
   :language: verilog
   :caption: ``docs/source/code_examples/synth_flow/techmap_01.v``

.. literalinclude:: /code_examples/synth_flow/techmap_01.ys
   :language: yoscrypt
   :caption: ``docs/source/code_examples/synth_flow/techmap_01.ys``

To see if it is correct we can use the following code:

.. todo:: replace inline yosys script code

.. code:: yoscrypt

    # read test design
    read_verilog techmap_01.v
    hierarchy -top test

    # create two version of the design: test_orig and test_mapped
    copy test test_orig
    rename test test_mapped

    # apply the techmap only to test_mapped
    techmap -map techmap_01_map.v test_mapped

    # create a miter circuit to test equivalence
    miter -equiv -make_assert -make_outputs test_orig test_mapped miter
    flatten miter

    # run equivalence check
    sat -verify -prove-asserts -show-inputs -show-outputs miter

Result:

.. code::

    Solving problem with 945 variables and 2505 clauses..
    SAT proof finished - no model found: SUCCESS!

AXI4 Stream Master
~~~~~~~~~~~~~~~~~~

The following AXI4 Stream Master has a bug. But the bug is not exposed if the
slave keeps ``tready`` asserted all the time. (Something a test bench might do.)

Symbolic Model Checking can be used to expose the bug and find a sequence of
values for ``tready`` that yield the incorrect behavior.

.. todo:: add/expand supporting text

.. literalinclude:: /code_examples/axis/axis_master.v
   :language: verilog
   :caption: ``docs/source/code_examples/axis/axis_master.v``

.. literalinclude:: /code_examples/axis/axis_test.v
   :language: verilog
   :caption: ``docs/source/code_examples/axis/axis_test.v``

.. literalinclude:: /code_examples/axis/axis_test.ys
   :language: yoscrypt
   :caption: ``docs/source/code_examples/axis/test.ys``

Result with unmodified ``axis_master.v``:

.. code::

    Solving problem with 159344 variables and 442126 clauses..
    SAT proof finished - model found: FAIL!

Result with fixed ``axis_master.v``:

.. code::

    Solving problem with 159144 variables and 441626 clauses..
    SAT proof finished - no model found: SUCCESS!