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yosys/docs/source/yosys_internals/extending_yosys/documenting.rst
Krystine Sherwin da65409d69
documenting.rst: Cell help v1 and v2 
Also a (commented) mention of v2a for the extra curious.
2025-11-21 12:30:54 +13:00

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Generated help messages and documentation
=========================================
All Yosys commands and built-in cell types should include help text, documenting
their functionality for users. This help text is made available through the
`help` command, and online via `ReadtheDocs`_ as part of the :doc:`/cmd_ref` and
:doc:`/cell_index` documentation. When running locally, any commands provided
by loaded plugins (either from the command line when calling ``yosys``, or
dynamically with the `plugin` command) will also be available to the `help`
command.
.. _ReadtheDocs: https://about.readthedocs.com/
.. note::
Since help text for commands is generated from compiled code, the online help
may differ from that produced by `help`. Some commands, like `abc`, may be
completely unavailable depending on compile flags; while others may limit
specific features, such as whether the `synth` script pass uses ABC.
Command help
------------
The first stop for command help text is the ``Pass::short_help``. This is a
short sentence describing the pass, and is set in the ``Pass`` constructor with
the name of the pass, as demonstrated here with `chformal`.
.. literalinclude:: /generated/chformal.cc
:language: c++
:start-at: public Pass {
:end-at: ChformalPass()
:caption: ``ChformalPass()`` from :file:`passes/cmds/chformal.cc`
:append:
// ...
} ChformalPass;
All currently available commands are listed with their ``short_help`` string
when calling `help` without arguments, and is more or less the same as the
unlisted :ref:`command index <commandindex>`. The string is also used when
hovering over links to commands in the documentation, and in section headings
like :ref:`chformal autocmd`.
The next section shows the complete help text for the `chformal` command. This
can be displayed locally by using `help <command>` (or ``yosys -h <command>``
from the command line). The general format is to show each usage signature (how
the command is called), followed by a paragraph describing what the pass does,
and a list of options or flags available. Additional arguments in the signature
or option may use square brackets (``[]``) to indicate optional parts, and angle
brackets (``<>``) for required parts. The pipe character (``|``) may be used to
indicate mutually exclusive arguments.
.. note::
Remember that when using ``Frontend`` and ``Backend`` the pass name will be
be prefixed with ``read_`` or ``write_`` respectively. Usage signatures must
match the pass name available in commands/scripts, which is available as
``Pass::pass_name``.
.. todo:: decide on a formatting style for pass options
.. _chformal autocmd:
.. autocmd:: chformal
:noindex:
Warning flags
~~~~~~~~~~~~~
In order to support commands which are not intended for general use, a number of
warning flags are provided to the ``Pass`` class. Take the
:ref:`internal_flag_example` as an example. In the body of the constructor, we
call ``Pass::internal()`` to set the warning flag that this is an internal; i.e.
one aimed at Yosys *developers* rather than users. Commands with the
``internal`` flag are often used for testing Yosys, and expose functionality
that would normally be abstracted. Setting this flag also ensures that commands
will be included in :doc:`/cmd/index_internal`.
.. literalinclude:: /generated/functional/test_generic.cc
:language: cpp
:start-at: FunctionalTestGeneric()
:end-at: }
:dedent:
:caption: `test_generic` pass constructor
:name: internal_flag_example
The other warning flag available is ``Pass::experimental()``, also to be called
during the constructor. This should used for experimental commands that may be
unstable, unreliable, incomplete, and/or subject to change. Experimental passes
also typically have the text ``(experimental)`` at the start of their
``short_help``, but this is not always the case.
.. todo:: should the experimental flag add ``(experimental)`` automatically?
In both cases, commands with these flags set will print additional warning text
in the help output. Calling commands with the ``experimental`` flag set, will
also call ``log_experimental()`` with the name of the pass, providing an
additional warning any time the pass is used.
.. note::
When testing the handling of expected error/warning messages with e.g.
`logger`, it is possible to disable the warnings for a given experimental
feature. This can be done by calling Yosys with ``--experimental
<feature>``, where ``<feature>`` is the name of the experimental pass.
The ``Pass::help()`` method
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Overriding this method is the original way to provide help text, and as of this
writing is still the most common. The ``log()`` function should be called
directly to print and format the help text, and each line should be limited to
80 (printed) characters. While it is possible to provide arbitrary formatting,
it is preferred to follow the guidelines here to maintain consistency with other
passes and to assist in correct parsing and formatting during RST generation
(i.e. these docs).
.. note::
It is good practice in the ``Pass::help`` method for each call to ``log()`` to
correspond to a single line, containing exactly one ``\n`` (at the end). This
allows the appearance in source to match the appearance in the terminal.
The first and last lines should always be empty, followed by the primary usage
signature for the command. Each usage signature should be indented with 4
spaces, and followed by an empty line. Each option or flag should start on a
new line indented with 4 spaces, followed by a description of the option which
is indented by a further 4 spaces, and then an empty line. Option descriptions
typically start with lower case, and may forgo a trailing period (``.``). Where
multiple options share a description the empty line between options should be
omitted.
.. note::
`Commands JSON`_ has more on how formatting in ``help()`` gets parsed.
The ``Pass::formatted_help()`` method
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The ``formatted_help`` method serves two purposes in help generation, both of
which are optional. In both cases, any pass which uses the method should
``#include "kernel/log_help.h"``, and begin the method by calling ``auto *help =
PrettyHelp::get_current();``. The method finishes by returning a boolean value.
``true`` means help content has been added to the current ``PrettyHelp``, while
``false`` indicates that ``Pass::help()`` should be called instead.
Setting a command group
^^^^^^^^^^^^^^^^^^^^^^^
Command groups are used when `dumping to JSON`_, so that related
commands can be presented together in documentation. For example, all of the
formal commands (which `chformal` is one of) are listed under
:doc:`/cmd/index_formal`, by using the ``autocmdgroup`` directive in
:file:`docs/source/cmd/index_formal.rst`. By default, commands are grouped by
their source location, such that the group is the same as the path to the source
file.
.. note::
Source location tracking requires :makevar:`ENABLE_HELP_SOURCE` to be set in
the makefile. Some passes, like the ``opt_*`` family, are able to be grouped
by the name of the pass; but most will be assigned the ``unknown`` group.
For frontends and backends, source code is structured such that different
formats are located in different folders. Default behavior is to instead
group all of these passes as :doc:`/cmd/index_frontends` and
:doc:`/cmd/index_backends` respectively. Without location tracking, the
fallback is to look for passes that start with ``read_`` or ``write_``.
It is possible to set the group of a command explicitly with the
``PrettyHelp::set_group()`` method. This allows grouping of commands which may
not share a common source location, as well as ensuring that commands are still
grouped when location tracking is disabled. Because ``Pass::formatted_help()``
returns if it produced help content, it is completely valid to override the
method, get the current instance of ``PrettyHelp``, set the command group, and
then return ``false``.
.. warning::
There is currently no warning available for groups that do not have a
corresponding ``autocmdgroup``. If you add a new command group, make sure
that it has a corresponding index page.
Rich help text
^^^^^^^^^^^^^^
The second purpose of ``Pass::formatted_help`` is to provide richer help
content which is able to take advantage of the reStructuredText formatting used
here in the web docs. It also provides a more fluid way of writing help text,
without getting caught up in the terminal-first spacing requirements of writing
for ``Pass::help()``.
Help content is a list of ``ContentListing`` nodes on a root node, which can be
found by calling ``PrettyHelp::get_root()``. Each node has a type, a body, and
its own list of children ``ContentListing``\ s. Adding content is done with the
``ContentListing::{usage, option, codeblock, paragraph}`` methods, which each
add a new child node with a type set to the calling method. Let's take a look
at the source code for `chformal`.
.. literalinclude:: /generated/chformal.cc
:language: c++
:start-at: bool formatted_help()
:end-before: void execute
:caption: ``ChformalPass::formatted_help()`` from :file:`passes/cmds/chformal.cc`
:name: chformal_source
We can see that each of the ``ContentListing`` methods have the body of the new
node as the first argument. For a ``usage`` node, this is how to call the
command (i.e. its usage signature). ``paragraph`` nodes contain a paragraph of
text with line breaks added automatically; the argument itself should contain
any line breaks, but the string can be broken across multiple lines as shown.
The body of a ``paragraph`` node is treated as raw RST, allowing for inline
formatting and references as if it were written in the RST file itself. As
shown in the example (and the :ref:`formatted output above <chformal autocmd>`),
this includes using single backticks for linking to cells or commands, and
double backticks for raw code.
The ``option`` method lists a single option for the command, usually starting
with a dash (``-``). An optional second argument can be provided with adds a
paragraph node as a child of the option, and is used for describing the option.
Where multiple options share a description, it should be added to the last
option.
.. note::
To add multiple paragraphs to an option's description,
``ContentListing::open_option()`` should be used instead. This method
returns the option node, which can then be used to call
``ContentList::paragraph()`` multiple times.
``codeblock`` content is displayed verbatim, and content should include line
breaks as desired. No extra formatting will be applied to the text, and it will
be rendered with a monospace font; making it perfect for code sections or ASCII
art diagrams which render the same on the web as they do in the terminal. An
optional second argument is available for specifying the language in RST output
for code syntax highlighting (use ``yoscrypt`` for yosys script syntax
highlighting).
..
not recommended since it (currently) doesn't render in the terminal
The final method available is ``ContentListing::open_usage``. As with
``open_option`` creates and returns a new node which can have additional content
added to it directly. For the usage node, this can be used for example to add
text/options specific to a given usage of the command. In the web documentation
any content added in this way will be indented under the usage signature.
..
When :makevar:`ENABLE_HELP_SOURCE` is set, each ``ContentListing`` node also
stores file path and line number of its source location. But I think this might
only be used when raising errors/warnings during ``autocmd``.
Command line rendering
~~~~~~~~~~~~~~~~~~~~~~
Rendering text for the command line is done by the ``Pass::help`` method. When
this method is not overridden, the default behavior is to call
``Pass::formatted_help()``. If this method is also left unimplemented, or the
return value is explicitly false, then a default message about missing help text
for the command is displayed. Returning true, however, will then call
``PrettyHelp::log_help()`` to convert the formatted help content into plain
text.
.. note::
Regardless of which help method is used, any `warning flags`_ set on the pass
will display a message to warn the user. These are regular messages, using
``log()`` rather than ``log_warning()``, meaning (for example) they will
be suppressed by the ``-q`` command line option.
Rendering rich help text as plain text is done by traversing over all the
``ContentListing`` nodes and printing the body text. ``usage`` nodes are
preceded by an empty line and indented one level (4 spaces). ``option`` nodes
are also indented one level, while their children are indented an extra level (8
spaces). Any ``codeblock`` nodes are rendered as-is at the current indentation,
with no further formatting applied.
``paragraph`` nodes are broken into words separated by spaces, and each word is
printed. If a word would cause the current line to exceed 80 characters
(controlled by ``MAX_LINE_LEN`` in :file:`kernel/log_help.cc`), then the word
will instead be placed on a new line with the same level of indentation. Special
handling is included for words that begin and end with a backtick (`````) so
that these are stripped when printing to the command line. Compare
:ref:`chformal_help` below with the :ref:`chformal autocmd` above. The content
is still the same, but for the command line it uses a fixed width.
.. todo:: spaces in backticks (``assert(...)`` vs ````assert(s_eventually ...)````)
.. literalinclude:: /generated/chformal.log
:lines: 2-
:name: chformal_help
:caption: Command line output for `help chformal`
Cell help
---------
Unlike commands, cell help text is generated at compile time, and is not
affected by platform or compile flags. This also means that it is not possible
to provide help content for custom cell types in plugins or technology
libraries.
Two verilog simulation libraries provide models for all built-in cell types.
These are defined in :file:`techlibs/common/simcells.v` (for
:doc:`/cell/index_gate`) and :file:`techlibs/common/simlib.v` (for
:doc:`/cell/index_word`). Each model is preceded by a structured comment block,
formatted as either :ref:`v1` or :ref:`v2`. These comment blocks are processed
by a python script, :file:`techlibs/common/cellhelp.py`, to generate the help
content used in :file:`kernel/register.cc`.
.. note::
Each verilog module (and its comment block) is parsed into a C++ ``dict``,
mapping the cell type (the name of the verilog module) to a ``SimHelper``
struct in :file:`kernel/register.cc` with ``#include``\ s. Calling `help
<celltype>` then retrieves the corresponding ``SimHelper`` and displays the
help text contained.
Calling `help -cells` will list all built-in cell types with their input/output
ports. There is again an unlisted :ref:`cell index <cellindex>` which shows all
cell types with their title. Unlike commands, providing a title is optional,
and only available with `v2`_ formatting, so most just use the name of the cell
(qualified with the containing group). It is also possible to display the
verilog simulation model by calling `help <celltype>+`.
.. _v1:
v1 (default)
~~~~~~~~~~~~
As mentioned previously, the verilog simulation models are preceded by a
structured comment block. Each line starting with ``//-`` is added to the
description of the next verilog module. Non-empty lines must have a space after
the dash before text, and should be limited to 80 characters (84 including the
``//-``). The description is rendered to the terminal as-is when calling `help
<celltype>`, while the web docs will render it as text, with empty lines being
used to separate paragraphs.
..
Descriptions can extend into the verilog module itself, including *all* comment
lines that start with a dash prior to the ``endmodule``. However, everything in
the ``module .. endmodule`` block is considered source code, so this is not
recommended.
.. note::
Most of the legacy cell descriptions include a signature line (``//-
$<celltype> (<ports>)``). More recent versions of the help generation will
automatically produce this signature from the verilog declaration, making
this an optional inclusion. Note that if a signature line *is* included, it
*must* start with at least 4 spaces (not tabs), and include one empty line
(``//-``) before and after.
Each cell type must also be assigned a group, failing to do so will produce an
error. This can be done by adding ``//* group <cellgroup>`` anywhere in the
comment block. As with commands, the group determines where the cell appears in
the Sphinx documentation, but does not otherwise impact the output of `help`. As
with commands, there is no warning produced if cells are assigned a group which
is not used in the documentation. Make sure to check :file:`docs/source/cell`
for the groups currently available.
For the cell models in :file:`techlibs/common/simcells.v`, it is possible to
provide a truth table at the end of the cell description which is rendered in
sphinx docs as a literal code block. We can look at the :ref:`NOT_module` to
see this in action.
.. literalinclude:: /generated/simcells._NOT_.v
:language: verilog
:start-at: //-
:end-at: module \$_NOT_
:name: NOT_module
:caption: `$_NOT_` cell comment block from :file:`techlibs/common/simcells.v`
..
v1 descriptions in :file:`techlibs/common/simcells.v` have their version
unconditionally changed to ``2a`` to facilitate the truth table rendering,
making use of the v2 handling of codeblocks with ``::``. This also means A.
using ``::`` on its own in a v1 (gate-level) description should be avoided, and
B. *all* text after the ``"Truth table:"`` line is included in the codeblock.
.. _v2:
v2 (more expressive)
~~~~~~~~~~~~~~~~~~~~
Fields can be directly assigned with a ``//* <name> <value>`` comment line. We
saw this in the `v1`_ format with the group, but this is actually possible with
*all* fields of the ``SimHelper`` struct. In order to use the extra fields,
``ver`` must be explicitly set as ``2``. The extra fields available are as
follows, with an example provided by the :ref:`nex_module`.
- title
A short title for the cell, equivalent to ``short_help`` in commands.
Rendered before the description and when hovering over links in
documentation.
- tags
A space-separated list of :doc:`/cell/properties`. Not used in `help`
output, but provided when dumping to JSON and in the Sphinx docs.
.. literalinclude:: /generated/simlib.nex.v
:language: verilog
:name: nex_module
:caption: `$nex` cell comment block from :file:`techlibs/common/simlib.v`
.. warning::
While it is possible to assign values to any of the ``SimHelper`` fields,
some fields are automatically assigned and explicitly setting them may result
in errors, or discarding of the assigned value. These fields are the name,
ports, code, source, and desc.
The cell description is provided in the same way as in `v1`_, with each line
starting with a ``//-``. When generating the Sphinx documentation, the cell
description is interpreted as raw RST. This allows both in-line formatting like
linking to commands or passes using backticks (`````), and literal code blocks
with the ``::`` marker as in the following example:
.. code-block:: verilog
//- text
//- ::
//-
//- monospaced text
//-
//- indentation and line length will be preserved, giving a scroll bar if necessary for the browser window
//-
//- more text
Note that the empty line after the ``::`` and before the text continues are
required, as is the indentation before the literal contents. When rendering to
the terminal with `help <celltype>`, the ``::`` line will be ignored, while
Sphinx displays the section verbatim like so:
text
::
monospaced text
indentation and line length will be preserved, giving a scroll bar if necessary for the browser window
more text
.. todo:: in line formatting for web docs isn't exclusive to v2,
but it does raise the question of if we should be doing something to prevent
v1 descriptions being treated as raw RST.
Dumping to JSON
---------------
Once compiled, Yosys is able to dump both the internal command and cell
libraries to a machine-readable JSON file. Primarily intended for building this
documentation (more on that in the next section), this feature is not advertised
within Yosys itself, and can be done with `help -dump-cmds-json <cmds.json>` and
`help -dump-cells-json <cells.json>` respectively.
Both JSON files are formatted very similarly, containing a single object. The
object has a ``version`` field which disambiguates between the two, a
``generator`` field which contains the Yosys version string used, a ``groups``
object which maps each group to the list of commands/cells in that group, and
finally a ``cmds`` or ``cells`` object which maps each command/cell to its help
content.
Commands JSON
~~~~~~~~~~~~~
Lets take a look at :ref:`chformal_json` as an example. We can see the bulk of
the object is taken up by the ``content`` field, which contains all the
``ContentListing`` nodes we added in :ref:`the formatted_help method for
chformal <chformal_source>`, maintaining the structure of those nodes. The
command's ``short_help`` is given in the ``title`` field, with other fields for
the `Warning flags`_, source location, source function, and corresponding group
(either implicit or explicit).
.. literalinclude:: /generated/cmds.json
:language: json
:start-at: "chformal": {
:end-before: "chparam": {
:caption: `chformal` in generated :file:`cmds.json`
:name: chformal_json
Every command registered in Yosys (including those from currently installed
plugins) has a corresponding object in the JSON dump. For commands where
``Pass::formatted_help()`` is unimplemented or returns false, ``ContentListing``
nodes will be generated by parsing the unformatted ``Pass::help()`` output. This
is largely the same as `Command line rendering`_ but in reverse, with a few
simple rules to try convert between raw text and the different node types.
To be parsed as a ``usage`` node, the current line:
+ must start with the name of the command (case sensitive), followed by a
space or a new line;
+ may have up to four characters of whitespace as indentation;
+ must be the first non-empty line, preceded by two empty lines, or
immediately following another usage signature with the same indentation.
Any lines immediately after a usage signature which is indented more than the
signature will be appended to the usage signature. This allows for breaking
arguments across lines in the terminal output while still producing a single
``usage`` node.
.. code-block:: cpp
:caption: Example code for a command with multiple usage signatures
log("\n");
log(" command\n");
log(" command -argument\n");
log(" -another argument\n");
log("\n");
log("\n");
log("command description.\n"); // not a signature because it is dedented
log("\n");
log("\n");
log(" command -different argument\n");
log("\n");
If a line is indented and starts with a dash (``-``), and does not immediately
follow a usage signature, it is parsed as an ``option`` node. Anything else is
parsed as a ``codeblock`` and added to either the root node or the current
option depending on the indentation. This allows yosys script syntax
highlighting for (most) options, while still respecting help content which
relies on the fixed-width rendering.
To enable syntax highlighting in synthesis command scripts, if the final block
of help output starts with the string ``"The following commands are executed by
this synthesis command:\n"``, then the rest of the code block is formatted as
``yoscrypt`` (e.g. `synth_ice40`). The caveat here is that if the ``script()``
calls ``run()`` on any commands *prior* to the first ``check_label`` then the
auto detection will break and revert to unformatted code (e.g.
`synth_fabulous`).
Cells JSON
~~~~~~~~~~
- effectively the ``SimHelper`` struct formatted as JSON (drops ``ver``, copies
tags to ``properties``)
- plus additional fields from the ``CellType``
+ inputs, outputs, property flags
.. literalinclude:: /generated/cells.json
:language: json
:start-at: "$nex": {
:end-at: },
:caption: `$nex` in generated :file:`cells.json`
:name: nex_json
Cells and commands in Sphinx
----------------------------
To support the rich documentation of commands and cells in Yosys, we use two
custom `Sphinx Domains`_.
.. _Sphinx Domains: https://www.sphinx-doc.org/en/master/usage/domains/index.html
- ``yoscrypt`` role allows inline code to have yosys script syntax highlighting
- default role of ``autoref``
+ any text in single backticks without an explicit role will be assigned this one
+ will convert to ``cell:ref`` if it begins with ``$``, otherwise ``cmd:ref``
+ to attempt linking there must be no spaces, and it must not begin with a
dash (``-``)
+ ```chformal``` (or ``:autoref:`chformal```) -> ``:cmd:ref:`chformal``` -> `chformal`
+ also works with two words, if the first one is ``help``
+ ```help $add``` -> ``:cell:ref:`help $add <$add>``` -> `help $add`
+ fallback to formatting as inline yoscrypt
+ ```-remove``` -> `-remove`
+ ```chformal -remove``` -> ``:yoscrypt:`chformal -remove``` -> `chformal -remove`
Using autodoc
~~~~~~~~~~~~~
- below is the raw RST output from ``autocmd`` (``YosysCmdDocumenter`` class in
:file:`docs/util/cmd_documenter.py`) for `chformal` command
- heading will be rendered as a subheading of the most recent heading (see
`chformal autocmd`_ above rendered under `Command help`_)
- ``.. cmd:def:: <cmd>`` line is indexed for cross references with ``:cmd:ref:``
directive (`chformal autocmd`_ above uses ``:noindex:`` option so that
`chformal` still links to the correct location)
+ ``:title:`` option controls text that appears when hovering over the
`chformal` link
- commands with warning flags (experimental or internal) add a ``.. warning``
block before any of the help content
- if a command has no ``source_location`` the ``.. note`` at the bottom will
instead link to :doc:`/cmd/index_other`
.. _showing autocmd generated rst:
.. autocmd_rst:: chformal
- command groups documented with ``autocmdgroup <group>``
+ with ``:members:`` option this is the same as calling ``autocmd`` for each
member of the group
- ``autocell`` and ``autocellgroup``
+ very similar to ``autocmd`` and ``autocmdgroup`` but for cells instead of
commands (``YosysCellDocumenter`` in :file:`docs/util/cell_documenter.py`)
+ optionally includes verilog source for cell(s) with ``:source:`` option
(plus ``:linenos:``)
+ cell definitions do not include titles
+ cells can have properties (:ref:`propindex`)
- bonus ``autocmd_rst``, used exclusively on this page for `showing autocmd
generated rst`_
Our custom Sphinx domains
~~~~~~~~~~~~~~~~~~~~~~~~~
- ``cmd`` and ``cell``
- Directives
+ ``cmd:def`` provide command definition
+ ``cmd:usage`` used by ``autocmd`` for command usage signatures
+ ``cell:def`` provide cell definition
+ ``cell:defprop`` provide cell property definition (used in
:doc:`/cell/properties`)
+ ``cell:source`` used by ``autocell`` for simulation models
- Roles
+ ``cmd:ref`` link to a ``cmd:def`` with the same name
+ ``cmd:title`` same as ``cmd:ref``, but includes the short help in the text
- ``:cmd:title:`chformal``` -> :cmd:title:`chformal`
+ ``cell:ref`` link to a ``cell:def`` with the same name
+ ``cell:title``
- ``:cell:title:`$nex``` -> :cell:title:`$nex`
+ ``cell:prop`` link to a ``cell:defprop`` of the same name
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