yosys/docs/source/yosys_internals/formats/rtlil_rep.rst

The RTL Intermediate Language (RTLIL)
=====================================

All frontends, passes and backends in Yosys operate on a design in RTLIL
representation. The only exception are the high-level frontends that use the AST
representation as an intermediate step before generating RTLIL data.

In order to avoid reinventing names for the RTLIL classes, they are simply
referred to by their full C++ name, i.e. including the ``RTLIL::`` namespace
prefix, in this document.

:numref:`Figure %s <fig:Overview_RTLIL>` shows a simplified Entity-Relationship
Diagram (ER Diagram) of RTLIL. In :math:`1:N` relationships the arrow points
from the :math:`N` side to the :math:`1`. For example one ``RTLIL::Design``
contains :math:`N` (zero to many) instances of ``RTLIL::Module`` . A two-pointed
arrow indicates a :math:`1:1` relationship.

The ``RTLIL::Design`` is the root object of the RTLIL data structure. There is
always one "current design" in memory which passes operate on, frontends add
data to and backends convert to exportable formats. But in some cases passes
internally generate additional ``RTLIL::Design`` objects. For example when a
pass is reading an auxiliary Verilog file such as a cell library, it might
create an additional ``RTLIL::Design`` object and call the Verilog frontend with
this other object to parse the cell library.

.. figure:: ../../../images/overview_rtlil.*
	:class: width-helper
	:name: fig:Overview_RTLIL

	Simplified RTLIL Entity-Relationship Diagram

There is only one active ``RTLIL::Design`` object that is used by all frontends,
passes and backends called by the user, e.g. using a synthesis script. The
``RTLIL::Design`` then contains zero to many ``RTLIL::Module`` objects. This
corresponds to modules in Verilog or entities in VHDL. Each module in turn
contains objects from three different categories:

-  ``RTLIL::Cell`` and ``RTLIL::Wire`` objects represent classical netlist data.

-  ``RTLIL::Process`` objects represent the decision trees (if-then-else statements,
   etc.) and synchronization declarations (clock signals and sensitivity) from
   Verilog always and VHDL process blocks.

-  ``RTLIL::Memory`` objects represent addressable memories (arrays).

Usually the output of the synthesis procedure is a netlist, i.e. all
``RTLIL::Process`` and ``RTLIL::Memory`` objects must be replaced by
``RTLIL::Cell`` and ``RTLIL::Wire`` objects by synthesis passes.

All features of the HDL that cannot be mapped directly to these RTLIL classes
must be transformed to an RTLIL-compatible representation by the HDL frontend.
This includes Verilog-features such as generate-blocks, loops and parameters.

The following sections contain a more detailed description of the different
parts of RTLIL and rationale behind some of the design decisions.

RTLIL identifiers
-----------------

All identifiers in RTLIL (such as module names, port names, signal names, cell
types, etc.) follow the following naming convention: they must either start with
a backslash (\) or a dollar sign ($).

Identifiers starting with a backslash are public visible identifiers. Usually
they originate from one of the HDL input files. For example the signal name
``\sig42`` is most likely a signal that was declared using the name ``sig42`` in
an HDL input file. On the other hand the signal name ``$sig42`` is an
auto-generated signal name. The backends convert all identifiers that start with
a dollar sign to identifiers that do not collide with identifiers that start
with a backslash.

This has three advantages:

-  First, it is impossible that an auto-generated identifier collides with an
   identifier that was provided by the user.

.. todo:: does opt_rmunused (still?) exist?

-  Second, the information about which identifiers were originally provided by
   the user is always available which can help guide some optimizations. For
   example the :cmd:ref:`opt_rmunused` tries to preserve signals with a
   user-provided name but doesn't hesitate to delete signals that have
   auto-generated names when they just duplicate other signals.

-  Third, the delicate job of finding suitable auto-generated public visible
   names is deferred to one central location. Internally auto-generated names
   that may hold important information for Yosys developers can be used without
   disturbing external tools. For example the Verilog backend assigns names in
   the form \_integer\_.

Whitespace and control characters (any character with an ASCII code 32 or less)
are not allowed in RTLIL identifiers; most frontends and backends cannot support
these characters in identifiers.

In order to avoid programming errors, the RTLIL data structures check if all
identifiers start with either a backslash or a dollar sign, and contain no
whitespace or control characters. Violating these rules results in a runtime
error.

All RTLIL identifiers are case sensitive.

Some transformations, such as flattening, may have to change identifiers
provided by the user to avoid name collisions. When that happens, attribute
``hdlname`` is attached to the object with the changed identifier. This
attribute contains one name (if emitted directly by the frontend, or is a result
of disambiguation) or multiple names separated by spaces (if a result of
flattening). All names specified in the ``hdlname`` attribute are public and do
not include the leading ``\``.

RTLIL::Design and RTLIL::Module
-------------------------------

The ``RTLIL::Design`` object is basically just a container for ``RTLIL::Module``
objects. In addition to a list of ``RTLIL::Module`` objects the
``RTLIL::Design`` also keeps a list of selected objects, i.e. the objects that
passes should operate on. In most cases the whole design is selected and
therefore passes operate on the whole design. But this mechanism can be useful
for more complex synthesis jobs in which only parts of the design should be
affected by certain passes.

Besides the objects shown in the :ref:`ER diagram <fig:Overview_RTLIL>` above,
an ``RTLIL::Module`` object contains the following additional properties:

-  The module name
-  A list of attributes
-  A list of connections between wires
-  An optional frontend callback used to derive parametrized variations of the
   module

The attributes can be Verilog attributes imported by the Verilog frontend or
attributes assigned by passes. They can be used to store additional metadata
about modules or just mark them to be used by certain part of the synthesis
script but not by others.

Verilog and VHDL both support parametric modules (known as "generic entities" in
VHDL). The RTLIL format does not support parametric modules itself. Instead each
module contains a callback function into the AST frontend to generate a
parametrized variation of the ``RTLIL::Module`` as needed. This callback then
returns the auto-generated name of the parametrized variation of the module. (A
hash over the parameters and the module name is used to prohibit the same
parametrized variation from being generated twice. For modules with only a few
parameters, a name directly containing all parameters is generated instead of a
hash string.)

.. _sec:rtlil_cell_wire:

RTLIL::Cell and RTLIL::Wire
---------------------------

A module contains zero to many ``RTLIL::Cell`` and ``RTLIL::Wire`` objects.
Objects of these types are used to model netlists. Usually the goal of all
synthesis efforts is to convert all modules to a state where the functionality
of the module is implemented only by cells from a given cell library and wires
to connect these cells with each other. Note that module ports are just wires
with a special property.

An ``RTLIL::Wire`` object has the following properties:

-  The wire name
-  A list of attributes
-  A width (buses are just wires with a width > 1)
-  Bus direction (MSB to LSB or vice versa)
-  Lowest valid bit index (LSB or MSB depending on bus direction)
-  If the wire is a port: port number and direction (input/output/inout)

As with modules, the attributes can be Verilog attributes imported by the
Verilog frontend or attributes assigned by passes.

In Yosys, busses (signal vectors) are represented using a single wire object
with a width > 1. So Yosys does not convert signal vectors to individual
signals. This makes some aspects of RTLIL more complex but enables Yosys to be
used for coarse grain synthesis where the cells of the target architecture
operate on entire signal vectors instead of single bit wires.

In Verilog and VHDL, busses may have arbitrary bounds, and LSB can have either
the lowest or the highest bit index. In RTLIL, bit 0 always corresponds to LSB;
however, information from the HDL frontend is preserved so that the bus will be
correctly indexed in error messages, backend output, constraint files, etc.

An ``RTLIL::Cell`` object has the following properties:

-  The cell name and type
-  A list of attributes
-  A list of parameters (for parametric cells)
-  Cell ports and the connections of ports to wires and constants

The connections of ports to wires are coded by assigning an ``RTLIL::SigSpec``
to each cell port. The ``RTLIL::SigSpec`` data type is described in the next
section.

.. _sec:rtlil_sigspec:

RTLIL::SigSpec
--------------

A "signal" is everything that can be applied to a cell port. I.e.

-  | Any constant value of arbitrary bit-width
   | 1em For example: ``1337, 16'b0000010100111001, 1'b1, 1'bx``

-  | All bits of a wire or a selection of bits from a wire
   | 1em For example: ``mywire, mywire[24], mywire[15:8]``

-  | Concatenations of the above
   | 1em For example: ``{16'd1337, mywire[15:8]}``

The ``RTLIL::SigSpec`` data type is used to represent signals. The ``RTLIL::Cell``
object contains one ``RTLIL::SigSpec`` for each cell port.

In addition, connections between wires are represented using a pair of
``RTLIL::SigSpec`` objects. Such pairs are needed in different locations.
Therefore the type name ``RTLIL::SigSig`` was defined for such a pair.

.. _sec:rtlil_process:

RTLIL::Process
--------------

When a high-level HDL frontend processes behavioural code it splits it up into
data path logic (e.g. the expression ``a + b`` is replaced by the output of an
adder that takes a and b as inputs) and an ``RTLIL::Process`` that models the
control logic of the behavioural code. Let's consider a simple example:

.. code:: verilog
   :number-lines:

   module ff_with_en_and_async_reset(clock, reset, enable, d, q);
   input clock, reset, enable, d;
   output reg q;
   always @(posedge clock, posedge reset)
       if (reset)
           q <= 0;
       else if (enable)
           q <= d;
   endmodule

In this example there is no data path and therefore the ``RTLIL::Module`` generated
by the frontend only contains a few ``RTLIL::Wire`` objects and an ``RTLIL::Process`` .
The ``RTLIL::Process`` in RTLIL syntax:

.. code:: RTLIL
   :number-lines:

   process $proc$ff_with_en_and_async_reset.v:4$1
       assign $0\q[0:0] \q
       switch \reset
           case 1'1
               assign $0\q[0:0] 1'0
           case
               switch \enable
                   case 1'1
                       assign $0\q[0:0] \d
                   case
               end
       end
       sync posedge \clock
           update \q $0\q[0:0]
       sync posedge \reset
           update \q $0\q[0:0]
   end

This ``RTLIL::Process`` contains two ``RTLIL::SyncRule`` objects, two
``RTLIL::SwitchRule`` objects and five ``RTLIL::CaseRule`` objects. The wire
``$0\q[0:0]`` is an automatically created wire that holds the next value of
``\q``. The lines 2..12 describe how ``$0\q[0:0]`` should be calculated. The
lines 13..16 describe how the value of ``$0\q[0:0]`` is used to update ``\q``.

An ``RTLIL::Process`` is a container for zero or more ``RTLIL::SyncRule``
objects and exactly one ``RTLIL::CaseRule`` object, which is called the root
case.

An ``RTLIL::SyncRule`` object contains an (optional) synchronization condition
(signal and edge-type), zero or more assignments (``RTLIL::SigSig``), and zero
or more memory writes (``RTLIL::MemWriteAction``). The always synchronization
condition is used to break combinatorial loops when a latch should be inferred
instead.

An ``RTLIL::CaseRule`` is a container for zero or more assignments
(``RTLIL::SigSig``) and zero or more ``RTLIL::SwitchRule`` objects. An
``RTLIL::SwitchRule`` objects is a container for zero or more
``RTLIL::CaseRule`` objects.

In the above example the lines 2..12 are the root case. Here ``$0\q[0:0]`` is
first assigned the old value ``\q`` as default value (line 2). The root case
also contains an ``RTLIL::SwitchRule`` object (lines 3..12). Such an object is
very similar to the C switch statement as it uses a control signal (``\reset``
in this case) to determine which of its cases should be active. The
``RTLIL::SwitchRule`` object then contains one ``RTLIL::CaseRule`` object per
case. In this example there is a case [1]_ for ``\reset == 1`` that causes
``$0\q[0:0]`` to be set (lines 4 and 5) and a default case that in turn contains
a switch that sets ``$0\q[0:0]`` to the value of ``\d`` if ``\enable`` is active
(lines 6..11).

A case can specify zero or more compare values that will determine whether it
matches. Each of the compare values must be the exact same width as the control
signal. When more than one compare value is specified, the case matches if any
of them matches the control signal; when zero compare values are specified, the
case always matches (i.e. it is the default case).

A switch prioritizes cases from first to last: multiple cases can match, but
only the first matched case becomes active. This normally synthesizes to a
priority encoder. The parallel_case attribute allows passes to assume that no
more than one case will match, and full_case attribute allows passes to assume
that exactly one case will match; if these invariants are ever dynamically
violated, the behavior is undefined. These attributes are useful when an
invariant invisible to the synthesizer causes the control signal to never take
certain bit patterns.

The lines 13..16 then cause ``\q`` to be updated whenever there is a positive
clock edge on ``\clock`` or ``\reset``.

In order to generate such a representation, the language frontend must be able
to handle blocking and nonblocking assignments correctly. However, the language
frontend does not need to identify the correct type of storage element for the
output signal or generate multiplexers for the decision tree. This is done by
passes that work on the RTLIL representation. Therefore it is relatively easy to
substitute these steps with other algorithms that target different target
architectures or perform optimizations or other transformations on the decision
trees before further processing them.

One of the first actions performed on a design in RTLIL representation in most
synthesis scripts is identifying asynchronous resets. This is usually done using
the :cmd:ref:`proc_arst` pass. This pass transforms the above example to the
following ``RTLIL::Process``:

.. code:: RTLIL
   :number-lines:

   process $proc$ff_with_en_and_async_reset.v:4$1
       assign $0\q[0:0] \q
       switch \enable
           case 1'1
               assign $0\q[0:0] \d
           case
       end
       sync posedge \clock
           update \q $0\q[0:0]
       sync high \reset
           update \q 1'0
   end

This pass has transformed the outer ``RTLIL::SwitchRule`` into a modified
``RTLIL::SyncRule`` object for the ``\reset`` signal. Further processing converts the
``RTLIL::Process`` into e.g. a d-type flip-flop with asynchronous reset and a
multiplexer for the enable signal:

.. code:: RTLIL
   :number-lines:

   cell $adff $procdff$6
       parameter \ARST_POLARITY 1'1
       parameter \ARST_VALUE 1'0
       parameter \CLK_POLARITY 1'1
       parameter \WIDTH 1
       connect \ARST \reset
       connect \CLK \clock
       connect \D $0\q[0:0]
       connect \Q \q
   end
   cell $mux $procmux$3
       parameter \WIDTH 1
       connect \A \q
       connect \B \d
       connect \S \enable
       connect \Y $0\q[0:0]
   end

Different combinations of passes may yield different results. Note that
``$adff`` and ``$mux`` are internal cell types that still need to be mapped to
cell types from the target cell library.

Some passes refuse to operate on modules that still contain ``RTLIL::Process`` 
objects as the presence of these objects in a module increases the complexity.
Therefore the passes to translate processes to a netlist of cells are usually
called early in a synthesis script. The proc pass calls a series of other passes
that together perform this conversion in a way that is suitable for most
synthesis tasks.

.. _sec:rtlil_memory:

RTLIL::Memory
-------------

For every array (memory) in the HDL code an ``RTLIL::Memory`` object is created.
A memory object has the following properties:

-  The memory name
-  A list of attributes
-  The width of an addressable word
-  The size of the memory in number of words

All read accesses to the memory are transformed to ``$memrd`` cells and all
write accesses to ``$memwr`` cells by the language frontend. These cells consist
of independent read- and write-ports to the memory. Memory initialization is
transformed to ``$meminit`` cells by the language frontend. The ``\MEMID``
parameter on these cells is used to link them together and to the
``RTLIL::Memory`` object they belong to.

The rationale behind using separate cells for the individual ports versus
creating a large multiport memory cell right in the language frontend is that
the separate ``$memrd`` and ``$memwr`` cells can be consolidated using resource
sharing. As resource sharing is a non-trivial optimization problem where
different synthesis tasks can have different requirements it lends itself to do
the optimisation in separate passes and merge the ``RTLIL::Memory`` objects and
``$memrd`` and ``$memwr`` cells to multiport memory blocks after resource
sharing is completed.

The memory pass performs this conversion and can (depending on the options
passed to it) transform the memories directly to d-type flip-flops and address
logic or yield multiport memory blocks (represented using ``$mem`` cells).

See :ref:`sec:memcells` for details about the memory cell types.

.. [1]
   The syntax ``1'1`` in the RTLIL code specifies a constant with a length of
   one bit (the first ``1``), and this bit is a one (the second ``1``).