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example_synth: proc and opt_expr

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Highlight `proc` blocks and intro `opt_expr`.
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Krystine Sherwin 2024-01-16 13:23:04 +13:00
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docs/source/getting_started/example_synth.rst
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@ -124,8 +124,7 @@ Our ``addr_gen`` circuit now looks like this:
``addr_gen`` module after :cmd:ref:`hierarchy`
.. todo:: how to highlight PROC blocks?
They seem to be replaced in ``show``, so the selection never matches
.. TODO:: pending https://github.com/YosysHQ/yosys/pull/4133
Simple operations like ``addr + 1`` and ``addr == MAX_DATA-1`` can be extracted
from our ``always @`` block in :ref:`addr_gen-v`. This gives us the highlighted
@ -141,13 +140,14 @@ To handle these, let us now introduce the next command: :doc:`/cmd/proc`.
modifying the design directly, it instead calls a series of other commands. In
the case of :cmd:ref:`proc`, these sub-commands work to convert the behavioral
logic of processes into multiplexers and registers. Let's see what happens when
we run it.
we run it. For now, we will call :yoscrypt:`proc -noopt` to prevent some
automatic optimizations which would normally happen.
.. figure:: /_images/code_examples/fifo/addr_gen_proc.*
:class: width-helper
:name: addr_gen_proc
``addr_gen`` module after :cmd:ref:`proc`
``addr_gen`` module after :yoscrypt:`proc -noopt`
There are now a few new cells from our ``always @``, which have been
highlighted. The ``if`` statements are now modeled with ``$mux`` cells, while
@ -166,25 +166,42 @@ Notice how in the top left of :ref:`addr_gen_proc` we have a floating wire,
generated from the initial assignment of 0 to the ``addr`` wire. However, this
initial assignment is not synthesizable, so this will need to be cleaned up
before we can generate the physical hardware. We can do this now by calling
:cmd:ref:`clean`:
:cmd:ref:`clean`. We're also going to call :cmd:ref:`opt_expr` now, which would
normally be called at the end of :cmd:ref:`proc`. We can call both commands
at the same time by separating them with a colon: :yoscrypt:`opt_expr; clean`.
.. figure:: /_images/code_examples/fifo/addr_gen_clean.*
:class: width-helper
:name: addr_gen_clean
``addr_gen`` module after :cmd:ref:`clean`
``addr_gen`` module after :yoscrypt:`opt_expr; clean`
You may also notice that the highlighted ``$eq`` cell input of ``255`` has been
converted to ``8'11111111``. Constant values are presented in the format
``<bit_width>'<bits>``, with 32-bit values instead using the decimal number.
This indicates that the constant input has been reduced from 32-bit wide to
8-bit wide. :cmd:ref:`opt_expr` performs simple expression rewriting and
constant folding, which we discuss in more detail in
:doc:`/using_yosys/synthesis/opt`.
.. TODO:: why doesn't the 32-bit value 1 get converted to 1'1?
.. note::
:doc:`/cmd/clean` can also be called with two semicolons after any command,
for example we could have called :yoscrypt:`proc;;` instead of
:yoscrypt:`proc` and then :yoscrypt:`clean`. It is generally beneficial to
run :cmd:ref:`clean` after each command as a quick way of removing
disconnected parts of the circuit which have been left over. You may notice
some scripts will end each line with ``;;``.
for example we could have called :yoscrypt:`opt_expr;;` instead of
:yoscrypt:`opt_expr; clean`. It is generally beneficial to run
:cmd:ref:`clean` after each command as a quick way of removing disconnected
parts of the circuit which have been left over. You may notice some scripts
will end each line with ``;;``.
.. todo:: consider a brief glossary for terms like adff
.. seealso:: Advanced usage docs for
- :doc:`/using_yosys/synthesis/proc`
- :doc:`/using_yosys/synthesis/opt`
The full example
^^^^^^^^^^^^^^^^