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docs: more on wreduce in synth starter

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Krystine Sherwin 2024-01-04 12:49:48 +13:00
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35
docs/source/getting_started/example_synth.rst
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@ -369,6 +369,8 @@ options is able to fold one of the ``$mux`` cells into the ``$adff`` to form an
Part 2
^^^^^^
The next group of commands performs a series of optimizations:
.. literalinclude:: /cmd/synth_ice40.rst
:language: yoscrypt
:start-at: wreduce
@ -377,12 +379,33 @@ Part 2
:caption: ``coarse`` section (part 2)
:name: synth_coarse2
The next three (new) commands are :doc:`/cmd/wreduce`, :doc:`/cmd/peepopt`, and
:doc:`/cmd/share`. None of these affect our design either, so let's skip over
them. :yoscrypt:`techmap -map +/cmp2lut.v -D LUT_WIDTH=4` optimizes certain
comparison operators by converting them to LUTs instead. The usage of
:cmd:ref:`techmap` is explored more in
:doc:`/using_yosys/synthesis/techmap_synth`.
First up is :doc:`/cmd/wreduce`. If we run this we get the following:
.. literalinclude:: /code_examples/fifo/fifo.out
:language: doscon
:start-at: yosys> wreduce
:end-before: yosys> select
:caption: output of :cmd:ref:`wreduce`
Looking at the data path for ``rdata``, the most relevant of these width
reductions are the ones affecting ``fifo.$flatten\fifo_reader.$add$fifo.v``.
That is the ``$add`` cell incrementing the fifo_reader address. We can look at
the schematic and see the output of that cell has now changed.
.. TODO:: pending bugfix in :cmd:ref:`wreduce` and/or :cmd:ref:`opt_clean`
.. figure:: /_images/code_examples/fifo/rdata_wreduce.*
:class: width-helper
:name: rdata_wreduce
``rdata`` output after :cmd:ref:`wreduce`
The next two (new) commands are :doc:`/cmd/peepopt` and :doc:`/cmd/share`.
Neither of these affect our design, and they're explored in more detail in
:doc:`/using_yosys/synthesis/opt`, so let's skip over them. :yoscrypt:`techmap
-map +/cmp2lut.v -D LUT_WIDTH=4` optimizes certain comparison operators by
converting them to LUTs instead. The usage of :cmd:ref:`techmap` is explored
more in :doc:`/using_yosys/synthesis/techmap_synth`.
Our next command to run is
:doc:`/cmd/memory_dff`.