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Docs: opt_expr

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Krystine Sherwin 2024-01-17 08:32:14 +13:00
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20
docs/source/getting_started/example_synth.rst
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@ -155,13 +155,6 @@ the register uses an ``$adff`` cell. If we look at the terminal output we can
also see all of the different ``proc_*`` commands being called. We will look at
each of these in more detail in :doc:`/using_yosys/synthesis/proc`.
.. TODO:: intro ``opt_expr``
:doc:`/cmd/opt_expr`
- by default called at the end of :cmd:ref:`proc`
- can be disabled with ``-noopt``
- done here for... reasons?
Notice how in the top left of :ref:`addr_gen_proc` we have a floating wire,
generated from the initial assignment of 0 to the ``addr`` wire. However, this
initial assignment is not synthesizable, so this will need to be cleaned up
@ -176,15 +169,14 @@ at the same time by separating them with a colon: :yoscrypt:`opt_expr; clean`.
``addr_gen`` module after :yoscrypt:`opt_expr; clean`
You may also notice that the highlighted ``$eq`` cell input of ``255`` has been
converted to ``8'11111111``. Constant values are presented in the format
You may also notice that the highlighted ``$eq`` cell input of ``255`` has
changed 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?
8-bit wide. This is a side-effect of running :cmd:ref:`opt_expr`, which
performs constant folding and simple expression rewriting. For more on why
this happens, refer to :doc:`/using_yosys/synthesis/opt` and the :ref:`section
on opt_expr <adv_opt_expr>`.
.. note::

14
docs/source/using_yosys/synthesis/opt.rst
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@ -21,10 +21,12 @@ to run this pass after each major step in the synthesis script. As listed in
:dedent:
:caption: Passes called by :cmd:ref:`opt`
Const folding and simple expression rewriting - :cmd:ref:`opt_expr`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. _adv_opt_expr:
This pass performs const folding on the internal combinational cell types
Constant folding and simple expression rewriting - :cmd:ref:`opt_expr`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This pass performs constant folding on the internal combinational cell types
described in :doc:`/yosys_internals/formats/cell_library`. This means a cell
with all constant inputs is replaced with the constant value this cell drives.
In some cases this pass can also optimize cells with some constant inputs.
@ -69,7 +71,11 @@ with a buffer.
Besides this basic const folding the :cmd:ref:`opt_expr` pass can replace 1-bit
wide ``$eq`` and ``$ne`` cells with buffers or not-gates if one input is
constant.
constant. Equality checks may also be reduced in size if there are redundant
bits in the arguments (i.e. bits which are constant on both inputs). This can,
for example, result in a 32-bit wide constant like ``255`` being reduced to the
8-bit value of ``8'11111111`` if the signal being compared is only 8-bit as in
:ref:`addr_gen_clean` of :doc:`/getting_started/example_synth`.
The :cmd:ref:`opt_expr` pass is very conservative regarding optimizing ``$mux``
cells, as these cells are often used to model decision-trees and breaking these