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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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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