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Docs: Update internal cells to autoref

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Krystine Sherwin 2024-05-03 13:14:25 +12:00
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docs/source/using_yosys/synthesis/opt.rst
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@ -31,7 +31,7 @@ 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.
.. table:: Const folding rules for ``$_AND_`` cells as used in :cmd:ref:`opt_expr`.
.. table:: Const folding rules for `$_AND_` cells as used in :cmd:ref:`opt_expr`.
:name: tab:opt_expr_and
:align: center
@ -54,7 +54,7 @@ In some cases this pass can also optimize cells with some constant inputs.
========= ========= ===========
:numref:`Table %s <tab:opt_expr_and>` shows the replacement rules used for
optimizing an ``$_AND_`` gate. The first three rules implement the obvious const
optimizing an `$_AND_` gate. The first three rules implement the obvious const
folding rules. Note that 'any' might include dynamic values calculated by other
parts of the circuit. The following three lines propagate undef (X) states.
These are the only three cases in which it is allowed to propagate an undef
@ -66,18 +66,18 @@ substitutions are possible they are performed first, in the hope that the 'any'
will change to an undef value or a 1 and therefore the output can be set to
undef.
The last two lines simply replace an ``$_AND_`` gate with one constant-1 input
The last two lines simply replace an `$_AND_` gate with one constant-1 input
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
wide `$eq` and `$ne` cells with buffers or not-gates if one input is
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 ``$mux``
The :cmd:ref:`opt_expr` pass is very conservative regarding optimizing `$mux`
cells, as these cells are often used to model decision-trees and breaking these
trees can interfere with other optimizations.
@ -100,7 +100,7 @@ identifies cells with identical inputs and replaces them with a single instance
of the cell.
The option ``-nomux`` can be used to disable resource sharing for multiplexer
cells (``$mux`` and ``$pmux``.) This can be useful as it prevents multiplexer
cells (`$mux` and `$pmux`.) This can be useful as it prevents multiplexer
trees to be merged, which might prevent :cmd:ref:`opt_muxtree` to identify
possible optimizations.
@ -141,16 +141,16 @@ Simplifying large MUXes and AND/OR gates - :cmd:ref:`opt_reduce`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This is a simple optimization pass that identifies and consolidates identical
input bits to ``$reduce_and`` and ``$reduce_or`` cells. It also sorts the input
bits to ease identification of shareable ``$reduce_and`` and ``$reduce_or``
input bits to `$reduce_and` and `$reduce_or` cells. It also sorts the input
bits to ease identification of shareable `$reduce_and` and `$reduce_or`
cells in other passes.
This pass also identifies and consolidates identical inputs to multiplexer
cells. In this case the new shared select bit is driven using a ``$reduce_or``
cells. In this case the new shared select bit is driven using a `$reduce_or`
cell that combines the original select bits.
Lastly this pass consolidates trees of ``$reduce_and`` cells and trees of
``$reduce_or`` cells to single large ``$reduce_and`` or ``$reduce_or`` cells.
Lastly this pass consolidates trees of `$reduce_and` cells and trees of
`$reduce_or` cells to single large `$reduce_and` or `$reduce_or` cells.
These three simple optimizations are performed in a loop until a stable result
is produced.
@ -160,7 +160,7 @@ Merging mutually exclusive cells with shared inputs - :cmd:ref:`opt_share`
This pass identifies mutually exclusive cells of the same type that:
a. share an input signal, and
b. drive the same ``$mux``, ``$_MUX_``, or ``$pmux`` multiplexing cell,
b. drive the same `$mux`, `$_MUX_`, or `$pmux` multiplexing cell,
allowing the cell to be merged and the multiplexer to be moved from
multiplexing its output to multiplexing the non-shared input signals.
@ -176,14 +176,14 @@ multiplexing its output to multiplexing the non-shared input signals.
Before and after :cmd:ref:`opt_share`
When running :cmd:ref:`opt` in full, the original ``$mux`` (labeled ``$3``) is
When running :cmd:ref:`opt` in full, the original `$mux` (labeled ``$3``) is
optimized away by :cmd:ref:`opt_expr`.
Performing DFF optimizations - :cmd:ref:`opt_dff`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This pass identifies single-bit d-type flip-flops (``$_DFF_``, ``$dff``, and
``$adff`` cells) with a constant data input and replaces them with a constant
This pass identifies single-bit d-type flip-flops (`$_DFF_`, `$dff`, and
`$adff` cells) with a constant data input and replaces them with a constant
driver. It can also merge clock enables and synchronous reset multiplexers,
removing unused control inputs.