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example_synth: more on DSP mapping

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Krystine Sherwin 2024-01-08 13:24:52 +13:00
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docs/source/getting_started/example_synth.rst
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@ -438,7 +438,11 @@ Part 3
^^^^^^
The third part of the :cmd:ref:`synth_ice40` flow is a series of commands for
mapping to DSPs.
mapping to DSPs. By default, the iCE40 flow will not map to the hardware DSP
blocks and will only be performed if called with the ``-dsp`` flag:
:yoscrypt:`synth_ice40 -dsp`. While our example has nothing that could be
mapped to DSPs we can still take a quick look at the commands here and describe
what they do.
.. literalinclude:: /cmd/synth_ice40.rst
:language: yoscrypt
@ -448,7 +452,28 @@ mapping to DSPs.
:caption: ``coarse`` section (part 3)
:name: synth_coarse3
.. TODO:: more on DSP mapping
:yoscrypt:`wreduce t:$mul` performs width reduction again, this time targetting
only cells of type ``$mul``. :yoscrypt:`techmap -map +/mul2dsp.v -map
+/ice40/dsp_map.v ... -D DSP_NAME=$__MUL16X16` uses :cmd:ref:`techmap` to map
``$mul`` cells to ``$__MUL16X16`` which are, in turn, mapped to the iCE40
``SB_MAC16``. Any multipliers which aren't compatible with conversion to
``$__MUL16X16`` are relabelled to ``$__soft_mul`` before :cmd:ref:`chtype`
changes them back to ``$mul``.
During the mul2dsp conversion, some of the intermediate signals are marked with
the attribute ``mul2dsp``. By calling :yoscrypt:`select a:mul2dsp` we restrict
the following commands to only operate on the cells and wires used for these
signals. :cmd:ref:`setattr` removes the now unnecessary ``mul2dsp`` attribute.
:cmd:ref:`opt_expr` we've already come across for const folding and simple
expression rewriting, the ``-fine`` option just enables more fine-grain
optimizations. Then we perform width reduction a final time and clear the
selection.
Finally we have :cmd:ref:`ice40_dsp`: similar to the :cmd:ref:`memory_dff`
command we saw in the previous section, this merges any surrounding registers
into the ``SB_MAC16`` cell. This includes not just the input/output registers,
but also pipeline registers and even a post-adder where applicable: turning a
multiply + add into a single multiply-accumulate.
.. seealso:: Advanced usage docs for
:doc:`/using_yosys/synthesis/techmap_synth`