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@ -19,24 +19,24 @@ FSM detection
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~~~~~~~~~~~~~
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The `fsm_detect` pass identifies FSM state registers. It sets the
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``\fsm_encoding = "auto"`` attribute on any (multi-bit) wire that matches the
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``fsm_encoding = "auto"`` attribute on any (multi-bit) wire that matches the
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following description:
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- Does not already have the ``\fsm_encoding`` attribute.
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- Does not already have the ``fsm_encoding`` attribute.
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- Is not an output of the containing module.
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- Is driven by single `$dff` or `$adff` cell.
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- The ``\D``-Input of this `$dff` or `$adff` cell is driven by a multiplexer
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- The ``D``-Input of this `$dff` or `$adff` cell is driven by a multiplexer
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tree that only has constants or the old state value on its leaves.
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- The state value is only used in the said multiplexer tree or by simple
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relational cells that compare the state value to a constant (usually `$eq`
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cells).
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This heuristic has proven to work very well. It is possible to overwrite it by
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setting ``\fsm_encoding = "auto"`` on registers that should be considered FSM
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state registers and setting ``\fsm_encoding = "none"`` on registers that match
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setting ``fsm_encoding = "auto"`` on registers that should be considered FSM
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state registers and setting ``fsm_encoding = "none"`` on registers that match
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the above criteria but should not be considered FSM state registers.
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Note however that marking state registers with ``\fsm_encoding`` that are not
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Note however that marking state registers with ``fsm_encoding`` that are not
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suitable for FSM recoding can cause synthesis to fail or produce invalid
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results.
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@ -44,7 +44,7 @@ FSM extraction
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~~~~~~~~~~~~~~
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The `fsm_extract` pass operates on all state signals marked with the
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(``\fsm_encoding != "none"``) attribute. For each state signal the following
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(``fsm_encoding != "none"``) attribute. For each state signal the following
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information is determined:
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- The state registers
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@ -109,7 +109,7 @@ The `fsm_opt` pass performs basic optimizations on `$fsm` cells (not including
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state recoding). The following optimizations are performed (in this order):
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- Unused control outputs are removed from the `$fsm` cell. The attribute
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``\unused_bits`` (that is usually set by the `opt_clean` pass) is used to
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``unused_bits`` (that is usually set by the `opt_clean` pass) is used to
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determine which control outputs are unused.
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- Control inputs that are connected to the same driver are merged.
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@ -196,7 +196,7 @@ Removing unused cells and wires - `opt_clean` pass
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This pass identifies unused signals and cells and removes them from the design.
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It also creates an ``\unused_bits`` attribute on wires with unused bits. This
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It also creates an ``unused_bits`` attribute on wires with unused bits. This
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attribute can be used for debugging or by other optimization passes.
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When to use `opt` or `clean`
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