macc: Stop using the B port #4817
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The B port is for single-bit summands. These can just as well be represented as an additional summand on the A port (which supports summands of arbitrary width). An upcoming `$macc_v2` cell won't be special-casing single-bit summands in any way. In preparation, make the following changes: * remove the `bit_ports` field from the `Macc` helper (instead add any single-bit summands to `ports` next to other summands) * leave `B` empty on cells emitted from `Macc::to_cell`
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I think some theoretical merit was mentioned in the past to the B port when techmapping to primitives that only support single-bit summands. Since I have no actual evidence of that being used by anyone, it's just something to keep in mind. Otherwise I'm in favor of the idea |
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What are the reasons/motivation for this change?
Explain how this is achieved.
The B port is for single-bit summands. These can just as well be represented as an additional summand on the A port (which supports summands of arbitrary width). An upcoming
$macc_v2cell won't be special-casing single-bit summands in any way.In preparation, make the following changes:
remove the
bit_portsfield from theMacchelper (instead add any single-bit summands toportsnext to other summands)leave
Bempty on cells emitted fromMacc::to_cellIf applicable, please suggest to reviewers how they can test the change.
An included test makes sure we interpret the B port on existing RTLIL exports.