Support DontCare in Mux and cloneSupertype #995
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From a user-facing perspective this looks fine to me. Is there a way to get Scala to understand that DontCare is a bottom type of Data, or is that not worth it / it introduces unwanted side effects / etc? |
edwardcwang
approved these changes
Jan 22, 2019
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The implementation of Nothing as a bottom type may be compiler magic, see this StackOverflow thread. I'm going to merge this as-is and we can improve later if we figure out how. |
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Sounds good. |
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Adds general support for DontCare to cloneSupertype, and add special case handling in Mux for the chisel style (DontCare object) to firrtl style (invalidate wire) conversion. Not sure how to actually test the DontCare case, but this tests the other case.
Note that it will still be difficult to use DontCare in a Mux, because Scala doesn't understand that DontCare should be a bottom type of Data, so it will infer the output type of Mux to be the common supertype of DontCare (which extends Element) and your Data, which usually means you'll get a Element or Data out, instead of something more specific eg UInt.
This also kind of feels like a game of whackamole, since DontCare support may be subtly broken in other contexts, though I guess it also makes no sense to do something like
1.U + DontCareRelated issue: resolves #848
Type of change: other enhancement
Impact: API addition (no impact on existing code)
Development Phase: implementation
Release Notes
Support DontCare in Muxes