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Update Initialization Coverage spec for zero-width #156

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Update Initialization Coverage spec for zero-width #156

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4 changes: 3 additions & 1 deletion spec.md
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Expand Up @@ -1616,8 +1616,10 @@ module MyModule :
```

This is an illegal FIRRTL circuit and an error will be thrown during compilation.
All wires, memory ports, instance ports, and module ports that can be connected to must be connected to under all conditions.
All non-zero-width wires, memory ports, instance ports, and module ports that can be connected to must be connected to under all conditions.
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Be mindful of the ambiguity in this sentence.

Do I "parse" this sentence as:

  • "All (non-zero-width wires), memory ports, instance ports, and module ports ...", or as
  • "All non-zero-width (wires, memory ports, instance ports, and module ports) ..."

?

Also, zero-width needs a definition (even if that definition isn't "obvious"). This is part of the ongoing initiative I have for the spec, where we need to be more explicit about semantics.

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This and the next are ambiguous as it's not clear (at least looking at each sentence in isolation) whether "non-zero-width" only applies to wires or to the entire list.

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@mwachs5 mwachs5 Dec 11, 2023 •
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i agree that there seems to be a difference in use case for 0-width elements (SInt, UInt, Analog) and 0-element Aggregates (Vec, bundle types). To me it seems like users should be responsible for initializing the first one, while the latter does make sense to allow to not happen in the spec. I'm actually not sure what the proposed spec change is currently trying to cover --both of these cases?

Registers do not need to be connected to under all conditions, as it will keep its previous value if unconnected.
Zero-width wires, memory ports, instance ports, and module ports do not need to be connected.
These are implicitly initialized with a value of zero.
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Is initialized the right way to frame this? Don't we elsewhere say that reads of zero-width are zero? If that is true why bring initialization into anything?


## Conditional Scopes

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