[SV][Prepare] Improve a namehint propagation and spilling logic

[uenoku]

This implements a name hint propagation to a wire op canonicalization.

%foo = sv.wire
%0 = comb.add ....
sv.assign %foo, %0
...

will be converted into

%0 = comb.add {sv.namehint = "foo"}
...

This also improves wire spilling logic by allowing expressions used only procedural assignments.

[mikeurbach]

I'm not sure about the change to PrepareForEmission, but the update in the wire canonicalizer looks great. I think this is always going to be an improvement, but I do wonder what about if the existing expression already had a namehint. I guess the name of the wire is a "stronger" hint, so I think what you have here looks good.

[mikeurbach]

Why was sv::PAssignOp removed here? Is this maybe an unrelated change that snuck in? I wouldn't think this is required for this change.

[uenoku]

This is unrelated to the PR but it's necessary for name propagation improvement. I'll split a PR.

[mikeurbach]

It doesn't have to be a separate PR if it's related, I'm just curious why it is necessary here, since we are only updating the canonicalizer or wires.

[uenoku]

The condition was intended to avoid spilling same expressions twice by skipping expressions that have single use of assignments. Therefore an expression used by procedural assignments was not spilled to a wire. For example,

%0 = comb.add {sv.namehint = "good_name"}
sv.always .. {
   sv.passign %reg, %0 : ..
}

In this case %0 was inlined to the assignment, but we want to create a wire for it.

[mikeurbach]

I see, so I think this makes sense then. In general, I guess it's all about tradeoffs, so trading off some inlining for better names sounds good to me.

[uenoku]

I do wonder what about if the existing expression already had a namehint. I guess the name of the wire is a "stronger" hint, so I think what you have here looks good.

Right, currently a name on a wire overwrites expressions's namehint. The intention is that the name on the wire is generally better than a namehint. I agree we might want to introduce a notion of priority of names.