Rationale for allowing 0-width type bit<0> ?

[jafingerhut]

I know I was there in the P4 language design work group meetings where this was discussed. I had no objections at the time, and I still don't actually have any objections. I do have a few questions, though:

• Has anyone used a type bit<0> in a P4 program for something? If yes, what for?
• If not, can you imagine a situation where it seems useful?

I am not sure, but my guess is that the primary reason for making a change to the spec to allow type bit<0> was along the lines of: well, we support structs and header type with no member fields, and tuples with 0 members, why not type bit<0>, also?

One of my reasons for asking is in thinking about whether it is considered important to make changes to the P4Runtime API specification to allow values of type bit<0> in control plane API messages, or whether to simply say "that type is not supported in the P4Runtime API": p4lang/p4runtime#428

[mihaibudiu]

For exeample

#define WIDTH 0 
struct H { bit<WIDTH> field; }

[jafingerhut]

Thanks, Mihai, for the example.

Are you intending that as an example of something you have found useful to include in a P4 program before?

Or is it more along the lines of: "I (Mihai) would guess that someone might find code like this useful?"

[jnfoster]

I think the point is that you might write a program with a macro that gets filled in according to a profile or configuration. If the value of the macro determines the width of a bitstring, and can assume the value 0, then having bit<0> may be useful. (It would be even better if the compiler optimized the code aggressively, so there was zero cost for using that feature instead of #ifdef-ing the code away by hand.)

[chrispsommers]

Thanks for the explanations. That makes sense: widths of zero might be the outcome of some profile, not so much a goal as much as a consequence.

[jnfoster]

Do we keep this open? Or has it converged?

[jafingerhut]

I think the issue has served its purpose of asking and answering the question. I will close it now.