Question: Are runtime_data and local types synonymous in bmv2 JSON config file?

[jafingerhut]

I can create an example if necessary, but it might be easy to determine why this is happening from perusal of the bmv2 back end.

I have a bmv2 JSON config file where in some cases, a primitive of an action will reference an action parameter using a JSON object with key "type" and value "runtime_data", and key "value" being an integer index into the array of action parameters.

In other cases, it appears that the same action parameters are being referenced via a JSON object with key "type" and value "local", and the same "value" as for "runtime_data" above.

Is there any semantic difference between these? If so, what? If not, should they all use the same value for "type"?

It appears that occurrences of type "local" are created here in the code: https://github.com/p4lang/p4c/blob/master/backends/bmv2/expression.cpp#L312

Occurences of type "runtime_data" are created here: https://github.com/p4lang/p4c/blob/master/backends/bmv2/expression.cpp#L427

[antoninbas]

when the argument to an action primitive is a simple expression, like a field value or a runtime-provided value (runtime_data), bmv2 encourages the compiler to treat them in a special way to avoid having to invoke the bmv2 expression-evaluation engine, which is a little expensive.
runtime_data is used to indicate that the argument provided to the action primitive comes directly from the runtime data of the table entry, and the expression-evaluation engine is bypassed. On the other hand, local is used in a "complex" expression to refer to runtime data.
The following should behave identically, although the first one should be faster:

{
  "op": "modify_field",
  "parameters": [
    {
      "type": "field",
      "value": ["ethernet", "srcAddr"]
    },
    {
      "type": "runtime_data",
       "value": 0
     }
   ]
}

and

  "op": "modify_field",
  "parameters": [
    {
      "type": "field",
      "value": ["ethernet", "srcAddr"]
    },
    {
      "type": "expression",
       "value": {
         "type": "local",
         "value": 0
     }
   ]
}

Note that local only refers to runtime data in the context of an expression evaluated inside an action. For expressions evaluated in another context, local may refer to something else.

[mihaibudiu]

local does not seem to be documented in the BMv2 JSON reference.

[antoninbas]

that's correct, I opened a PR for this: https://github.com/p4lang/behavioral-model/pull/383/files

[jafingerhut]

Thanks for the explanation. Are there any plans to use local to refer to things other than runtime data (i.e. action parameters)? If so, do you have an example in mind?

[antoninbas]

The only other use that comes to mind is the P4_14 Python compiler using it to handle variable-length field extraction. There, locals are fields in the header being extracted.

[jafingerhut]

Thanks for the answers, and for adding to the bmv2 JSON config file documentation. Closing this issue.