Clarification on the calling convention of directionless, extern parameter

[jaehyun1ee]

In the below parser type declaration, the first parameter, packet_in b is an directionless extern object.

parser Top(packet_in b, out Parsed_headers headers) {

Spec section 6.8. Calling convention: call by copy in/copy out discusses the calling convention on directionless parameter as follows:

• The meaning of parameters with no direction depends upon the kind of entity the parameter is for:
  • For anything other than an action, e.g. a control, parser, or function, a directionless parameter means that the value supplied as an argument in a call must be a compile-time known value (see Section 18.1).
  • For an action, a directionless parameter indicates that it is “action data”. See Section 14.1 for the meaning of action data, but its meaning includes the following possibilities:
    • The parameter's value is provided in the P4 program. In this case, the parameter behaves as if the direction were in. Such an argument expression need not be a compile-time known value.
    • The parameter's value is provided by the control plane software when an entry is added to a table that uses that action. See Section 14.1.

Yet, it does not discuss how the parameter packet_in b should actually be passed to the parser block.

Given that the packet_in object is stateful (+ instantiated by the target), it seems reasonable to assume that passing it by reference would be the appropriate approach, rather than copying the entire object.

I suggest that the specification clarify this point by providing guidance on how directionless extern parameters should be passed to parsers, controls, or functions.

[jafingerhut]

I agree that at least for extern object instances passed in this way, passing by reference seems to be the only thing that makes sense, and a clarification saying so explicitly would be a good thing. There might already be somewhere in the language spec that says this, but if so, I do not recall off-hand where it would be.

Are you interested in creating a pull request with a suggested change?

[jaehyun1ee]

Are you interested in creating a pull request with a suggested change?

I'd be happy to, but I'm afraid I am unsure of what exactly to specify and at which part of the specification.

Would this change in bold be acceptable?

• The meaning of parameters with no direction depends upon the kind of entity the parameter is for:
  • For anything other than an action, e.g. a control, parser, or function, a directionless parameter means that the value supplied as an argument in a call must be a compile-time known value (see Section 18.1).
    • A directionless parameter of extern object type is passed by reference.
  • For an action, a directionless parameter indicates that it is “action data”. See Section 14.1 for the meaning of action data, but its meaning includes the following possibilities:
    • The parameter's value is provided in the P4 program. In this case, the parameter behaves as if the direction were in. Such an argument expression need not be a compile-time known value.
    • The parameter's value is provided by the control plane software when an entry is added to a table that uses that action. See Section 14.1.

[jafingerhut]

That suggested addition looks like a good starting point for discussion by the language design work group to me.

[jaehyun1ee]

Thank you for the help, closed as merged