Add explanation on passing directionless, extern object argument

[jaehyun1ee]

I propose adding some clarifications to the spec, discussing how directionless, extern objects are passed to a control/parser. (#1276)

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.

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