Question

Argument passing for imported functions is ruled by the WYSIWYG principle: What You Specify Is What You Get, see Section 27.5.1.1. The evaluation order of formal arguments follows general SystemVerilog rules.

Argument compatibility and coercion rules are the same as for native SystemVerilog functions. If a coercion is needed, a temporary variable is created and passed as the actual argument. For input and inout arguments, the temporary variable is initialized with the value of actual argument with the appropriate coercion; for output or inout arguments, the value of the temporary variable is assigned to the actual argument with the appropriate conversion. The assignments between a temporary and the actual argument follow general SystemVerilog rules for assignments and automatic coercion.

On the SystemVerilog side of the interface, the values of actual arguments for formal input arguments of imported functions shall not be affected by the callee; the initial values of formal output arguments of imported functions are unspecified (and can be implementation-dependent), and the necessary coercions, if any, are applied as for assignments. imported functions shall not modify the values of their input arguments.

For the SystemVerilog side of the interface, the semantics of arguments passing is as if input arguments are passed by copy-in, output arguments are passed by copy-out, and inout arguments were passed by copy-in, copy-out. The terms copy-in and copy-out do not impose the actual implementation; they refer only to “hypothetical assignment”.

The actual implementation of argument passing is transparent to the SystemVerilog side of the interface. In particular, it is transparent to SystemVerilog whether an argument is actually passed by value or by reference. The actual argument passing mechanism is defined in the foreign language layer. See Annex E for more details.