Question

Some DPI imported tasks or functions require that the context of their call is known. It takes special instrumentation of their call instances to provide such context; for example, an internal variable referring to the “current instance” might need to be set. To avoid any unnecessary overhead, imported task or function calls in SystemVerilog code are not instrumented unless the imported task or function is specified as context.

All DPI exported tasks or functions require that the context of their call is known. This occurs since SystemVerilog task or function declarations always occur in instantiable scopes, hence allowing a multiplicity of unique task or function instances.

For the sake of simulation performance, an imported task or function call shall not block SystemVerilog compiler optimizations. An imported task or function not specified as context shall not access any data objects from SystemVerilog other than its actual arguments. Only the actual arguments can be affected (read or written) by its call. Therefore, a call of a non-context task or function is not a barrier for optimizations. A context imported task or function, however, can access (read or write) any SystemVerilog data objects by calling PLI/VPI, or by calling an export task or function. Therefore, a call to a context task or function is a barrier for SystemVerilog compiler optimizations.

Only calls of context imported tasks or functions are properly instrumented and cause conservative optimizations; therefore, only those tasks or functions can safely call all tasks or functions from other APIs, including PLI and VPI functions or exported SystemVerilog tasks or functions. For imported tasks or functions not specified as context, the effects of calling PLI or VPI functions or SystemVerilog tasks or functions can be unpredictable and such calls can crash if the callee requires a context that has not been properly set. However note that declaring an import context task or function does not automatically make any other simulator interface automatically available. For VPI access (or any other interface access) to be possible, the appropriate implementation defined mechanism must still be used to enable these interface(s). Note also that DPI calls do not automatically create or provide any handles or any special environment that can be needed by those other interfaces. It is the user’s responsibility to create, manage or otherwise manipulate the required handles/ environment(s) needed by the other interfaces.

Context imported tasks or functions are always implicitly supplied a scope representing the fully qualified instance name within which the import declaration was present. This scope defines which exported SystemVerilog tasks or functions can be called directly from the imported task or function; only tasks or functions defined and exported from the same scope as the import can be called directly. To call any other exported SystemVerilog tasks or functions, the imported task or function shall first have to modify its current scope, in essence performing the foreign language equivalent of a SystemVerilog hierarchical task or function call.

Special DPI utility functions exist that allow imported task or functions to retrieve and operate on their scope. See Annex E for more details.