Question

Also refer to Section 27.4.3.

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

All DPI export tasks and functions require that the context of their call is known. This occurs since SystemVerilog task and function declarations always occur in instantiable scopes, hence allowing a multiplicity of unique task and function instances in the simulator’s elaborated database. Thus, there is no such thing as a non-context export task or function.

For the sake of simulation performance, a non-context imported task or function call shall not block System- Verilog compiler optimizations. An imported task or function not specified as context shall not access any data objects from SystemVerilog other then its actual arguments. Only the actual arguments can be affected (read or written) by its call. Therefore, a call of non-context imported 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, nor by calling an embedded export task or function. Therefore, a call to a context task or function is a barrier for SystemVerilog compiler optimizations.

Only the calls of context imported tasks and functions are properly instrumented and cause conservative optimizations; therefore, only those tasks and functions can safely call all functions from other APIs, including PLI and VPI functions or exported SystemVerilog functions. For imported task or functions not specified as context, the effects of calling PLI, VPI, or SystemVerilog functions can be unpredictable and such calls can crash if the callee requires a context that has not been properly set.

Special DPI utility functions exist that allow imported task and functions to retrieve and operate on their context. For example, the C implementation of an imported task or function can use svGetScope() to retrieve an svScope corresponding to the instance scope of its corresponding SystemVerilog import declaration. See Annex E.8 for more details.