Question

Special properties can be specified for an imported task or function: as pure or as context (see also Section 27.4.2 or 27.4.3 ).

A function whose result depends solely on the values of its input arguments and with no side effects can be specified as pure. This can usually allow for more optimizations and thus can result in improved simulation performance. Section 27.4.2 details the rules that must be obeyed by pure functions. An imported task can never be declared pure.

An imported task or function that is intended to call exported tasks or functions or to access SystemVerilog data objects other then its actual arguments (e.g. via VPI or PLI calls) must be specified as context. Calls of context tasks and functions are specially instrumented and can impair SystemVerilog compiler optimizations; therefore simulation performance can decrease if the context property is specified when not necessary. A task or function not specified as context shall not read or write any data objects from SystemVerilog other then its actual arguments. For tasks or functions not specified as context, the effects of calling PLI, VPI, or exported SystemVerilog tasks or functions can be unpredictable and can lead to unexpected behavior; such calls can even crash. Section 27.4.3 details the restrictions that must be obeyed by non-context tasks or functions.

If neither the pure nor the context attribute is used on an imported task or function, the task or function shall not access SystemVerilog data objects, however it can perform side-effects such as writing to a file or manipulating a global variable.