Question

See also Section 27.4.2.

Only non-void functions with no output or inout arguments can be specified as pure. Functions specified as pure in their corresponding SystemVerilog import declarations shall have no side effects; their results need to depend solely on the values of their input arguments. Calls to such functions can be removed by SystemVerilog compiler optimizations or replaced with the values previously computed for the same values of the input arguments.

Specifically, a pure function is assumed not to directly or indirectly (i.e., by calling other functions):

— perform any file operations

— read or write anything in the broadest possible meaning, includes i/o, environment variables, objects from the operating system or from the program or other processes, shared memory, sockets, etc.

— access any persistent data, like global or static variables.

If a pure function does not obey the above restrictions, SystemVerilog compiler optimizations can lead to unexpected behavior, due to eliminated calls or incorrect results being used.