The best practice is to use a tcl file:
database -open waves -shm probe -create your_top_level -depth all -all -shm -database waves run exit
run with :
irun -access +r testcase.sv -input shm.tcl
Alternatively the following can be added to the simulation:
initial begin $shm_open("waves.shm"); $shm_probe("AS"); end
irun -access +r testcase.sv
NB: I have had trouble getting this last version to work, the tcl file method is more reliable.