How to Get Num Particles in GPU Compute Sim

I want to reference a Num Particle in Custom HLSL, but I can’t get the value right.

So I checked the sheet to see what kind of value is contained, and it seems to be 0.
However, in CPU Sim mode, the attribute had a proper Num Particles value.

How can I get Num Particles in GPU Compute Sim?

Thanks.