what is criteria shader requires loop unrolling?

Hello, everyone,

I'm implementing Volume Rendering using this.

And then, this example code uses the tex3dlod function to sample 3d texture.

When I use other function to sample 3d texture for example tex3d or tex.Sample(https://docs.unity3d.com/2019.4/Documentation/Manual/SL-SamplerStates.html), compiler requires loop unrolling.

tex3dlod don't reqire loop unrollling. What is the different criteria to tex3dlod and others?,Hello, everyone,

I'm implementing Volume Rendering using this.

And then, this example code uses the tex3dlod function to sample 3d texture.

When I use other function to sample 3d texture for example tex3d or tex.Sample(https://docs.unity3d.com/2019.4/Documentation/Manual/SL-SamplerStates.html), compiler requires loop unrolling.

tex3dlod don't reqire loop unrollling.

What is the different criteria to tex3dlod and others?