Getting model bounds in vertex shader

Is there a way to get the bounds of the model in the vertex shader? Or just calculate max/min vertex.xyz ?

for example i want to change color of model depending on current fragment height (in model space)

 uniform float max_y = 0;        
     
     v2f vert (appdata_full v)
     {
         v2f o;
         
         
         o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
         o.uv    = v.texcoord.xy;
             
         if(v.vertex.y > max_y) max_y = v.vertex.y;
         
         o.height = v.vertex.y / max_y;
         
         return o;
     }

but max_y always zero. If i set height in code - all works fine.