Question Compute shader does not work on Vulkan

Hi There I have the following compute shader, which basically sorts a series of lights into the nearest 4 for a given 'tile' centre. This works fine in the editor and runs fine on Quest 2 in OpenGL, but something is not right when I build using Vulkan.

Does anyone have any theory looking at the shader code as to why this might be? I suspect it is the branching but I have not been able to verify that and I've tried various forms of the branching and evne included the [branch] attribute but the results are the same... I don't think it ever goes into any of the IF statements.

If not the branching then maybe my code, literals or other are a not quite right and this is being tolerated by OpenGL but not Vulkan?

Code (CSharp):

1. // Each #kernel tells which function to compile; you can have many kernels
2. #pragma kernel CSMain
3.  
4. // Create a RenderTexture with enableRandomWrite flag and set it
5. // with cs.SetTexture
6. RWTexture2D<float4> Lighting;
7.  
8. struct csLight
9. {
10.     float3 position;
11.     float intensity;
12.     float3 colour;
13.     float overbright;
14. };
15.  
16. struct Important
17. {
18.     float index;
19.     float distance;
20. };
21.  
22. RWStructuredBuffer<csLight> Lights;
23.  
24. float LightCount;
25.  
26. [numthreads(8,8,1)]
27. void CSMain (uint3 id : SV_DispatchThreadID)
28. {
29.     Important imp[4];
30.  
31.     imp[0].index = 0;
32.     imp[0].distance = 9000000.0f;
33.  
34.     imp[1].index = 0;
35.     imp[1].distance = 9000000.0f;
36.  
37.     imp[2].index = 0;
38.     imp[2].distance = 9000000.0f;
39.  
40.     imp[3].index = 0;
41.     imp[3].distance = 9000000.0f;
42.  
43.     float x = id.x;
44.     float y = id.y;
45.  
46.     float3 centre = float3(0.75f + (x * 1.5f), 0.75f, 0.75f + (y * 1.5f));
47.     int i = 1;
48.     for (i = 1; i < LightCount; ++i)
49.     {
50.         csLight l = Lights[i];
51.  
52.         float dist = distance(l.position, centre);
53.  
54.         if (imp[0].distance > dist)
55.         {
56.             imp[3] = imp[2];
57.             imp[2] = imp[1];
58.             imp[1] = imp[0];
59.  
60.             imp[0].distance = dist;
61.             imp[0].index = i;
62.         }
63.         else
64.         {
65.             if (imp[1].distance > dist)
66.             {
67.                 imp[3] = imp[2];
68.                 imp[2] = imp[1];
69.  
70.                 imp[1].distance = dist;
71.                 imp[1].index = i;
72.             }
73.             else
74.             {
75.                 if (imp[2].distance > dist)
76.                 {
77.                     imp[3] = imp[2];
78.  
79.                     imp[2].distance = dist;
80.                     imp[2].index = i;
81.                 }
82.                 else
83.                 {
84.                     if (imp[3].distance > dist)
85.                     {
86.                         imp[3].distance = dist;
87.                         imp[3].index = i;
88.                     }
89.                 }
90.             }
91.         }
92.     }
93.  
94.     Lighting[id.xy] = float4(imp[0].index / 255.0f, imp[1].index / 255.0f, imp[2].index / 255.0f, imp[3].index / 255.0f);
95. }

Appreciate that there may be ways to do this with structured buffers, I have yet to explore that but it's up next, hopefully once I have this solved.

Thanks
Ian H