Shader Compiler Error when SV_POSITION is a Half4 on Mali GPUs

I found a very specific shader error when testing a shader on android, causing it to render magenta on the device.



I tested on 3 devices: two Galaxy S8 with different GPUs (Mali-G71 and Adreno 540), and a Galaxy S7 with a Mali-T880. The shader works fine on Adreno, but doesn't work on the Mali devices.

Here's a test shader which you can use to reproduce the error. In order to trigger the issue, you need 3 or more TEXCOORDs in your V2F and have SV_POSITION be declared as a Half4.

Code (HLSL):

1. Shader "Unlit/testHalf"
2. {
3.     Properties
4.     {
5.     }
6.     SubShader
7.     {
8.         Tags { "RenderType"="Opaque" }
9.  
10.         Pass
11.         {
12.             CGPROGRAM
13.             #pragma vertex vert
14.             #pragma fragment frag
15.  
16.             #include "UnityCG.cginc"
17.  
18.             struct appdata
19.             {
20.                 half4 vertex : POSITION;
21.             };
22.  
23.             struct v2f
24.             {
25.                 half2 uv : TEXCOORD0;
26.                 half2 uv1 : TEXCOORD1;
27.                 half2 uv2 : TEXCOORD2;
28.                 half4 vertex : SV_POSITION;
29.             };
30.  
31.             v2f vert (appdata v)
32.             {
33.                 v2f o;
34.                 o.vertex = UnityObjectToClipPos(v.vertex);
35.                 return o;
36.             }
37.  
38.             half4 frag (v2f i) : SV_Target
39.             {
40.                 half4 col = half4(1,1,1,1);
41.                 return col;
42.             }
43.             ENDCG
44.         }
45.     }
46. }
47.  

Changing SV_POSITION to a float4 or having 2 or less TEXCOORDs solves the issue:



but this is boggling my mind. What could be the cause for this behaviour?

 

Just hit this same issue in 2019.4.6f1, it seems having 2 TEXCOORDS and SV_POSITION suddenly introduces a requirement for gl_ClipDistance in the compiled shader that breaks some compilers, in my case, the Apple simulator.

Was a bug ever recorded for this and fixed, or am I seeing something similar but unrelated?