Bug [WebGL] for loop in URP standard shaders kills Safari performance

Hi There,

X-Posting from https://forum.unity.com/threads/pointlight-kills-safari-performance-2020-3-25-urp.1216254/
and bug https://fogbugz.unity3d.com/default.asp?1391874_s87affehutkvq4jf

The Simple Lit shader exhibits terrible performance in WebGL, but only on Safari.

I noticed it only when using pixel (rather than vertex) lighting. Digging into the shader code in

Lighting.hlsl

Code (CSharp):

1. half4 UniversalFragmentBlinnPhong(InputData inputData, half3 diffuse, half4 specularGloss, half smoothness, half3 emission, half alpha)
2.  

If I replace the for loop that iterates over the pixel lights

Code (CSharp):

1. for (uint lightIndex = 0u; lightIndex < pixelLightCount; ++lightIndex)
2. {
3. ...
4. }

With anything else (even just repeating the code 4x, for 4 pixel lights) the performance in safari becomes significantly better.

My guess is however apple have implemented the for-loop for gles 3 is horribly imperformant.

The question is, what is a performant way of re-implementing the for loop. Would using if statements

e.g (pseduo code)

Code (CSharp):

1. col += numLights < 0 ? PixelLighting(Light0) : 0;
2. col += numLights < 1 ? PixelLighting(Light1) : 0;

cause too much branching logic and impact perf?

Thanks in advance!