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Here is a simple fragment shader:

vec3 redColor = vec3(1.0, 0.0, 0.0);

vec3 getColor()
{
    return redColor;
}
void main(void) {
    vec3 color = getColor();
    gl_FragColor = vec4(color, 1.0);
}

When we use the global variable redColor in getColor() method, after translated it to metal shader language, all variables defined in global scope will be skipped. It will make redColor undefined in getColor() method in msl:
float3 getColor () { return redColor; }

It is a bit complicate and will take more time to fix the issues, thus we don't take this into account for the time being.

This post explain the problems encountered when using an array of textures in msl.

Finally that issues was fixed by upgrading my phone to iOS12.0.1. For macOS, the version should at least macOS 10.13. Due to its restrict on equipments, We temporary no support an array of textures in msl.

It is strange that, the same GLSL shader can not be converted to metal shader sometimes. The usage of glsl-optimizer codes is here.

I think it is because some parameters are not initialized, so it works sometimes.