(In reply to Marek Olšák from comment #10)
> (In reply to Michel Dänzer from comment #9)
> > Note that AFAIK using things like derivatives in non-uniform control flow
> > isn't supported by GLSL. What's the original GLSL shader?
> 
> True.
> 
> The GLSL shader is using discard followed by fwidth. This is undefined
> behavior.
> 
> Therefore, it's an application bug.
> 
> The easy workaround would be to disable register allocation in st/mesa to
> get a quasi-SSA form and trivially move KILL_IF to the end of the shader if
> the app is detected to be Talos Principle.

I forwarded this information to Croteam and Dean Sekulic answered:
> Oh, just found out that discard instruction is treated as dynamic flow control
> for some reason... :/
>
> Will fix this!
>
>
> On 19-02-16 8:21, Dean Sekulic wrote:
>> I can't find any reference anywhere that doing fwidth() (or ddx/ddy for
>> that matter) after discard should be undefined behavior... :/
>>
>> GPUs should handle that case completely fine.
>>
>> Can you elaborate, please?

Looks like there might be an update for The Talos Principle which will address
this.