How to force GLSL branching?

Question

I'm working on a fragment shader. It works, but it still needs some optimalizations.

As far as I know, most of the cases branches in GLSL are flatten, so both cases are executed. I've eliminated most of the if-else conditions, but there are some of them, which have to stay as they are, because both branches are expensive to execute. I know, that in HLSL there is a [branch] keyword for this problem. But how is it possible to solve it in GLSL?

My code looks like this (the conditions are not uniform, their results depend on calculations in the shader):

if( condition ) {
    expensive calculations...
}

if( condition2 ) {
    expensive calculations...
}

if( condition3 ) {
    expensive calculations...
}

...

One "expensive calculation" can modify the variables, on which a condition will depend. It is possible, that more than one calculation is executed.

I know, that there are older or mobile GPU-s, which does not support branching at all. In that case, there is nothing to do with this issue

You've already ruled out most of the simple optimisation techniques for your case. I think you have to completely rethink and its going to be tricky. Showing what kind of expensive calculations you are doing might help further. — codetiger
I cannot avoid branching, if I do something like ray tracing, or ray marching — Iter Ator
The way modern GPGPU's work requires them to execute all branches of a given if statement in sequence (possibly causing a group of cores to idle in the meantime). It's therefore a limitation that unfortunately cannot be elimiated. As @codetiger rightfully pointed out, there's probably no easy solution here, but you should try to find one (or an approximation) nonetheless. — Bartvbl
If I write: if( condition ) { ...some code...; return; } ... other calculations ..., will the calculations be after the return statement also executed? — Iter Ator
That depends. Cores are typically organised in groups, which share execution code, but apply their operations on different pieces of data. As such, if only a single core needs to execute the else clause whereas all other course are executing the "if" clause, all cores need to wait until that one particular core is finished. The same is true for return statements. Branches can therefore leave a number of cores just idling, waiting for others to finish. As such you can't expect a speedup by just inserting a return statement. GPUs work in that sense quite differently compared to CPUs. — Bartvbl

Nicol Bolas Nicol Bolas · Accepted Answer · 2016-09-12T12:21:54

3

votes

GLSL has no mechanism to enforce branching (or to enforce flattening a branch).

How to force GLSL branching?

1 Answers