GLSL/HLSL - Multiple single line conditional statements as opposed to single block

Question

Doing some research into antialiasing methods, I stumbled upon this piece of code (grabbed from Nvidia's FXAA shader):

if(!pairN) lumaN = lumaS; if(!pairN) gradientN = gradientS; if(!pairN) lengthSign *= -1.0;

Is there a good reason it's not written as the following instead?

if (!pairN) { lumaN = lumaS; gradientN = gradientS; lengthSign *= -1.0; }

I assume it's an optimization of some sort? Just seems really counterintuitive...

Sergey K. · Accepted Answer · 2014-03-07 16:03:48Z

It is trivial for a GLSL compiler to convert this code:

if(!pairN) lumaN = lumaS; if(!pairN) gradientN = gradientS; if(!pairN) lengthSign *= -1.0;

into this branchless code:

lumaN = pair ? lumaN : lumaS; gradientN = pair ? gradientN : gradientS; lengthSign *= pair ? 1.0 : -1.0;

It is branchless in the sense it can be expressed as:

float a = float( pair ); float b = 1.0 - a; lumaN = a * lumaN + b * lumaS; gradientN = a * gradientN + b * gradientS; lengthSign *= a * 1.0 + b * -1.0;

This is somewhat efficient optimization on mobile GPUs.

Automatically optimizing an entire if block is not so trivial.

Would modern GPU compilers not optimize the intuitive version then?
It looks like they are unable to optimize a large if block.
@skiwi If it's one thing I've learned about GLSL implementations over the years... the compiler does a lot of unexpected stuff. Some optimizations blow my mind and others I can hardly believe were missed. Trying both ways and looking at the intermediate output via glGetProgramBinary is as good as you can get.

Collectives™ on Stack Overflow

GLSL/HLSL - Multiple single line conditional statements as opposed to single block

1 Answer 1

3 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Related