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.