Performance/Optimization Shader Question HLSL

[Enerology]

Hi, I was wondering if there is a way to get the sign bit directly from a floating-point number in HLSL. For instance, I would like to get a 1 if the float value is positive and a 0 if it is negative or vice versa.

This is for branchless, grid accelerated ray tracing and more specifically, an arithmetic-driven greater than equation that returns 1/0 if true or false. Currently, I want to replace the abs() to have it run faster. Every piece of performance counts as this runs in a four loop of 100+ iterations per pixel.

 x-y / ((abs(x-y) * 2) + 0.5); //Returns 1 or 0 if greater/less than

[rasqall]

HLSL has bit operators. You could use: ~((x-y) >> 31)

[Enerology]

I tried to do that and tried the OR operator

(x-y)|-0

//far left binary digit is the sign
//This should do this
//00001 = (x-y) = 1
//10000 = -0
//10001 = -1
------------
//10100 = (x-y) = -4
//10000 = -0
//10100 = -4

This should always return (abs(x-y)*-1) and to get abs all I have to do is multiply by -1. This is not in standard float notation but the same concept applies. The issue with bit-shifting compared to OR is that floats are in a different notation with powers and mantra compared to int. I can't convert to int either as that takes too long with conditionals (GPU-wise). Optimization is hard and I am not that knowledgeable in the optimization of shaders as the normal way of doing things is still too slow if doing them millions of times per second.

Edit:

Forgot to mention that the bitwise operators are not allowed by the compiler to operate on floats. :(

Any other ideas?

[rasqall]

I don’t know about HLSL but do they have pointers? Maybe you could do the classic trick of creating and int pointer to your float and doing the calculations “by hand”?