Any fast alternative to sine in GLSL?

Question

Should I just use the built-in sin() function or my custom sine function?
I'm concerned about performance here. I don't care about accuracy much here because I use it to just get wave effects in my shader.

The shader is for mobile platforms (OpenGL ES)

Here's my code

float customSin(float x){
   x = fract( 0.75 + x*0.159155 )*2.0 - 1.0; 
   return x*x * (6.0 - 4.0*x) - 1.0;
}

Here's my algorithm

x is the input, y is the output

r = 0.75 + x/(π*2)
a = | r- floor(r) - 0.5 |*2
y = ( 3(a^2) - 2(a^3) )*2 - 1

I checked the graph, and it's pretty close to the actual sine.

Which one should I go with?

Answer 1

ATI made an announcement that the sine / cosine instructions in their newest GPUs now requires only a single clock cycle... in 2004! Modern GPUs in fact have special function units specifically for calculating trigonometric functions.

So any hand-written sine or cosine function is unlikely to perform any better than the intrinsic sin() / cos() function, no matter how many accuracy concessions you make. They are just insanely optimized in hardware.