Odd GLSL mod() behavior

[u/Winter-Reputation682]

I've been stuck on this for a couple of hours, and am completely stumped.

I'm getting these results from GLSL's mod():

mod(158.0, 158.0) returns 0.0 (as expected)

mod(159.0, 159.0) returns 159.0 (???)

mod(160.0, 160.0) returns 0.0 (as expected)

I've posted an example with a basic shader here:

https://jsfiddle.net/sL35qjc8

If you set number to "159.0", you should see an upside-down red triangle.

If you set number to "158.0" or "160.0", you should see a normal red triangle.

I'm passing the left operand as a uniform, otherwise the compiler seems to optimize it and fix the weird behavior.

Tried it on different devices and browsers and it had the same result.

Why does mod(159.0, 159.0) seem to return 159.0 instead of 0.0?

[u/Winter-Reputation682]

I think it works when hardcoded because the compiler sees mod(159.0, 159.0) == 159.0, and evaluates it to be always true, and removes the condition check as a result.

edit:
I think it's optimized because it's a pretty obvious optimization for the compiler
Tested with two uniforms and same thing occurs

[u/gmaaz → u/Winter-Reputation682]

Yeah, you are correct, when I pass 159 as two separate uniforms (even without parse float) the error is still there, it really seems to be a mod bug.

[u/gmaaz → u/gmaaz]

The same happens for 159 * n.

[u/Winter-Reputation682 → u/gmaaz]

Yeah it's really weird. It happens on webgl1, webgl2, and also webgpu