[p5.js 2.0+ Bug Report]: p5.strands loop-carried variables can read values overwritten earlier in the same iteration #9377

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• WebGPU
• p5.strands
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

p5.js version

2.4.0-rc.5

Web browser and version

Google Chrome 152

Operating system

Fedora Linux

Steps to reproduce this

Steps

1. Use two loop-carried values inside a p5.strands hook.
2. Swap them inside a for loop.
3. Use the result for the output color.

filterColor.begin();

let a = 0.25;
let b = 0.75;

for (let i = 0; i < 1; i++) {
  const t = a;
  a = b;
  b = t;
}

filterColor.set([a, b, 0, 1]);
filterColor.end();

Expected

a = 0.75
b = 0.25

roughly [191, 64, 0].

Actual

a = 0.75
b = 0.75

roughly [191, 191, 0], with no error.

I can reproduce the same ordering issue with rotations and Fibonacci-style updates. It happens on WebGL and WebGPU on 2.4.0-rc.5, current main (8831d5baf), and 2.3.4.

The ordering is already present before backend-specific code generation, so this seems to be in the loop-carried state handling.

I'd be happy to work on a fix if this is confirmed.