Prevent loop unrolling in shader program when using ANGLE

Question

in my WEBGL shader I am using loop of unknown length (on compile).

    do {
        sample = texture(uTex, posXY).a;
        accumulated += aSample * uAMultiplier;
    } while (accumulated < 0.8);

This works as expected on OpenGL browsers, but by default windows chrome/ff uses angle, that unrolls loops, which isn't possible in this case, causing compile errors.

Error: Error compiling generate
Error: Cannot link program
Info log:
C:\fakepath(110,28-106): error X4014: cannot have gradient operations inside loops with divergent flow control

C:\fakepath(59,12-96): warning X3570: gradient instruction used in a loop with varying iteration, forcing loop to unroll
C:\fakepath(118,1-82): error X3511: unable to unroll loop, loop does not appear to terminate in a timely manner (1024 iterations)

Is there a solution to prevent unrolling or otherwise bypass having to loop?

Answer 1

You can try to declare a static loop with an exit. For example

float accumulated = 0.0;
#define ITERATION_LIMIT 100;
for (int i = 0; i < ITERATION_LIMIT; ++i) {
  sample = texture(uTex, posXY).a;
  accumulated += aSample * uAMultiplier;
  if (accumulated >= 0.8) {
    break;
  }
}

Of course it's best to pick a reasonable number for the limit