[RFC] Local comptime definitions

[iwoplaza]

Main ideas:

• Being able to alias a comptime value with a name in a specific scope - a function or a block
• Being more explicit when defining default parameters, and how they should appear in the final WGSL code

Local definitions

function average() {
  'use gpu';
  const length = tgpu.comptimeValue(expensiveCalculation());
  let sum = d.f32(0);
  for (let i = 0; i < length; i++) {
    sum += sample(i);
  }
  return sum / length;
}

// ...
fn average() {
  var sum = 0f;
  for (var i = 0; i < 512i; i++) {
    sum += sample(i);
  }
  return sum / 512f;
}

Default parameters

Parameters initialized with a value wrapped in tgpu.comptimeValue would be available at comptime.

function interpolate(t: number, cyclic: boolean = tgpu.comptimeValue(false)) {
  'use gpu';
  if (cyclic) {
    return std.fract(t);
  }
  return std.saturate(t);
}

function main() {
  'use gpu';
  return interpolate(0.5);
}

fn interpolate(t: f32) -> f32 {
  return saturate(t);
}

fn main() {
  return interpolate(0.5f);
}

Parameters passed into shellless functions

I am unsure if we should allow for this, since it can change how the callee's body looks quite dramatically

function interpolate(t: number, cyclic: boolean) {
  'use gpu';
  if (cyclic) {
    return std.fract(t);
  }
  return std.saturate(t);
}

function main() {
  'use gpu';
  // The caller determines if a parameter is comptime-known
  return interpolate(0.5, tgpu.comptimeValue(false));
}

fn interpolate(t: f32) -> f32 {
  return saturate(t);
}

fn main() {
  return interpolate(0.5f);
}