Description

Related: #30759

A computed node created outside the loop and used multiple times within it is not stored in a temporary variable. In other cases, such nodes are stored in a temporary variable if they are used multiple times.

With #31459, we don't have to explicitly call toVar() before assigning to a variable, and the number of uses is automatically tracked by TSL so we usually don't have to use toVar() to create a mutable node, or look for multiple uses and add caching statements by calling toVar() or toConst(). But with the behavior above, and in a fairly complex node graph, it's very hard to track which nodes rely on a node created outside of the loop, so I'm inclined to add toVar() or toConst() for a safety reason when I cannot afford to debug the transpiled code. But doing this obscures the intent of the code and take less advantage of the automatic optimization.

Example

// Some expensive function:
const g = Fn(() => {
  const sum = vec3(0)
  Loop({ start: 0, end: 10_000, condition: '<' }, () => {
    sum.addAssign(0.0001)
  })
  return sum
})

// A loop relies on the result of the above function.
const f = Fn(() => {
  const t = g()
  const sum = vec3(0)
  Loop({ start: 0, end: 10, condition: '<' }, () => {
    sum.addAssign(t)
  })
  return sum
})

outoutNode = f()

The above outoutNode is transpiled as:

// vars

var nodeVar0 : vec3<f32>;
var nodeVar1 : vec3<f32>;

// flow
// code

nodeVar0 = vec3<f32>( 0.0, 0.0, 0.0 );

for ( var i : i32 = 0; i < 10; i ++ ) {

	nodeVar1 = vec3<f32>( 0.0, 0.0, 0.0 );

	for ( var i : i32 = 0; i < 10000; i ++ ) {

		nodeVar1 = ( nodeVar1 + vec3<f32>( 0.0001 ) );

	}

	nodeVar0 = ( nodeVar0 + nodeVar1 );

}

// result

output.color = vec4<f32>( nodeVar0, 1.0 );

Calling toVar() or toConst() on t creates a temporary variable outside of the loop, as stated before.

If we have another loop, the latter loop uses the result in the previous loop, which is also a weird behavior:

const f = Fn(() => {
  const t = g()
  const sum = vec3(0)
  Loop({ start: 0, end: 10, condition: '<' }, () => {
    sum.addAssign(t)
  })
  Loop({ start: 0, end: 10, condition: '<' }, () => {
    sum.addAssign(t)
  })
  return sum
})

This is transpiled as:

var nodeVar0 : vec3<f32>;
var nodeVar1 : vec3<f32>;

// flow
// code

nodeVar0 = vec3<f32>( 0.0, 0.0, 0.0 );

for ( var i : i32 = 0; i < 10; i ++ ) {

  nodeVar1 = vec3<f32>( 0.0, 0.0, 0.0 );

  for ( var i : i32 = 0; i < 10000; i ++ ) {

    nodeVar1 = ( nodeVar1 + vec3<f32>( 0.0001 ) );

  }

  nodeVar0 = ( nodeVar0 + nodeVar1 );

}

for ( var i : i32 = 0; i < 10; i ++ ) {

  nodeVar0 = ( nodeVar0 + nodeVar1 );

}

// result

output.color = vec4<f32>( nodeVar0, 1.0 );

Solution

If a node used in a loop satisfies the following conditions, for example:

• It is not a variable or constant node.
• It has no dependencies on the nodes in the loop stack.

then assume it may be used multiple times, and store it in a temporary variable outside of the loop stack.

Alternatives

Add documentation on the limitation about the automatic optimization.

Additional context

No response