TSL: Fix double expressions.

[Mugen87]

Fixed #32589.

Description

I'm not sure this is the best fix but it seems calling toStack() is only required in the method chaining case e.g. when using .discard(). In all other cases, the call can be removed.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.36 84.47	355.36 84.47	+0 B +0 B
WebGPU	621.49 172.57	621.47 172.58	-24 B +8 B
WebGPU Nodes	620.1 172.32	620.07 172.33	-24 B +8 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	487.23 119.23	487.23 119.23	+0 B +0 B
WebGPU	692.41 187.81	692.41 187.81	-2 B +1 B
WebGPU Nodes	642.21 175.03	642.21 175.04	-2 B +6 B

[Mugen87]

Unfortunately, statements like break are now missing at other places. I don't understand yet why the behavior differs. Closing for now as it seems the fix must be done elsewhere, potentially in deeper parts of TSL.

[mrdoob]

Claude (after 3 tries) suggested doing this... 👀

diff --git a/src/nodes/code/ExpressionNode.js b/src/nodes/code/ExpressionNode.js
--- a/src/nodes/code/ExpressionNode.js
+++ b/src/nodes/code/ExpressionNode.js
@@ -38,7 +38,19 @@ class ExpressionNode extends Node {
 
 		if ( type === 'void' ) {
 
-			builder.addLineFlowCode( snippet, this );
+			// Prevent duplicate generation of void expressions (e.g., discard, return, break, continue)
+			// This can happen when the same expression is both in stack.nodes (via .toStack())
+			// and stack.outputNode (returned from a callback like If(cond, () => Discard()))
+
+			const nodeData = builder.getDataFromNode( this, builder.shaderStage );
+
+			if ( nodeData.generated !== true ) {
+
+				nodeData.generated = true;
+
+				builder.addLineFlowCode( snippet, this );
+
+			}
 
 		} else {

/cc @sunag

[sunag]

What makes TSL a middle ground between a shading language and a node system are the stacks; when added a node to the stack, it's equivalent to writing a line of code, while a pure node system creates a dynamic and independent flow.

The problem with the code below is that when we use () => Discard() we are saying that the conditional is returning a value:

If(diffuseColor.a.mul(alphaFactor).lessThanEqual(0.1337), () => Discard())

Which is the same as:

If(diffuseColor.a.mul(alphaFactor).lessThanEqual(0.1337), () => {

	return Discard();

} )

The correct approach would be to not return a value but using void syntax, like the example below:

If(diffuseColor.a.mul(alphaFactor).lessThanEqual(0.1337), () => void Discard())

Claude (after 3 tries) suggested doing this... 👀

I never cease to be amazed by AI. Certainly an approach we use for caching and avoid repetition. My only question is whether this should be linked to the stack and not necessarily to the node expression, because we might have other void nodes that could present an equivalent problem.