perf: better effect pruning (#16625)

* tweak

* prune effects where possible

* tweak

* simplify

* simplify

* changeset

* reset parent if necessary

.changeset/tasty-lizards-care.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: prune effects without dependencies

packages/svelte/src/internal/client/reactivity/effects.js

@@ -133,19 +133,29 @@ function create_effect(type, fn, sync, push = true) {
 		schedule_effect(effect);
 	}
 
-	// if an effect has no dependencies, no DOM and no teardown function,
-	// don't bother adding it to the effect tree
-	var inert =
+	if (push) {
+		/** @type {Effect | null} */
+		var e = effect;
+
+		// if an effect has already ran and doesn't need to be kept in the tree
+		// (because it won't re-run, has no DOM, and has no teardown etc)
+		// then we skip it and go to its child (if any)
+		if (
 			sync &&
-		effect.deps === null &&
-		effect.first === null &&
-		effect.nodes_start === null &&
-		effect.teardown === null &&
-		(effect.f & EFFECT_PRESERVED) === 0;
+			e.deps === null &&
+			e.teardown === null &&
+			e.nodes_start === null &&
+			e.first === e.last && // either `null`, or a singular child
+			(e.f & EFFECT_PRESERVED) === 0
+		) {
+			e = e.first;
+		}
+
+		if (e !== null) {
+			e.parent = parent;
 
-	if (!inert && push) {
 			if (parent !== null) {
-			push_effect(effect, parent);
+				push_effect(e, parent);
 			}
 
 			// if we're in a derived, add the effect there too
@@ -155,7 +165,8 @@ function create_effect(type, fn, sync, push = true) {
 				(type & ROOT_EFFECT) === 0
 			) {
 				var derived = /** @type {Derived} */ (active_reaction);
-			(derived.effects ??= []).push(effect);
+				(derived.effects ??= []).push(e);
+			}
 		}
 	}
 
@@ -242,7 +253,7 @@ export function inspect_effect(fn) {
  */
 export function effect_root(fn) {
 	Batch.ensure();
-	const effect = create_effect(ROOT_EFFECT, fn, true);
+	const effect = create_effect(ROOT_EFFECT | EFFECT_PRESERVED, fn, true);
 
 	return () => {
 		destroy_effect(effect);
@@ -256,7 +267,7 @@ export function effect_root(fn) {
  */
 export function component_root(fn) {
 	Batch.ensure();
-	const effect = create_effect(ROOT_EFFECT, fn, true);
+	const effect = create_effect(ROOT_EFFECT | EFFECT_PRESERVED, fn, true);
 
 	return (options = {}) => {
 		return new Promise((fulfil) => {
@@ -375,7 +386,7 @@ export function block(fn, flags = 0) {
  * @param {boolean} [push]
  */
 export function branch(fn, push = true) {
-	return create_effect(BRANCH_EFFECT, fn, true, push);
+	return create_effect(BRANCH_EFFECT | EFFECT_PRESERVED, fn, true, push);
 }
 
 /**