From 348844d758a631033510c67e62dafb2e2b3480ad Mon Sep 17 00:00:00 2001
From: ljodea <liam.j.odea@gmail.com>
Date: Wed, 29 Jul 2026 16:37:50 -0500
Subject: [PATCH] perf: O(1) membership in sort_const_tags topological sort
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DFS over {@const} tags used sorted.includes(node) (O(n) per check → O(n²)
overall). Track visited nodes with a Set so membership is O(1) while keeping
the same order and cycle handling.

Closes #18599
---
 .changeset/sort-const-tags-set.md                   |  5 +++++
 .../svelte/src/compiler/phases/3-transform/utils.js |  6 +++++-
 .../samples/const-tag-reverse-diamond/_config.js    | 13 +++++++++++++
 .../samples/const-tag-reverse-diamond/main.svelte   | 10 ++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/sort-const-tags-set.md
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/_config.js
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/main.svelte

diff --git a/.changeset/sort-const-tags-set.md b/.changeset/sort-const-tags-set.md
new file mode 100644
index 0000000000..2c1771fb5b
--- /dev/null
+++ b/.changeset/sort-const-tags-set.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: O(1) membership in `{@const}` topological sort (`sort_const_tags`)
diff --git a/packages/svelte/src/compiler/phases/3-transform/utils.js b/packages/svelte/src/compiler/phases/3-transform/utils.js
index c7fce61ff9..f04911822f 100644
--- a/packages/svelte/src/compiler/phases/3-transform/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/utils.js
@@ -87,9 +87,12 @@ function sort_const_tags(nodes, state) {
 	/** @type {AST.ConstTag[]} */
 	const sorted = [];
 
+	/** @type {Set<AST.ConstTag>} */
+	const seen = new Set();
+
 	/** @param {Tag} tag */
 	function add(tag) {
-		if (sorted.includes(tag.node)) {
+		if (seen.has(tag.node)) {
 			return;
 		}
 
@@ -98,6 +101,7 @@ function sort_const_tags(nodes, state) {
 			if (dep_tag) add(dep_tag);
 		}
 
+		seen.add(tag.node);
 		sorted.push(tag.node);
 	}
 
diff --git a/packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/_config.js b/packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/_config.js
new file mode 100644
index 0000000000..ed7bb52573
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/_config.js
@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+// sum is declared before left/right; sort_const_tags must emit deps first.
+// Also exercises multi-parent membership (sum → left and sum → right).
+export default test({
+	html: '<p>23</p>',
+
+	async test({ component, target, assert }) {
+		component.n = 1;
+
+		assert.htmlEqual(target.innerHTML, '<p>5</p>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/main.svelte b/packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/main.svelte
new file mode 100644
index 0000000000..ffb5191f80
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/const-tag-reverse-diamond/main.svelte
@@ -0,0 +1,10 @@
+<script>
+	export let n = 10;
+</script>
+
+{#if true}
+	{@const sum = left + right}
+	{@const left = n + 1}
+	{@const right = n + 2}
+	<p>{sum}</p>
+{/if}