perf: optimize compiler analysis phase (#17823)

## Summary

Two optimizations to the compiler's analysis phase:

- **Cache `ignore_stack` snapshots instead of `structuredClone` on every
node.** The universal `_` visitor in the analysis walk runs on every AST
node and calls `structuredClone(ignore_stack)` each time. In practice,
`svelte-ignore` comments are rare (0–5 per component), so 99%+ of nodes
deep-clone an unchanged stack. This adds a copy-on-write cache that only
re-creates the snapshot when `push_ignore`/`pop_ignore` actually change
the stack.

- **Walk the CSS stylesheet once instead of once per element.**
`prune()` was called in a loop for each element, each time doing a full
`walk()` of the stylesheet AST. This restructures the loop so the
stylesheet is walked once, and the element iteration happens inside the
`ComplexSelector` visitor.

## Benchmarks

Compiled each component 500 times (after 50 warmup iterations),
measuring average time per `compile()` call:

| Component | Before | After | Speedup |
|---|---|---|---|
| `has` (80+ CSS selectors, 12 elements) | 3.405 ms | 2.680 ms | **21%
faster** |
| `siblings-combinator-each-nested` (65 CSS rules, 15 elements) | 2.034
ms | 1.575 ms | **23% faster** |
| synthetic (100 CSS rules, 50 elements) | 10.099 ms | 4.564 ms | **55%
faster** |

The CSS pruning optimization scales with `elements × CSS rules` — the
more elements a component has, the bigger the win since we go from N
stylesheet walks down to 1. The `structuredClone` fix helps every
component regardless of CSS, eliminating ~500–2000 deep clones per
compile (one per AST node) and replacing them with 0–5 (one per
`svelte-ignore` comment).

For typical real-world components with 10–20 elements and some CSS,
expect roughly **20–30% faster compilation** in the analysis phase.

## Test plan

- [x] Full test suite passes (7329 tests, 0 failures)
- [x] CSS pruning tests pass (selector matching, scoping, unused rule
detection)
- [x] `svelte-ignore` behavior unchanged (snapshot is consumed read-only
via `.has()`/`.some()`)


🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.changeset/tidy-brooms-train.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+perf: optimize compiler analysis phase

packages/svelte/src/compiler/phases/2-analyze/css/css-prune.js

@@ -125,9 +125,9 @@ const seen = new Set();
 /**
  *
  * @param {Compiler.AST.CSS.StyleSheet} stylesheet
- * @param {Compiler.AST.RegularElement | Compiler.AST.SvelteElement} element
+ * @param {Iterable<Compiler.AST.RegularElement | Compiler.AST.SvelteElement>} elements
  */
-export function prune(stylesheet, element) {
+export function prune(stylesheet, elements) {
 	walk(/** @type {Compiler.AST.CSS.Node} */ (stylesheet), null, {
 		Rule(node, context) {
 			if (node.metadata.is_global_block) {
@@ -139,17 +139,19 @@ export function prune(stylesheet, element) {
 		ComplexSelector(node) {
 			const selectors = get_relative_selectors(node);
 
-			seen.clear();
+			for (const element of elements) {
+				seen.clear();
 
-			if (
-				apply_selector(
-					selectors,
-					/** @type {Compiler.AST.CSS.Rule} */ (node.metadata.rule),
-					element,
-					BACKWARD
-				)
-			) {
-				node.metadata.used = true;
+				if (
+					apply_selector(
+						selectors,
+						/** @type {Compiler.AST.CSS.Rule} */ (node.metadata.rule),
+						element,
+						BACKWARD
+					)
+				) {
+					node.metadata.used = true;
+				}
 			}
 
 			// note: we don't call context.next() here, we only recurse into

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -21,7 +21,7 @@ import { prune } from './css/css-prune.js';
 import { hash, is_rune } from '../../../utils.js';
 import { warn_unused } from './css/css-warn.js';
 import { extract_svelte_ignore } from '../../utils/extract_svelte_ignore.js';
-import { ignore_map, ignore_stack, pop_ignore, push_ignore } from '../../state.js';
+import { ignore_map, get_ignore_snapshot, pop_ignore, push_ignore } from '../../state.js';
 import { ArrowFunctionExpression } from './visitors/ArrowFunctionExpression.js';
 import { AssignmentExpression } from './visitors/AssignmentExpression.js';
 import { AnimateDirective } from './visitors/AnimateDirective.js';
@@ -134,7 +134,7 @@ const visitors = {
 			push_ignore(ignores);
 		}
 
-		ignore_map.set(node, structuredClone(ignore_stack));
+		ignore_map.set(node, get_ignore_snapshot());
 
 		const scope = state.scopes.get(node);
 		next(scope !== undefined && scope !== state.scope ? { ...state, scope } : state);
@@ -856,9 +856,7 @@ export function analyze_component(root, source, options) {
 		analyze_css(analysis.css.ast, analysis);
 
 		// mark nodes as scoped/unused/empty etc
-		for (const node of analysis.elements) {
-			prune(analysis.css.ast, node);
-		}
+		prune(analysis.css.ast, analysis.elements);
 
 		const { comment } = analysis.css.ast.content;
 		const should_ignore_unused =

packages/svelte/src/compiler/state.js

@@ -82,16 +82,38 @@ export let ignore_stack = [];
  */
 export let ignore_map = new Map();
 
+/**
+ * Cached snapshot of the ignore_stack. Only re-created when the stack changes
+ * (i.e. when push_ignore or pop_ignore is called), avoiding a structuredClone
+ * on every node visit during analysis.
+ * @type {Set<string>[] | null}
+ */
+let cached_ignore_snapshot = null;
+
+/**
+ * Returns a snapshot of the current ignore_stack, reusing a cached copy
+ * when the stack hasn't changed since the last call.
+ * @returns {Set<string>[]}
+ */
+export function get_ignore_snapshot() {
+	if (cached_ignore_snapshot === null) {
+		cached_ignore_snapshot = ignore_stack.map((s) => new Set(s));
+	}
+	return cached_ignore_snapshot;
+}
+
 /**
  * @param {string[]} ignores
  */
 export function push_ignore(ignores) {
 	const next = new Set([...(ignore_stack.at(-1) || []), ...ignores]);
 	ignore_stack.push(next);
+	cached_ignore_snapshot = null;
 }
 
 export function pop_ignore() {
 	ignore_stack.pop();
+	cached_ignore_snapshot = null;
 }
 
 /**
@@ -141,4 +163,5 @@ export function adjust(state) {
 
 	ignore_stack = [];
 	ignore_map.clear();
+	cached_ignore_snapshot = null;
 }