fix: remove `WAS_MARKED` flag in favor of `Set` (#18127)

Removes the `WAS_MARKED` logic in favor of a simple `Set` heuristic: If
`mark_reactions` goes beyond a certain count, initialize it, otherwise
keep it `null`. Balances the common case of not having many transitive
dependencies with the edge case of cyclic dependencies as was seen in
#16658 (I rechecked that reproduction and it remains fast with this
change).

I ran the benchmark against this and it doesn't hit the `Seen` heuristic
once (i.e. even the benchmark doesn't have this extreme level of
dependencies where it becomes noticeable).

Fixes #18123, supersedes #18124

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/grumpy-pens-press.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: remove `WAS_MARKED` flag in favor of `Set`

benchmarking/benchmarks/reactivity/index.js

@@ -13,7 +13,7 @@ import {
 	sbench_create_4to1,
 	sbench_create_signals
 } from './sbench.js';
-import { fileURLToPath } from 'node:url';
+import { fileURLToPath, pathToFileURL } from 'node:url';
 import { create_test } from './util.js';
 
 // This benchmark has been adapted from the js-reactivity-benchmark (https://github.com/milomg/js-reactivity-benchmark)
@@ -39,7 +39,8 @@ for (const file of fs.readdirSync(`${dirname}/tests`)) {
 
 	const name = file.replace('.bench.js', '');
 
-	const module = await import(`${dirname}/tests/${file}`);
+	const module_url = pathToFileURL(path.join(dirname, 'tests', file));
+	const module = await import(module_url.href);
 	const { owned, unowned } = create_test(name, module.default);
 
 	reactivity_benchmarks.push(owned, unowned);

packages/svelte/src/internal/client/constants.js

@@ -51,15 +51,6 @@ export const EFFECT_PRESERVED = 1 << 19;
 export const USER_EFFECT = 1 << 20;
 export const EFFECT_OFFSCREEN = 1 << 25;
 
-// Flags exclusive to deriveds
-/**
- * Tells that we marked this derived and its reactions as visited during the "mark as (maybe) dirty"-phase.
- * Will be lifted during execution of the derived and during checking its dirty state (both are necessary
- * because a derived might be checked but not executed). This is a pure performance optimization flag and
- * should not be used for any other purpose!
- */
-export const WAS_MARKED = 1 << 16;
-
 // Flags used for async
 export const REACTION_IS_UPDATING = 1 << 21;
 export const ASYNC = 1 << 22;

packages/svelte/src/internal/client/dev/debug.js

@@ -15,7 +15,6 @@ import {
 	MAYBE_DIRTY,
 	RENDER_EFFECT,
 	ROOT_EFFECT,
-	WAS_MARKED,
 	MANAGED_EFFECT
 } from '#client/constants';
 import { snapshot } from '../../shared/clone.js';
@@ -204,7 +203,6 @@ export function log_reactions(signal) {
 		if ((flags & DIRTY) !== 0) names.push('DIRTY');
 		if ((flags & MAYBE_DIRTY) !== 0) names.push('MAYBE_DIRTY');
 		if ((flags & CONNECTED) !== 0) names.push('CONNECTED');
-		if ((flags & WAS_MARKED) !== 0) names.push('WAS_MARKED');
 		if ((flags & INERT) !== 0) names.push('INERT');
 		if ((flags & DESTROYED) !== 0) names.push('DESTROYED');
 

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

@@ -9,7 +9,6 @@ import {
 	EFFECT_PRESERVED,
 	STALE_REACTION,
 	ASYNC,
-	WAS_MARKED,
 	DESTROYED,
 	CLEAN,
 	REACTION_RAN,
@@ -365,7 +364,6 @@ export function execute_derived(derived) {
 
 			stack.push(derived);
 
-			derived.f &= ~WAS_MARKED;
 			destroy_derived_effects(derived);
 			value = update_reaction(derived);
 		} finally {
@@ -375,7 +373,6 @@ export function execute_derived(derived) {
 		}
 	} else {
 		try {
-			derived.f &= ~WAS_MARKED;
 			destroy_derived_effects(derived);
 			value = update_reaction(derived);
 		} finally {

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

@@ -25,10 +25,7 @@ import {
 	MAYBE_DIRTY,
 	BLOCK_EFFECT,
 	ROOT_EFFECT,
-	ASYNC,
+	ASYNC
-	WAS_MARKED,
-	CONNECTED,
-	REACTION_IS_UPDATING
 } from '#client/constants';
 import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
@@ -171,6 +168,17 @@ export function set(source, value, should_proxy = false) {
 	return internal_set(source, new_value, legacy_updates);
 }
 
+/**
+ * A set of signals we have already seen while traversing in mark_reactions.
+ * Not always set to balance the common case of sources only having a couple
+ * of (transitive) dependencies (where always creating a Set would be bad for perf)
+ * with the edge case of extremely deep or wide dependency arrays with cycles.
+ * @type {Set<any> | null}
+ */
+var seen = null;
+/** Number of transitive dependencies, see {@link seen} for more info */
+var count_deps = 0;
+
 /**
  * @template V
  * @param {Source<V>} source
@@ -240,7 +248,10 @@ export function internal_set(source, value, updated_during_traversal = null) {
 
 		// For debugging, in case you want to know which reactions are being scheduled:
 		// log_reactions(source);
+		seen = null;
+		count_deps = 0;
 		mark_reactions(source, DIRTY, updated_during_traversal);
+		seen = null;
 
 		// It's possible that the current reaction might not have up-to-date dependencies
 		// whilst it's actively running. So in the case of ensuring it registers the reaction
@@ -347,6 +358,18 @@ function mark_reactions(signal, status, updated_during_traversal) {
 	var runes = is_runes();
 	var length = reactions.length;
 
+	count_deps += length;
+	// Activate the `seen` Set if we think from the unusually high number of deps that
+	// there might be cycles in the graph, to avoid repeated lookups for reactions
+	// Example: https://github.com/sveltejs/svelte/issues/16658 has a graph with one source
+	// reaching ~10000 distinct deriveds/effects each, resulting in 65 million walks through repeated visits.
+	if (count_deps > 100000 && seen === null) seen = new Set();
+
+	if (seen !== null) {
+		if (seen.has(signal)) return;
+		seen.add(signal);
+	}
+
 	for (var i = 0; i < length; i++) {
 		var reaction = reactions[i];
 		var flags = reaction.f;
@@ -370,18 +393,7 @@ function mark_reactions(signal, status, updated_during_traversal) {
 			var derived = /** @type {Derived} */ (reaction);
 
 			batch_values?.delete(derived);
-
+			mark_reactions(derived, MAYBE_DIRTY, updated_during_traversal);
-			if ((flags & WAS_MARKED) === 0) {
-				// Only connected deriveds being executed outside the update cycle can be reliably unmarked right away
-				if (
-					flags & CONNECTED &&
-					(active_effect === null || (active_effect.f & REACTION_IS_UPDATING) === 0)
-				) {
-					reaction.f |= WAS_MARKED;
-				}
-
-				mark_reactions(derived, MAYBE_DIRTY, updated_during_traversal);
-			}
 		} else if (not_dirty) {
 			var effect = /** @type {Effect} */ (reaction);
 

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

@@ -1,24 +1,7 @@
-/** @import { Derived, Effect, Value } from '#client' */
+/** @import { Effect } from '#client' */
-import { CLEAN, DERIVED, DIRTY, MAYBE_DIRTY, WAS_MARKED } from '#client/constants';
+import { CLEAN, DIRTY, MAYBE_DIRTY } from '#client/constants';
 import { set_signal_status } from './status.js';
 
-/**
- * @param {Value[] | null} deps
- */
-function clear_marked(deps) {
-	if (deps === null) return;
-
-	for (const dep of deps) {
-		if ((dep.f & DERIVED) === 0 || (dep.f & WAS_MARKED) === 0) {
-			continue;
-		}
-
-		dep.f ^= WAS_MARKED;
-
-		clear_marked(/** @type {Derived} */ (dep).deps);
-	}
-}
-
 /**
  * @param {Effect} effect
  * @param {Set<Effect>} dirty_effects
@@ -31,10 +14,6 @@ export function defer_effect(effect, dirty_effects, maybe_dirty_effects) {
 		maybe_dirty_effects.add(effect);
 	}
 
-	// Since we're not executing these effects now, we need to clear any WAS_MARKED flags
-	// so that other batches can correctly reach these effects during their own traversal
-	clear_marked(effect.deps);
-
 	// mark as clean so they get scheduled if they depend on pending async state
 	set_signal_status(effect, CLEAN);
 }

packages/svelte/src/internal/client/runtime.js

@@ -21,7 +21,6 @@ import {
 	REACTION_IS_UPDATING,
 	STALE_REACTION,
 	ERROR_VALUE,
-	WAS_MARKED,
 	MANAGED_EFFECT,
 	REACTION_RAN
 } from './constants.js';
@@ -164,10 +163,6 @@ export function is_dirty(reaction) {
 		return true;
 	}
 
-	if (flags & DERIVED) {
-		reaction.f &= ~WAS_MARKED;
-	}
-
 	if ((flags & MAYBE_DIRTY) !== 0) {
 		var dependencies = /** @type {Value[]} */ (reaction.deps);
 		var length = dependencies.length;
@@ -408,11 +403,8 @@ function remove_reaction(signal, dependency) {
 	) {
 		var derived = /** @type {Derived} */ (dependency);
 
-		// If we are working with a derived that is owned by an effect, then mark it as being
-		// disconnected and remove the mark flag, as it cannot be reliably removed otherwise
 		if ((derived.f & CONNECTED) !== 0) {
 			derived.f ^= CONNECTED;
-			derived.f &= ~WAS_MARKED;
 		}
 
 		// In a fork it's possible that a derived is executed and gets reactions, then commits, but is