perf: skip repeatedly traversing the same derived

We introduce a new flag which marks a derived during the mark_reaction phase and lift it during the execution phase.

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

@@ -1,3 +1,4 @@
+// General flags
 export const DERIVED = 1 << 1;
 export const EFFECT = 1 << 2;
 export const RENDER_EFFECT = 1 << 3;
@@ -5,13 +6,13 @@ export const BLOCK_EFFECT = 1 << 4;
 export const BRANCH_EFFECT = 1 << 5;
 export const ROOT_EFFECT = 1 << 6;
 export const BOUNDARY_EFFECT = 1 << 7;
-export const UNOWNED = 1 << 8;
-export const DISCONNECTED = 1 << 9;
 export const CLEAN = 1 << 10;
 export const DIRTY = 1 << 11;
 export const MAYBE_DIRTY = 1 << 12;
 export const INERT = 1 << 13;
 export const DESTROYED = 1 << 14;
+
+// Flags exclusive to effects
 export const EFFECT_RAN = 1 << 15;
 /** 'Transparent' effects do not create a transition boundary */
 export const EFFECT_TRANSPARENT = 1 << 16;
@@ -20,6 +21,11 @@ export const HEAD_EFFECT = 1 << 18;
 export const EFFECT_PRESERVED = 1 << 19;
 export const USER_EFFECT = 1 << 20;
 
+// Flags exclusive to deriveds
+export const UNOWNED = 1 << 8;
+export const DISCONNECTED = 1 << 9;
+export const WAS_MARKED = 1 << 15;
+
 // Flags used for async
 export const REACTION_IS_UPDATING = 1 << 21;
 export const ASYNC = 1 << 22;

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

@@ -10,7 +10,8 @@ import {
 	MAYBE_DIRTY,
 	STALE_REACTION,
 	UNOWNED,
-	ASYNC
+	ASYNC,
+	WAS_MARKED
 } from '#client/constants';
 import {
 	active_reaction,
@@ -326,6 +327,7 @@ export function execute_derived(derived) {
 
 			stack.push(derived);
 
+			derived.f &= ~WAS_MARKED;
 			destroy_derived_effects(derived);
 			value = update_reaction(derived);
 		} finally {
@@ -335,6 +337,7 @@ 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

@@ -27,7 +27,8 @@ import {
 	MAYBE_DIRTY,
 	BLOCK_EFFECT,
 	ROOT_EFFECT,
-	ASYNC
+	ASYNC,
+	WAS_MARKED
 } from '#client/constants';
 import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
@@ -331,7 +332,8 @@ function mark_reactions(signal, status) {
 			set_signal_status(reaction, status);
 		}
 
-		if ((flags & DERIVED) !== 0) {
+		if ((flags & DERIVED) !== 0 && (flags & WAS_MARKED) === 0) {
+			reaction.f |= WAS_MARKED;
 			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
 		} else if (not_dirty) {
 			if ((flags & BLOCK_EFFECT) !== 0) {

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

@@ -20,7 +20,8 @@ import {
 	DISCONNECTED,
 	REACTION_IS_UPDATING,
 	STALE_REACTION,
-	ERROR_VALUE
+	ERROR_VALUE,
+	WAS_MARKED
 } from './constants.js';
 import { old_values } from './reactivity/sources.js';
 import {
@@ -161,6 +162,10 @@ export function is_dirty(reaction) {
 		var dependencies = reaction.deps;
 		var is_unowned = (flags & UNOWNED) !== 0;
 
+		if (flags & DERIVED) {
+			reaction.f &= ~WAS_MARKED;
+		}
+
 		if (dependencies !== null) {
 			var i;
 			var dependency;