perf: reuse the cached value in the `<option>`/`<select>` value guard (#18713)

One fewer evaluation of the value expression per update.

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/eighty-moons-happen.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: reuse the cached value in the `<option>`/`<select>` value guard

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -701,28 +701,32 @@ function build_element_special_value_attribute(
 	);
 
 	const evaluated = context.state.scope.evaluate(value);
-	const assignment = b.assignment('=', b.member(node_id, '__value'), value);
 
-	const set_value_assignment = b.assignment(
-		'=',
-		b.member(node_id, 'value'),
-		evaluated.is_defined ? assignment : b.logical('??', assignment, b.literal(''))
-	);
+	/** @param {Expression} value */
+	const build_update = (value) => {
+		const assignment = b.assignment('=', b.member(node_id, '__value'), value);
 
-	const update = b.stmt(
-		is_select_with_value
-			? b.sequence([
-					set_value_assignment,
-					// This ensures a one-way street to the DOM in case it's <select {value}>
-					// and not <select bind:value>. We need it in addition to $.init_select
-					// because the select value is not reflected as an attribute, so the
-					// mutation observer wouldn't notice.
-					b.call('$.select_option', node_id, value)
-				])
-			: synthetic
-				? assignment
-				: set_value_assignment
-	);
+		const set_value_assignment = b.assignment(
+			'=',
+			b.member(node_id, 'value'),
+			evaluated.is_defined ? assignment : b.logical('??', assignment, b.literal(''))
+		);
+
+		return b.stmt(
+			is_select_with_value
+				? b.sequence([
+						set_value_assignment,
+						// This ensures a one-way street to the DOM in case it's <select {value}>
+						// and not <select bind:value>. We need it in addition to $.init_select
+						// because the select value is not reflected as an attribute, so the
+						// mutation observer wouldn't notice.
+						b.call('$.select_option', node_id, value)
+					])
+				: synthetic
+					? assignment
+					: set_value_assignment
+		);
+	};
 
 	if (has_state) {
 		const id = b.id(state.scope.generate(`${node_id.name}_value`));
@@ -733,9 +737,14 @@ function build_element_special_value_attribute(
 		const init = element === 'option' ? b.object([]) : undefined;
 
 		state.init.push(b.var(id, init));
-		state.update.push(b.if(b.binary('!==', id, b.assignment('=', id, value)), b.block([update])));
+
+		// the guard already evaluated `value` into `id`, so read that back rather than
+		// evaluating the same expression (and its signal reads) a second time
+		state.update.push(
+			b.if(b.binary('!==', id, b.assignment('=', id, value)), b.block([build_update(id)]))
+		);
 	} else {
-		state.init.push(update);
+		state.init.push(build_update(value));
 	}
 
 	if (is_select_with_value) {

packages/svelte/tests/snapshot/samples/select-with-rich-content/_expected/client/index.svelte.js

@@ -74,7 +74,7 @@ export default function Select_with_rich_content($$anchor) {
 			$.set_text(text, $.get(item));
 
 			if (option_5_value !== (option_5_value = $.get(item))) {
-				option_5.__value = $.get(item);
+				option_5.__value = option_5_value;
 			}
 		});
 
@@ -137,7 +137,7 @@ export default function Select_with_rich_content($$anchor) {
 			$.set_text(text_1, $.get(x));
 
 			if (option_8_value !== (option_8_value = $.get(x))) {
-				option_8.__value = $.get(x);
+				option_8.__value = option_8_value;
 			}
 		});
 
@@ -175,7 +175,7 @@ export default function Select_with_rich_content($$anchor) {
 			$.set_text(text_2, $.get(item));
 
 			if (option_10_value !== (option_10_value = $.get(item))) {
-				option_10.__value = $.get(item);
+				option_10.__value = option_10_value;
 			}
 		});
 
@@ -240,7 +240,7 @@ export default function Select_with_rich_content($$anchor) {
 					$.set_text(text_4, $.get(item));
 
 					if (option_13_value !== (option_13_value = $.get(item))) {
-						option_13.__value = $.get(item);
+						option_13.__value = option_13_value;
 					}
 				});