add comments to explain perf sensitive code

src/compiler/preprocess/index.ts

@@ -149,16 +149,16 @@ export default async function preprocess(
 			filename
 		});
 
-		if (processed) {
+		if (!processed) continue;
-			if (processed.dependencies) dependencies.push(...processed.dependencies);
+
-			source = processed.code;
+		if (processed.dependencies) dependencies.push(...processed.dependencies);
-			if (processed.map) {
+		source = processed.code;
-				sourcemap_list.unshift(
+		if (processed.map) {
-					typeof(processed.map) === 'string'
+			sourcemap_list.unshift(
-						? JSON.parse(processed.map)
+				typeof(processed.map) === 'string'
-						: processed.map
+					? JSON.parse(processed.map)
-				);
+					: processed.map
-			}
+			);
 		}
 	}
 

src/compiler/utils/string_with_sourcemap.ts

@@ -59,6 +59,8 @@ function merge_tables<T>(this_table: T[], other_table: T[]): [T[], number[], boo
 }
 
 function pushArray<T>(_this: T[], other: T[]) {
+	// We use push to mutate in place for memory and perf reasons
+	// We use the for loop instead of _this.push(...other) to avoid the JS engine's function argument limit (65,535 in JavascriptCore)
 	for (let i = 0; i < other.length; i++) {
 		_this.push(other[i]);
 	}