perf: optimize assign-depths in compilation

Author: alexander-akait

lib/Compilation.js

@@ -3895,28 +3895,30 @@ Or do you want to use the entrypoints '${name}' and '${runtime}' independently o
 	assignDepths(modules) {
 		const moduleGraph = this.moduleGraph;
 
-		/** @type {Set<Module | number>} */
+		/** @type {Set<Module>} */
 		const queue = new Set(modules);
-		queue.add(1);
+		// Track these in local variables so that queue only has one data type
+		let nextDepthAt = queue.size;
 		let depth = 0;
 
 		let i = 0;
 		for (const module of queue) {
-			i++;
-			if (typeof module === "number") {
-				depth = module;
-				if (queue.size === i) return;
-				queue.add(depth + 1);
-			} else {
-				moduleGraph.setDepth(module, depth);
-				for (const { module: refModule } of moduleGraph.getOutgoingConnections(
-					module
-				)) {
-					if (refModule) {
-						queue.add(refModule);
-					}
+			moduleGraph.setDepth(module, depth);
+			// Some of these results come from cache, which speeds this up
+			const connections = moduleGraph.getOutgoingConnectionsByModule(module);
+			// connections will be undefined if there are no outgoing connections
+			if (connections) {
+				for (const refModule of connections.keys()) {
+					if (refModule) queue.add(refModule);
 				}
 			}
+			i++;
+			// Since this is a breadth-first search, all modules added to the queue
+			// while at depth N will be depth N+1
+			if (i >= nextDepthAt) {
+				depth++;
+				nextDepthAt = queue.size;
+			}
 		}
 	}