From 5ef225853c0f81ba0a7d1ce888ab3b2d283eae0a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bartek=20Iwa=C5=84czuk?= <biwanczuk@gmail.com>
Date: Wed, 14 Jun 2023 16:21:06 +0200
Subject: [PATCH] perf: don't run microtask checkpoint if macrotask callback
 did no work (#19492)

Most of the time there's no firing timers, nor pending promise
rejections, so it's wasteful to run microtask checkpoint
additionally twice on each tick of the event loop.

Closes https://github.com/denoland/deno/issues/18871
Ref https://github.com/denoland/deno/issues/19451
---
 core/01_core.js       | 9 +++++++++
 ext/web/02_timers.js  | 4 +++-
 runtime/js/99_main.js | 6 ++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/core/01_core.js b/core/01_core.js
index 13aa17c7ed..d4a6508cb2 100644
--- a/core/01_core.js
+++ b/core/01_core.js
@@ -187,7 +187,16 @@
       const cb = macrotaskCallbacks[i];
       while (true) {
         const res = cb();
+
+        // If callback returned `undefined` then it has no work to do, we don't
+        // need to perform microtask checkpoint.
+        if (res === undefined) {
+          break;
+        }
+
         ops.op_run_microtasks();
+        // If callback returned `true` then it has no more work to do, stop
+        // calling it then.
         if (res === true) {
           break;
         }
diff --git a/ext/web/02_timers.js b/ext/web/02_timers.js
index 19ebfaa0e4..27e2e953db 100644
--- a/ext/web/02_timers.js
+++ b/ext/web/02_timers.js
@@ -54,8 +54,10 @@ const timerTasks = [];
 let timerNestingLevel = 0;
 
 function handleTimerMacrotask() {
+  // We have no work to do, tell the runtime that we don't
+  // need to perform microtask checkpoint.
   if (timerTasks.length === 0) {
-    return true;
+    return undefined;
   }
 
   const task = ArrayPrototypeShift(timerTasks);
diff --git a/runtime/js/99_main.js b/runtime/js/99_main.js
index 8fd9a6bd94..511368141c 100644
--- a/runtime/js/99_main.js
+++ b/runtime/js/99_main.js
@@ -351,6 +351,12 @@ function promiseRejectCallback(type, promise, reason) {
 }
 
 function promiseRejectMacrotaskCallback() {
+  // We have no work to do, tell the runtime that we don't
+  // need to perform microtask checkpoint.
+  if (pendingRejections.length === 0) {
+    return undefined;
+  }
+
   while (pendingRejections.length > 0) {
     const promise = ArrayPrototypeShift(pendingRejections);
     const hasPendingException = ops.op_has_pending_promise_rejection(