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

1 files changed, 9 insertions, 0 deletions

diff --git a/core/01_core.js b/core/01_core.js
index 13aa17c7e..d4a6508cb 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;
         }