[V8 perf] Loop callback changing is throttling

Version

v24.20.0

Platform

Windows 10

Subsystem

No response

What steps will reproduce the bug?

• Clone the repo https://github.com/AlttiRi/loop-callback-throttling-bug
• Install deps ("sharp")
• Run demo-1.ts and demo-2.ts

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~8 secs) ✅
• LANCZOS (~8 secs) ✅

What do you see instead?

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~30 secs) ❌
• LANCZOS (~30 secs) ❌

Additional information

I have downsample1D_V1 function (it's very unoptimized, but it works, and it's perfect for the demonstration), which executes the passed callback const weight = fn(x); billions of times (3_107_687_040 times per 1 downsample2D call).

There are 2 types of the passed callback function — lanczos3 and box:

function lanczos3(x: number): number {
  if (-3 <= x && x < 3) {
    return sinc(x) * sinc(x / 3);
  }
  return 0;
}

function sinc(x: number): number {
  if (x === 0) {
    return 1;
  }
  x = Math.PI * x;
  return Math.sin(x) / x;
}


function box(x: number): number {
  if (x > -0.5 && x <= 0.5) {
    return 1;
  }
  return 0;
}

I call downsample2D multiple times with the follow callback params (demo-1.ts):

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~30 secs) ❌
• LANCZOS (~30 secs) ❌

The first 2 calls work with the expected speed, for ~5 second.
But when the callback is changed, downsample2D works multiple times slower than expected, for ~30 seconds.

In reverse order I also face the throttling (demo-2.ts):

• LANCZOS (~8 secs) ✅
• LANCZOS (~8 secs) ✅
• BOX (~30 secs) ❌
• BOX (~30 secs) ❌

The problem appears on both Node.js (24) and Deno (2.9).

Bun (1.3/1.4) works fine:

• BOX (~5 secs) ✅
• BOX (~5 secs) ✅
• LANCZOS (~8 secs) ✅
• LANCZOS (~8 secs) ✅

---

Related keywords: performance v8 polymorphic inline cache

Beginne damit, die verknüpfte Reproduktion zu klonen und demo-1.ts sowie demo-2.ts unter Node.js v24 auszuführen, wobei du die Timings beim Wechseln der Callbacks vergleichst. Untersuche das berichtete polymorphe Inline-Cache-Verhalten von V8 und überprüfe das Ergebnis anhand der Fälle BOX und LANCZOS. Als abgeschlossen gilt die Aufgabe, wenn das Ändern von Callbacks nicht mehr dazu führt, dass die wiederholten downsample2D-Aufrufe gegenüber den erwarteten Timings langsamer werden.