When debugging in Unreal Engine, the ensure() macro is a powerful tool to detect and report unexpected behavior without crashing the game. However, developers may observe a noticeable hitch when the first ensure() is triggered, especially in non-editor builds.
Why the First ensure() Causes a Hitch
On its first invocation, ensure() performs a stack trace to help identify where the unexpected condition occurred. To do this, Unreal loads debug symbols and initializes the platform stack walking system. This process is expensive and can cause a significant frame drop—sometimes long enough to trigger a hitch warning or impact gameplay.
This cost only happens once—subsequent calls to ensure() are much faster because the symbol loading and stack walking systems are already initialized.
How to Avoid the Hitch
To avoid the performance impact during gameplay, you can manually initialize the stack walking system early—typically during game startup. This ensures that the necessary systems and symbol resolution are already warmed up before any ensure() is triggered in a critical path.
Solution
Call the following function during your game’s startup sequence:
FPlatformStackWalk::InitStackWalking();
This proactively initializes the stack walking system, so when an ensure() is later triggered, the engine can resolve the stack trace with minimal overhead.
When to Use This
This technique is particularly useful for:
- Shipping builds where unexpected
ensure()s can severely affect player experience. - Games with high frame rate sensitivity, like competitive shooters or VR.
- Debugging sessions where you want smooth behavior despite many
ensure()calls.
Caveats
- This does introduce a slight startup cost as the symbol handler is initialized. In most cases, this cost is negligible compared to the runtime benefit.
- Be aware that this does not replace good practice: remove or resolve
ensure()s in production paths where possible.