Optimize `Object::notification` method performance

[CrazyRoka]

Revision 1

Optimize Object::notification method performance

This PR optimizes the Object::notification() function by splitting it into two specialized functions: notification_forward() and notification_reverse(). This change significantly reduces the CPU usage of this frequently called function.

Key changes:

• Split Object::notification() into notification_forward() and notification_reverse()
• Update notification() to act as a simple dispatcher to the appropriate specialized function

Revision 2

Refactor Object::notification using if constexpr and templates

Key changes:

• Implemented the optimization using C++17's if constexpr and template specialization.
• The compiler now generates separate, optimized code paths for reversed and non-reversed cases.

Before

Before: 5.7% CPU usage inside Object::notification(). Hotspot is referring to this line.

After

After: 1.49% CPU usage inside Object::_notification_fast()

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This optimization results in a significant reduction in CPU usage for this function.

The performance improvement is achieved by:

1. Eliminating conditional branching within the function
2. Allowing better compiler optimizations for each specialized case
3. Improving code locality and potential for inlining

Testing

Tested locally with a scene that contains 20K cubes, all of them were moving around. Link to the test project: https://github.com/CrazyRoka/godot-benchmark-moving-cubes/tree/main

[alvinhochun]

Would it make sense to use template and constexpr if to avoid duplicating the code inside the function?

[AThousandShips]

Please squash your commits into one, see here

[CrazyRoka]

Please squash your commits into one, see here

Done

[RandomShaper]

I see you're introducing a new convention for template arguments (t_ prefix). Not a big deal since we don't have uniformity in that regard across the codebase. However, it may be better to choose one of the already used ones (like the suggested all-caps, or CamelCase).

That said, t_ looks great to me as a potential new convention for the whole codebase later.

[lawnjelly]

Good idea, we use these template functions in the same way already in a few places for optimization. :+1:

[akien-mga]

Superseded by #104381. Thanks for the contribution!