PhysX VS Bullet

I know you mean PhysX how it was WAYYY back (& GPU accel), but it's been open source for ages.

Unity’s default physics engine is literally PhysX (https://github.com/NVIDIAGameWorks/PhysX).

It all gets petty hairy.. which is why writing physics engines is Hard, and you should be pretty skeptical when someone is like: "Hay gauis I made a new phYsics engine!! you should totally use it!" Even the big guys thrash around in this space.. as someone mentioned in another comment... although there is sometimes a case to be made for rolling your own, if you're valve for instance.. All that said, I'm really partial to Bullet just because it's so damn battle tested (unity/blender and countless other engines) But I'm generally pro havok/physX and also the newer Jolt physics library. https://github.com/NVIDIAGameWorks/PhysX https://github.com/ashconnell/physx-js https://jrouwe.github.io/JoltPhysics/

Then what about this? https://github.com/NVIDIAGameWorks/PhysX/tree/4.1/physx

PhysX has gone open source quite a while ago as well: https://github.com/NVIDIAGameWorks/PhysX https://github.com/NVIDIAGameWorks/UnityPhysXPlugin

The pull request can be found here: https://github.com/NVIDIAGameWorks/PhysX/pull/577

For typical game physics engines... not that much. Math libraries like Eigen or Blaze use lots of template metaprogramming techniques under the hood that can help when you're doing large batched matrix multiplications (since it can remove temporary allocations at compile-time and can also fuse operations efficiently, as well as applying various SIMD optimizations), but it doesn't really help when you need lots of small operations (with mat3 / mat4 / vec3 / quat / etc.). Typical game physics engines tend to use iterative algorithms for their solvers (Gauss-Seidel, PBD, etc...) instead of batched "matrix"-oriented ones, so you'll get less benefits out of Eigen / Blaze compared to what you typically see in deep learning / scientific computing workloads.

The codebases I've seen in many game physics engines seem to all roll their own math libraries for these stuff, or even just use SIMD (SSE / AVX) intrinsics directly. Examples: PhysX (https://github.com/NVIDIA-Omniverse/PhysX), Box2D (https://github.com/erincatto/box2d), Bullet (https://github.com/bulletphysics/bullet3)...

I suspect most C++ physics libraries like Box2D (https://github.com/erincatto/box2d) or Bullet3 (https://github.com/bulletphysics/bullet3) could really benefit a lot from SIMD.

No, they replaced Bullet with Jolt. That is considerably more than "some adjustment", regardless of what you think of the result.

Vulkan graphics via Intel GVK, and physics via Bullet

The first two have a bunch of great examples, and I’m tying them together by refactoring some of the THREE examples to fit the ECS paradigm defined in AFrame. then upping the ante by adding physics using AMMO, which is more challenging since it’s only a partial implementation of Bullet, and already poorly documented (yet popular) physics engine.

Plus, SM uses a system called bullet physics, I imagine this would be rather complex to rework into a modern engine such as Unreal or Unity (after all, the majority of performance issues come from the physics engine rather than the graphics engine)

The physics in HDT-SMP are actually being calculated outside of Skyrim's engine with Bullet, an open-source physics engine. So this isn't some limitation of Skyrim's engine.