Comment by rwmj
3 years ago
It's how often reference counts are adjusted on hot paths that matters (including in libraries), and back to the original point, reference counting doesn't let you free groups of objects in one go (unlike a tracing GC).
Also it'd be nice if the reference counts were stored separately from the objects. Storing them alongside the object being tracked is a classic mistake made by reference count implementations (it spreads the writes over a large number of cache lines). I was actually surprised that Rust doesn't get this right.
Another issue with manual memory management is that you can't compact the heap.
The amount of reference-counted pointers in most Rust code is a tiny fraction compared to boxes or compiler-tracked-lifetime references.
Yes in theory it would be more efficient to store all the reference counts together, but that's in theory. In practice most Rust apps will not call clone on a shared pointer on a hot path and if they do it's usually 1 such pointer and they do something with the data as well (so it's all 1 cache line anyway)
You can't compare Rust/C++ with Swift/Nim when it comes to RC, there just aren't enough reference count operations for it to matter much (unless you're in a shitty OO C++ codebase like me that pretends it is java with std::shared_ptr everywhere)
Apps where heap compaction would be relevant in a low-level language like Rust or C++ will typically use a bump allocator which will trounce any kind of GC.