U Language Introduces New Memory Management System Without Garbage Collection

Garbage Collection Elimination

The U language manages memory without a garbage collector. It enforces ownership through a Directed Acyclic Graph (DAG), where strong references flow from parent to child, preventing upward or cyclical dependencies. When an owner's reference count reaches zero, its entire memory subtree is deallocated. This method avoids the overhead of tracing, mark-sweep algorithms, and associated pauses, with deallocation cost proportional to the memory allocated by that specific owner.

Ownership and Back-References

Back-references are annotated with '+R(parent)' and are treated as weak references by the compiler, meaning they do not contribute to the reference count and resolve to 'none' if the referent is deallocated. A linter uses Tarjan's SCC algorithm to ensure the DAG structure, rejecting programs with cycles that lack a '+R(parent)' edge.

Slab Chain Allocator

Each owner in U utilizes a slab chain allocator. Allocations occur by bumping a pointer within the current slab. When a slab becomes full, a new slab, double the size of the previous one, is linked into the chain. This structure means that for 'n' total bytes, there are at most 'log₂(n/initial)' slabs, each being a power-of-two allocation that system allocators can handle efficiently. Allocation is a simple pointer increment and comparison, avoiding per-object free-list traversals or general-purpose allocation metadata for owner-scoped objects. Deallocation involves walking the chain and freeing each slab, resulting in an O(log n) cost for 'n' total bytes.

NaN-Boxed Tagged Values

Dynamic values within Lists, Maps, and Trees can use an 8-byte tagged representation via NaN-boxing. This allows a single machine word to represent various types, including real doubles, small integers, pointers, booleans (true/false), and 'none' or 'tombstone' states. Floating-point NaNs are canonicalized to prevent confusion with tagged payloads. This approach means leaf values do not require separate heap allocation, and tags/payloads are extracted using masks, shifts, and comparisons.

List Implementation

Lists in U employ stable power-of-two slabs for storage. Elements maintain their positions even as the list grows, as appending new elements does not cause existing elements to move.