Phase 3 (#4) increment 1: precise transitive reachability (mark-phase core)#10
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Phase 3 (#4) increment 1: precise transitive reachability (mark-phase core)#10usiegj00 wants to merge 1 commit into
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… core)
Add `Crystal::RTTI.reachable_from(root) : Set(UInt64)` — a precise transitive
walk of the object graph from a root via the RTTI reference layout, returning
every reachable Crystal object's address. This is the core operation of a
precise GC's mark phase (the foundation for `-Dgc=immix`), and immediately a
retainer/leak-analysis tool ("what is reachable from X?") on the default Boehm
GC — directly serving the leak-diagnosis use case behind the epic.
- Refactor `each_outgoing_reference` to delegate to a pointer-based core
(`each_outgoing_reference_at(base_ptr, type_id)`), since a transitive walk
holds raw `Void*` pointers and cannot cast to the abstract `Reference`.
- `reachable_from` does an iterative DFS, following a yielded pointer only when
its header is a valid type id (so raw Array buffers / non-object pointers are
leaves; Array *elements* are still reached via the buffer scan).
- Specs (red→green): transitive closure over an object graph (excludes a node
reachable only from the stack), and reaching references through Array elements.
Verified: rtti_spec 13/0; format + ameba clean; rtti.cr parses under Crystal
1.0.0 (forward-compat). Behind `-Drtti`; the full Immix allocator/sweep are
subsequent increments on this proven precise-mark foundation.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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First increment of Phase 3 (#4, precise GC). Rather than land a full Immix collector at once (allocator + blocks/lines + sweep + fiber stacks — a multi-increment effort), this delivers the core of a precise GC's mark phase, built on the RTTI from #8/#9.
What
Crystal::RTTI.reachable_from(root) : Set(UInt64)— a precise transitive walk of the object graph fromrootvia the RTTI reference layout, returning every reachable Crystal object's address. This is precise marking from a root; it also immediately serves retainer/leak analysis ("what's reachable from X?") on the default Boehm GC — the diagnosis the epic was built for.each_outgoing_referenceto delegate to a pointer-based core (each_outgoing_reference_at(base_ptr, type_id)), since a transitive walk holds rawVoid*and can't cast to the abstractReference.reachable_fromfollows a yielded pointer only when its header is a valid type id (raw Array buffers / non-object pointers are leaves; Array elements are still reached via the buffer scan).Verification
spec/compiler/codegen/rtti_spec.cr(red→green): transitive closure over a graph (excludes a node reachable only from the stack); reaching references through Array elements. 13 examples, 0 failures.rtti.crparses under Crystal 1.0.0 (forward-compat checked proactively).Scope
Behind
-Drtti. The full Immix allocator / sweep / fiber-stack root scanning are subsequent increments on top of this proven precise-mark foundation. Fork-only; builds on merged #9.