GC roadmap: make minor GC structurally cheap

[andrewtdiz]

Goal

Move Perry from tuned generational mark-sweep toward cheap, precise minor GC.

Target invariant:

minor GC cost = live young objects + dirty old cards/pages + mutable roots
not arena size + malloc table + block persistence + sweep walk

Why

Perry already has many of the right pieces: NaN-boxed values, a shadow stack, generation metadata, write-barrier infrastructure, remembered tracking, optional evacuation, and arena reset/deallocation. The next dramatic improvement is to make minor GC structurally cheap instead of continuing to tune the current sweep/block-persistence path.

Child Work

This umbrella tracks the staged work for:

• GC telemetry and heap-shape counters
• Defaultable cheap write barriers and dirty page/card tracking
• Mutable runtime root visitors
• Policy-driven evacuation
• Copying nursery prototype
• Layout descriptors and pointer masks

Child issues will be linked once created.

Verification Bar

Ship the roadmap only when the benchmark matrix proves:

bench_json_roundtrip RSS down materially
bench_gc_pressure pause time down
object_create / binary_trees no regression
perf-comprehensive no mid-round GC outliers
conservatively_pinned_bytes low and trending down
minor pause flat as dead nursery allocation grows

[andrewtdiz]

Child tickets created:

• GC telemetry: expose per-cycle phase timings and heap-shape counters #1091 — GC telemetry: expose per-cycle phase timings and heap-shape counters
• GC barriers: replace object remembered set with dirty page/card tracking #1092 — GC barriers: replace object remembered set with dirty page/card tracking
• GC roots: add mutable runtime root visitor API #1093 — GC roots: add mutable runtime root visitor API
• GC evacuation: make nursery-to-old movement policy-driven #1094 — GC evacuation: make nursery-to-old movement policy-driven
• GC nursery: prototype copying minor collector with wholesale reset #1095 — GC nursery: prototype copying minor collector with wholesale reset
• GC scanning: add layout descriptors and pointer masks #1096 — GC scanning: add layout descriptors and pointer masks

Requested assignee for umbrella and child tickets: @andrewtdiz. GitHub rejected the assignee mutation from this token/account, so a maintainer needs to apply the assignment in the UI.