Moves the meta-GC from greedy (always verify) to adaptive: track a
scaled EMA of recent escape rate; when rate exceeds 50% (128/256),
SKIP the verifier and let the heap grow until natural GC; every 16
skipped arenas, force a verify as a probe to re-sample the rate.
Two correctness fixes uncovered while testing adaptive:
1. gc_mark_env was picking up 24-byte strings and closures as if
they were env nodes (size check alone is ambiguous). Now also
requires offset 0 to be tagged TAG_SYM, which env nodes always
are and strings/closures never are.
2. gc_mark_drain's vector/hash-table dispatch walked `length`
elements without sanity-checking that `8 + length*8` fits in
the block. A 25-char string (40-byte payload) misinterpreted as
a 25-element vector walked 200 bytes off the end, reading
adjacent blocks' bytes as tagged roots and setting mark bits on
wrong things. Both paths now validate the header's payload-size
against the claimed length / nbuckets before walking.
New builtin:
(arena-set-mode 0|1) — 0 = greedy baseline, 1 = adaptive (default)
arena-stats extended to six fields:
(calls resets escapes skipped bytes-reclaimed ema-rate)
Bench (tests/bench-gc-adaptive.sh, one process per phase to isolate
a separate latent cross-phase bug we haven't cracked, N=1000 per
phase, i5-8350U):
workload mode time_ms resets escapes skipped
friendly greedy 732 1000 0 0
friendly adapt 691 1000 0 0
hostile greedy 568 0 1000 0
hostile adapt 607 0 1000 11
mixed greedy 1981 17 1983 0
mixed adapt 1694 14 1986 2
Adaptive wins on friendly (-6%) and mixed (-17%). On fully hostile
workloads both modes are dominated by implicit full-GC firings
(982/1000 arenas trigger heap overflow that clears arena_active
before reaching the policy), so adaptive barely activates and
greedy happens to edge out by ~7%. The mixed result is the clear
adaptive win — and the one that matches the pattern the policy was
designed for: probe-and-adapt as the workload shifts.
137 asm (no-GC) + 137 asm (GC) + 189 shared functional tests all
still pass.
russell@unturf.com
3d55092037