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phm: grow the bucket array past load factor 2 (map-read 4.5x recovery)

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The phm had a FIXED 8 buckets, so a 100-entry map was a ~12-entry linear
scan per lookup — and phm-get walked the bucket twice (contains? then find).
This went mostly unnoticed until the canonical zipmap (batch 2) started
returning phms where kvs->map had built structs for scalar keys, regressing
the map-read bench ~7x (jolt-s3y).

phm-assoc now rehashes into a doubled bucket array when the count passes 2
entries/bucket (done on the fresh copy, so persistence is untouched);
phm-get is single-pass with a presence flag (nil values still distinguish
from missing); key= tries identity/scalar equality before paying for
canonicalization; the bucket count is derived from (length (m :buckets)),
not a constant, so any already-marshaled map keeps working. core-contains?'s
phm branch goes through phm-contains? instead of poking buckets directly.

map-read 48.5 -> 10.9 ms (the residual vs the pre-batch-2 6.7 is the
canonicalizing-representation constant); map-build steady; bench TOTAL 4457
vs 4565 on main back-to-back. New unit case crosses the resize boundary at
500 entries: every key found, nil values present, collection keys canonical,
dissoc + persistence intact. Gate: jpm exit 0, conformance 326x3.
This commit is contained in:
Yogthos 2026-06-10 17:06:03 -04:00
parent 1d373299a4
commit 24a8522b51
3 changed files with 76 additions and 24 deletions
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17
test/unit/persistent-map-test.janet
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@ -72,4 +72,21 @@
(assert (= 42 (ct-eval ctx "(get big-map :k42)")) "get k42"))
(print " passed")
(print "6: bucket resize (jolt-s3y)...")
(let [ctx (init-cached)]
# Crossing the load-factor boundary several times: every key still found,
# nil values preserved, collection keys still canonical, dissoc intact.
(eval-string ctx "
(def m (reduce (fn [m i] (assoc m i (* 10 i))) (hash-map) (range 500)))")
(assert (= 500 (ct-eval ctx "(count m)")) "count survives rehash")
(assert (= true (ct-eval ctx "(every? (fn [i] (= (* 10 i) (get m i))) (range 500))"))
"every key found after rehash")
(assert (= true (ct-eval ctx "(let [m2 (assoc m :nilv nil)] (and (contains? m2 :nilv) (nil? (get m2 :nilv :miss))))"))
"nil value present after rehash")
(assert (= :hit (ct-eval ctx "(get (assoc m [1 2] :hit) [1 2])"))
"collection key canonical after rehash")
(assert (= 499 (ct-eval ctx "(count (dissoc m 0))")) "dissoc after rehash")
(assert (= 500 (ct-eval ctx "(count m)")) "persistence: source unchanged"))
(print " passed")
(print "\nAll PersistentHashMap tests passed!")