Bloom-filter fast-path for the dense tag store (O(1) insertion)

[dougqh]

What

Adds a per-map bloom-style presence filter (one long, knownBloom) to OptimizedTagMap's dense known-tag store. A clear bit means a known tag is definitely absent, so an append skips the O(n) knownIndexOf linear scan — the common per-span-build case (each tag set once) becomes O(1).

Correctness — superset invariant

The filter is a superset of the present ids' bits: set on every add, never cleared on remove (a stale-set bit only costs an unnecessary scan, never a wrong answer). The linear scan stays authoritative — a set bit means "maybe present, scan to confirm." So correctness is independent of the position→bit collision rate; only the fast-path hit rate depends on it. All 339 TagMap + dense fuzz tests pass.

Position→bit mapping

Crude 1L << (fieldPos & 63) to start. fieldPos (not globalSerial) is deliberate: it's the axis a future per-type graph-coloring optimizes — coloring co-occurring tags to distinct positions raises the fast-path hit rate without touching correctness.

Why (evidence)

On the stacked id-comparison benchmark, the bloom brings dense id-insertion to HashMap parity at typical tag counts (0.99× at 7 tags, up from 0.91× without it), while dense still allocates ~half of HashMap. Cost: +8 B/op (the single long field). The demonstrating benchmark lands in the stacked id PR.

Stacking

Dense-store id line: #11814 (Entry-less storage) → this (bloom fast-path) → id API + comparison benchmark. Based on dougqh/dense-store.

Follow-ups

Per-type graph coloring (raises hit rate); multi-level transitive prune for read-through (skip parents whose bit is clear).

🤖 Generated with Claude Code

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	13.96 s	13.83 s	[+0.3%; +1.5%] (maybe worse)
startup:insecure-bank:tracing:Agent	12.87 s	12.98 s	[-1.5%; -0.1%] (maybe better)
startup:petclinic:appsec:Agent	17.49 s	17.50 s	[-1.4%; +1.3%] (no difference)
startup:petclinic:iast:Agent	17.43 s	17.50 s	[-1.0%; +0.2%] (no difference)
startup:petclinic:profiling:Agent	17.42 s	17.49 s	[-1.3%; +0.6%] (no difference)
startup:petclinic:sca:Agent	17.49 s	17.44 s	[-0.5%; +1.1%] (no difference)
startup:petclinic:tracing:Agent	16.45 s	16.64 s	[-2.2%; -0.1%] (maybe better)

Commit: 428e72ff · CI Pipeline · Benchmarking Platform UI

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