Bloom-filter fast-path for the dense tag store (O(1) insertion) (phase 2)

[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	14.00 s	13.90 s	[-0.0%; +1.5%] (no difference)
startup:insecure-bank:tracing:Agent	12.88 s	12.94 s	[-1.2%; +0.2%] (no difference)
startup:petclinic:appsec:Agent	16.86 s	16.83 s	[-0.8%; +1.1%] (no difference)
startup:petclinic:iast:Agent	16.91 s	16.96 s	[-1.2%; +0.5%] (no difference)
startup:petclinic:profiling:Agent	16.32 s	16.87 s	[-7.8%; +1.2%] (no difference)
startup:petclinic:sca:Agent	16.43 s	16.91 s	[-7.2%; +1.5%] (no difference)
startup:petclinic:tracing:Agent	16.17 s	16.04 s	[-0.1%; +1.8%] (no difference)

Commit: 75950bee · CI Pipeline · Benchmarking Platform UI

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[datadog-prod-us1-4]

🎯 Code Coverage (details)
• Patch Coverage: 92.86%
• Overall Coverage: 57.32% (-0.00%)

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