Add `StringIndex`: a generic open-addressed string set by dougqh · Pull Request #11660 · DataDog/dd-trace-java

dougqh · 2026-06-17T11:03:26Z

What Does This Do

Adds StringIndex an open-indexed immutable hash set -- that can also work in conjunction with a separate data array as an immutable map.

In terms of ops/sec, StringIndex performs on par or better than HashSet and HashMap while consuming less memory.
In terms of ops/sec StringIndex out performs Set.of but consumes more memory.

StringIndex excels in situations where map values are primitive types or multiple map values are needed for the same keys.

Motivation

Data structure that can be reused throughout dd-trace-java that helps reduce tracer overhead - throughput impact and memory footprint

Additional Notes

StringIndex (datadog.trace.util, alongside the custom Hashtable) — a flat, allocation-free, open-addressed string set / index:

EmbeddingSupport — the static algorithm over raw int[] hashes / String[] names. Held in static final fields the refs fold to constants (the hot path).
Data — a build-time carrier {int[] hashes, String[] names} (pull into your own fields).
an instance wrapper for convenience (of/contains/indexOf).

2×-oversized (load factor ≤ 0.5), linear probe + wraparound, hash gates equals, interned == fast path, 0 = empty sentinel. Generic — no payload baked in; it just knows names. The headline capability is indexOf, which assigns each known string a stable dense slot; consumers attach a parallel array (e.g. long[] ids) indexed by that slot. Membership (contains) falls out as indexOf >= 0.

(Renamed from TagSet — the structure is more general than tags; a fixed name→id / membership index is just one of its uses.)

The static tier is named EmbeddingSupport and the sizing helper capacityFor, matching the rest of the flat-collections family (LightMap, Hashtable) so the "static functions over a caller-owned array/spine" tier is named identically across all of them.

Benchmarks

The motivating use is a fast, declared name→id table — a "pit of success" alternative to hand-rolled string switches and per-name caches — but the structure is general. Benchmarks (Apple M1, JDK 17, @Fork(5), @Threads(8); M ops/s):

ImmutableSetBenchmark (membership, hit): static EmbeddingSupport 2320 ≳ HashSet 2198 > instance StringIndex 2098 > Set.copyOf/SetN 1914, and ~2.5–3.5× the array/sortedArray/treeSet forms. The folded EmbeddingSupport path is the fastest membership structure (~6% over HashSet); the instance wrapper costs ~10% (a field load), landing near HashSet.
ImmutableMapBenchmark (name→value get): static EmbeddingSupport 1498 (interned key 2081) > instance 1363 > HashMap 1216 > Map.copyOf/MapN 1049 — StringIndex-as-map is the fastest get, and a parallel long[]/int[] avoids the boxing a HashMap<String, Long> pays.
StringIndexSwitchBenchmark (name→id vs a hand-written string switch): on a runtime, varied key — the realistic regime — StringIndex is ~1.85× the switch (2147 vs 1161) and flat across inline/key-shape; the switch only matches it when the key is a compile-time constant (the JIT const-folds the switch away), which production keys never are.

Footprint (StringIndexFootprintTest, JOL): ~9% lighter than HashSet (no per-element Nodes) but ~27% heavier than Set.copyOf/SetN (it carries the cached int[] hashes + a 2×-oversized table) — so vs the JDK compact immutables the edge is speed + the indexOf→parallel-array capability, not footprint.

StringIndexTest covers hashing/zero-sentinel, probe + wraparound, table-full, and the parallel-payload usage.

🤖 Generated with Claude Code

datadog-datadog-prod-us1 · 2026-06-17T11:05:16Z

🎯 Code Coverage (details)
• Patch Coverage: 95.65%
• Overall Coverage: 58.40% (+0.01%)

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 1541293 | Docs | View more details | Give us feedback!}

dd-octo-sts · 2026-06-25T14:20:21Z

🟢 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	14.75 s	14.71 s	[-0.6%; +1.1%] (no difference)
startup:insecure-bank:tracing:Agent	13.59 s	13.70 s	[-1.8%; +0.2%] (no difference)
startup:petclinic:appsec:Agent	17.48 s	17.20 s	[+0.7%; +2.6%] (maybe worse)
startup:petclinic:iast:Agent	17.45 s	17.62 s	[-1.8%; -0.1%] (maybe better)
startup:petclinic:profiling:Agent	17.48 s	17.46 s	[-1.0%; +1.3%] (no difference)
startup:petclinic:sca:Agent	17.25 s	17.25 s	[-1.0%; +0.9%] (no difference)
startup:petclinic:tracing:Agent	16.58 s	16.65 s	[-1.4%; +0.5%] (no difference)

Commit: 15412934 · CI Pipeline · Benchmarking Platform UI

Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

- Consume the StringIndex surface API now in #11660: FIXED_HANDLER_IDS via Support.mapIntValues; handlerId resolves via Support.lookup (id, or 0 on miss -- the not-intercepted sentinel, so no separate slot check). - Make TagInterceptor final (nothing extends it; aids JIT devirtualization). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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bric3

Note, in general the generated comments, are not that readable. So I'd rather have human crafted comments.

E.g this way of aligning words is difficult to process, one sentence would be ok, but since all comments are like that :

 * <p>Slot 0-value is the empty sentinel: {@link Support#hash} never returns 0, so {@code hashes[i]
 * == 0} unambiguously means an empty slot.

datadog-datadog-prod-us1

More details

All 166 adversarial scenarios executed against the new StringIndex implementation pass cleanly — empty-string key (remapped hash 0xDD06), empty index, duplicate deduplication, full wraparound probe chains crossing different-hash occupants, parallel long[]/int[]/T[] payloads with realistic tracer tag sets, non-interned string lookups, and typed-array return from mapValues. The open-addressing algorithm (zero sentinel, linear probe, LF ≤ 0.5) is internally consistent and handles every edge case the diff introduces.

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bric3 · 2026-07-16T10:52:07Z

coverage is violated:

Rule violated for class datadog.trace.util.StringIndex: instructions covered ratio is 0.7, but expected minimum is 0.8

dougqh · 2026-07-16T14:26:47Z

coverage is violated:

Rule violated for class datadog.trace.util.StringIndex: instructions covered ratio is 0.7, but expected minimum is 0.8

@bric3 Yes, I was working on plugging that gap yesterday. I added a few convenience methods at the end and that dropped the coverage level. I'll fix it.

datadog-datadog-prod-us1

Large capacity requests can return Integer.MIN_VALUE. This result violates the helper contract and can cause NegativeArraySizeException during allocation.

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bric3

Pre-approving, code looks good, but there's some light tweak around javadoc, and method overloads that take hashes or not.

…actor ceil(n / loadFactor) was narrowed to int before checking bounds, so a sufficiently large n or small loadFactor silently saturated the narrowing conversion and the subsequent << 1 wrapped to a negative capacity, producing NegativeArraySizeException downstream instead of a clear error. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

datadog-datadog-prod-us1

More details

The new index keeps the required open-addressing rules. The static review found no concrete failure in its set, index, mapping, or lookup behavior.

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Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

put is the only array-mutating entry point in EmbeddingSupport; keeping it internal to the package (used only by create) enforces that callers treat the arrays a StringIndex/EmbeddingSupport hands them as build-once/immutable, and removes the raw-hash misuse surface a public put would otherwise expose to external callers. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Aligns naming with EmbeddingSupport (renamed from Support for consistency with the in-flight LightMap class) across benchmark methods, javadoc result tables, and prose. No benchmarks re-run -- existing numbers are relabeled. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

The stringIndex_embedded rename left columns ragged; realign to the new longest label. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

dd-octo-sts · 2026-08-20T21:02:20Z

/merge

gh-worker-devflow-routing-ef8351 · 2026-08-20T21:02:25Z

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Since those jobs are not marked as being allowed to fail, the pipeline will most likely fail.
Therefore, and to allow other builds to be processed, this merge request has been rejected and the pipeline got canceled.

JOL's GraphLayout relies on HotSpot-specific Unsafe internals and throws IllegalStateException on IBM/Semeru JVMs, which are part of the CI test matrix. Mirrors the guard already used in ScopeAndContinuationLayoutTest.

dd-octo-sts · 2026-08-21T14:47:32Z

/merge

gh-worker-devflow-routing-ef8351 · 2026-08-21T14:47:36Z

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dougqh added comp: core Tracer core tag: ai generated Largely based on code generated by an AI or LLM tag: no release notes Changes to exclude from release notes type: feature Enhancements and improvements labels Jun 17, 2026

dougqh changed the title ~~Add TagSet: a generic open-addressed string set~~ Add StringIndex: a generic open-addressed string set Jun 23, 2026

This was referenced Jun 23, 2026

Overhaul set benchmarks: split Immutable / SingleThreaded, add Set.copyOf #11721

Merged

Resolve TagInterceptor tags via StringIndex (handlerId + handleIntercept) #11742

Closed

dougqh mentioned this pull request Jun 26, 2026

[Experiment, shelved] Functional TagHandler[] dispatch for TagInterceptor #11748

Closed

dougqh added the tag: performance Performance related changes label Jun 30, 2026

dougqh marked this pull request as ready for review June 30, 2026 20:16

dougqh requested a review from a team as a code owner June 30, 2026 20:16

dougqh requested a review from ygree June 30, 2026 20:16

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bric3 reviewed Jul 9, 2026

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bric3 changed the title ~~Add StringIndex: a generic open-addressed string set~~ Add StringIndex: a generic open-addressed string set Jul 9, 2026

dougqh commented Jul 13, 2026

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dougqh force-pushed the dougqh/tagset branch from f4a2a07 to 1d8fd80 Compare July 15, 2026 19:41

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bric3 approved these changes Aug 20, 2026

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dougqh requested a review from a team as a code owner August 20, 2026 19:18

Add usage example to StringIndex class javadoc

aa8f1be

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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dougqh and others added 6 commits August 20, 2026 15:26

Add hot-path usage example to EmbeddingSupport javadoc

de984b4

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Document raw-hash contract on EmbeddingSupport.indexOf's 4-arg overload

e9dcbac

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Add missing javadoc to hash-less EmbeddingSupport.indexOf overload

fd16e58

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Fix column alignment in StringIndex benchmark result tables

fdb1f23

The stringIndex_embedded rename left columns ragged; realign to the new longest label. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

dougqh enabled auto-merge August 20, 2026 20:12

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github-merge-queue Bot removed this pull request from the merge queue due to failed status checks Aug 20, 2026

Skip StringIndexFootprintTest on J9-based JVMs

1541293

JOL's GraphLayout relies on HotSpot-specific Unsafe internals and throws IllegalStateException on IBM/Semeru JVMs, which are part of the CI test matrix. Mirrors the guard already used in ScopeAndContinuationLayoutTest.

dougqh added this pull request to the merge queue Aug 21, 2026

github-merge-queue Bot removed this pull request from the merge queue due to failed status checks Aug 21, 2026

gh-worker-dd-mergequeue-cf854d Bot merged commit aa35904 into master Aug 21, 2026
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gh-worker-dd-mergequeue-cf854d Bot deleted the dougqh/tagset branch August 21, 2026 15:40

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What Does This Do

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Benchmarks

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🟢 Java Benchmark SLOs — All performance SLOs passed

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