Add per-operation self-tuning dense-store sizing (SizingHint)

[dougqh]

What

Sizes each span's dense TagMap store from a per-operation sizing hint instead of a generic default, then feeds the actual known-tag count back on span finish so the hint converges to that operation's real high-water mark. Over-provisioning shrinks toward the operation's true shape without any hand-tuned per-type tables.

This is the SizingHint layer of the dense-store stack, sitting between the generator (dougqh/generator-v2) and the id-keyed TagMap.set(long) work (dougqh/tag-id-api, #11901).

How

• FlatHashtable — open-addressed find-or-create over self-contained entries; static-polymorphism via a concrete-typed Helper singleton so the JIT devirtualizes/inlines hash/matches/create per call site. Single-reference publish → torn-free under racy access.
• SizingHint / SizingHelper — opaque per-operation hint (label + cached hash + tuned size) and its String-key helper.
• SizingHintTable — process-wide, pure-static two-lane (entry vs child span) registry keyed by operation name. Fixed-capacity and deliberately lock-free/racy: a lost update or double-mint only mis-sizes an array (over/under-provision) for a span or two — it never corrupts tag data.
• Wiring — CoreTracer.buildSpanContext resolves the hint by operation name + entry-ness and hands it to DDSpanContext, which sizes its TagMap from it and records the final size back via DDSpan.finishAndAddToTrace.

Testing

• FlatHashtableTest, SizingHintTableTest (incl. the self-tuning recordSize loop and content-keying so a UTF8BytesString operation name resolves to the same hint as its String form).
• Benchmarks: FlatHashtableBenchmark; a buildMapSized arm in DenseStoreAllocBenchmark.
• spotlessJavaCheck, :internal-api:spotbugsMain, :dd-trace-core:spotbugsMain all green.

Notes

• No behavior change to tag semantics — this only sizes the backing store.
• The three delegating test-only DDSpanContext ctors are marked @Deprecated @VisibleForTesting to steer new code to the full ctor.

🤖 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	13.97 s	13.96 s	[-0.7%; +0.8%] (no difference)
startup:insecure-bank:tracing:Agent	12.97 s	12.95 s	[-0.7%; +0.9%] (no difference)
startup:petclinic:appsec:Agent	16.83 s	16.77 s	[-0.7%; +1.5%] (no difference)
startup:petclinic:iast:Agent	16.92 s	16.92 s	[-0.8%; +0.8%] (no difference)
startup:petclinic:profiling:Agent	16.57 s	16.79 s	[-2.6%; +0.1%] (no difference)
startup:petclinic:sca:Agent	16.90 s	16.25 s	[-0.4%; +8.4%] (no difference)
startup:petclinic:tracing:Agent	16.13 s	16.00 s	[-0.3%; +1.8%] (no difference)

Commit: 956f2c9a · 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.

[datadog-datadog-prod-us1-2]

Bits has a CI fix ready

🟢 Investigated · 🟢 Fix prepared · ⚪ Validation skipped · 🟠 Ready

Registered appsec.agentic_onboarding in AppSec configuration and read it with an empty default during Config initialization, so the setting is emitted in configuration telemetry.

Commit fix to this PR

---

View in Datadog | Reviewed commit 956f2c9 · Any feedback? Reach out in #deveng-pr-agent