[dougqh]

What Does This Do

TagMap keeps known tags in an IdStore (by id) and custom tags in a HashStore (by name), instead of one bucket array.

• A name is classified once, at the boundary, and routed to its store. Reads by id go straight to the id store.
• Read-through works per store. Known and custom tags never share a key, so precedence never crosses stores.
• The name store is created lazily, on the first custom write. Until then, a map borrows its read-through parent's store read-only.
• stream() and spliterator() yield stable Entrys, and getEntryReader gives a momentary reader for typed reads.
• TagMap.Internals stays for now, writing straight to the id store. It goes away in the next PR, when the span owns its stores.
• Store access is restricted to core. The forbidden-API filter blocks instrumentation from using the stores and TagMap's new view methods.

Motivation

This is the second of three PRs that replace the span's single TagMap with two stores split by tag kind. TagMap had picked up name canonicalization, known-versus-custom classification and direction awareness. Moving storage into the stores leaves TagMap a thin facade, and lets the span use the same stores directly in the next PR.

Additional Notes

• No behavior change. The TagMap suites pass as they are, including the Internals tests, which now exercise the shim. Entry objects are still shared across copies.
• TagStoresTest covers the stores through TagMap, and IdStoreContractTest gains the view cases.
• Tests: internal-api and dd-trace-core pass. The only failures are the two TracerConnectionReliabilityTest cases, which need Docker.

Where this is going

This PR is one step in replacing the span's single TagMap with storage that knows each tag's kind. The goal is to cut what every span allocates on the application thread, without changing behavior.

The end state

• A tag is classified once, at the boundary. A known tag becomes an id (from the tag registry, Resolve direction-scoped OpenTelemetry names at the OpenTelemetry boundary (otlp - tag registry - phase 3) #12731), and a custom tag stays a name. Instrumentation sets known tags by id (Route intercepted tags by id instead of by name (otlp - tag registry - phase 3) #12817), so the JIT can constant-fold the routing.
• The span owns its tag stores directly. Known tags are kept by id and custom tags by name. TagMap becomes a thin view for code outside the span.
• Known tags live in a dense store by default. Parallel arrays of serials and values replace an Entry object per tag, and each span's store is presized from a per-operation hint. With the flag on, a web-request-shaped trace allocates about 30% less on the application thread than on master.
• Core reads and writes a span through SpanContents, not a TagMap. That covers post-processors now and serializers later, so no TagMap view gets built for them. SpanContents starts with four methods and grows a name, id and primitive API as more readers move to it.

Later, not in this stack: cheaper read-through to parent spans, trace-level tags in a per-trace holder (TraceSegment), and primitive tag values without boxing.

The stack. This replaces #12918, split into pieces that can each be reviewed, and measured, on their own.

PR	Step	Allocation per 7-span trace (flag off unless noted)
#12999	A null tag value removes the tag in TagMap. This is the stack's only behavior change, kept separate on purpose.	unchanged
#12992	Bucket known tags by slot in TagMap, and add TraceShapeBenchmark	−96 B vs master
#13001	SpanContents for the eager post-processors	unchanged
#13002	Extract TagMap's bucket mechanics into HashSupport (a pure move)	unchanged
#13006	Add the id and name tag stores, not yet used	unchanged
#13007	Make TagMap a facade over the two stores	unchanged
#13003	Have the span own its tag stores directly	+104 B (+1.3%)
#13004	Dense id store behind trace.experimental.dense.tags.enabled (off by default)	flag on: −30.6%
#12926	Turn the dense store on by default, add sizing hints and bulk prototype seeding	flag on: about 3% less again

Every PR after #12999 is behavior-preserving.

The pieces' allocation figures come from a deterministic per-thread probe (ThreadMXBean allocated bytes) that builds the same 7-span trace as TraceShapeBenchmark. The #12992 and #12926 figures come from TraceShapeBenchmark itself.

🤖 Generated with Claude Code

[datadog-datadog-prod-us1-2]

🎯 Code Coverage (details)
• Patch Coverage: 95.85%
• Overall Coverage: 59.97% (+0.19%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 75ec723 | Docs | Give us feedback!}

[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	14.01 s	13.99 s	[-0.7%; +0.9%] (no difference)
startup:insecure-bank:tracing:Agent	12.92 s	12.98 s	[-1.2%; +0.3%] (no difference)
startup:petclinic:appsec:Agent	48.51 s	27.82 s	[-81.1%; +229.9%] (unstable)
startup:petclinic:iast:Agent	50.62 s	30.12 s	[-63.8%; +199.8%] (unstable)
startup:petclinic:profiling:Agent	31.33 s	53.77 s	[-115.4%; +31.9%] (unstable)
startup:petclinic:sca:Agent	33.15 s	31.54 s	[-61.8%; +72.1%] (unstable)
startup:petclinic:tracing:Agent	29.31 s	43.62 s	[-115.9%; +50.2%] (unstable)

Commit: 75ec723a · CI Pipeline · Benchmarking Platform UI

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