[dougqh]

What Does This Do

Adds the two tag stores the span will own, without using them yet.

• IdStore keeps known tags by id. Its one implementation today is HashIdStore, built through IdStoreFactory. A JVM loads a single IdStore subclass, so class hierarchy analysis devirtualizes every call on it.
• HashStore keeps custom tags by name.
• Both reuse HashSupport's bucket mechanics (Extract TagMap's bucket mechanics into HashSupport (dense store - part 4) #13002) and read through to a frozen parent's store. Each hides an inherited tag with a tombstone:
  • in the id store, one shared, immutable Entry per known tag (Entry.tombstoneOf);
  • in the name store, a set of removed names.
• HashIdStore has no slot masks. Its buckets are the exact state.

Nothing uses the stores yet. TagMap and the span keep their current storage. The only TagMap changes are to the nested Entry, EntryReader and BucketGroup classes, which the stores share: a tombstone flag and a lookup of a bucket chain by id.

Also in this PR:

• HashSupport.checkBuckets now indexes a group's entries by entry instead of by bucket, so the integrity check really inspects groups.
• The tag registry gains _dd.agent_psr as a trace-level known tag.

Motivation

This is the first of three PRs that replace the span's single TagMap with two stores split by tag kind. Here the stores arrive on their own, pinned by IdStoreContractTest, so they can be reviewed against a contract before anything depends on them.

Additional Notes

• No behavior change: TagMap is untouched apart from the nested classes above, and the existing suites pass unchanged.
• IdStoreContractTest runs against the stores directly. Its cases that need a TagMap view arrive with the facade PR.

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

[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	13.94 s	[-0.3%; +1.1%] (no difference)
startup:insecure-bank:tracing:Agent	12.87 s	12.87 s	[-0.5%; +0.6%] (no difference)
startup:petclinic:appsec:Agent	17.14 s	17.03 s	[-0.2%; +1.4%] (no difference)
startup:petclinic:iast:Agent	17.07 s	16.48 s	[-0.7%; +7.8%] (no difference)
startup:petclinic:profiling:Agent	16.78 s	16.92 s	[-1.8%; +0.1%] (no difference)
startup:petclinic:sca:Agent	17.12 s	16.98 s	[+0.0%; +1.6%] (maybe worse)
startup:petclinic:tracing:Agent	16.16 s	16.21 s	[-1.3%; +0.7%] (no difference)

Commit: ec6ec40e · 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.