[dougqh]

What Does This Do

A pure move. TagMap's loops over one bucket array become static methods of a new HashSupport class, moved verbatim:

• find, put, remove, copy and merge;
• the forEach variants, fillMap and fillStringMap;
• count, isEmpty, the bucket-position check and toInternalString.

Each method reports what it did through its return value: the entry that put replaced, the entry that remove removed, and how much merge grew the map. TagMap keeps its fields and its size counter, plus the read-through and tombstone handling around those calls.

Motivation

The next PR in the stack splits a span's tags into a known-tag store and a custom-tag store. Both reuse these mechanics, so this moves them out first and reviews the move on its own, separate from the split.

Additional Notes

• No new objects or fields, and no behavior change. Allocation per trace matches the base, and the TagMap and core span suites pass unchanged.
• The bucket-integrity check was copied as-is. It still has a pre-existing indexing bug (it uses the outer bucket index inside the group loop). The split PR next in the stack fixes it, so this one stays a verbatim move.

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	13.96 s	13.91 s	[-0.2%; +1.0%] (no difference)
startup:insecure-bank:tracing:Agent	12.95 s	12.99 s	[-1.2%; +0.4%] (no difference)
startup:petclinic:appsec:Agent	17.18 s	17.07 s	[-0.3%; +1.7%] (no difference)
startup:petclinic:iast:Agent	17.04 s	16.93 s	[-0.3%; +1.6%] (no difference)
startup:petclinic:profiling:Agent	16.06 s	16.67 s	[-9.2%; +1.8%] (unstable)
startup:petclinic:sca:Agent	17.04 s	16.80 s	[+0.6%; +2.3%] (maybe worse)
startup:petclinic:tracing:Agent	16.11 s	16.16 s	[-1.3%; +0.7%] (no difference)

Commit: 4c83a5c3 · 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.