Remove LegacyTagMap; OptimizedTagMap is the sole TagMap implementation

[dougqh]

What

OptimizedTagMap has been the default for some time (the optimized.map.enabled flag defaulted to true). With Legacy effectively dead, this removes it so OptimizedTagMap is the sole TagMap implementor — satisfying CHA's unique-implementor precondition (a prerequisite for upcoming read-through / dense-store work).

Changes

• Delete LegacyTagMap and the whole TagMapFactory abstraction (the abstract class + its lone OptimizedTagMapFactory); TagMap.create()/create(int)/EMPTY now construct OptimizedTagMap directly.
• Delete the now-trivial TagMapType / TagMapTypePair test enums and de-parameterize their tests to plain @Test. TagMapScenario (parameterizes by size) is kept.
• Remove the optimized.map.enabled flag — Config field/getter, GeneralConfig constant, and the metadata/supported-configurations.json entry. Per the Config team, removing the registry entry is detected automatically at the next release (FPD + public docs get a Max Version: v1.xx.0 marker).

Notable

• Fixes a latent static-init cycle that removing the flag exposed: TagMap (interface with default methods) ↔ OptimizedTagMap could leave TagMap.EMPTY null when OptimizedTagMap initialized first. A lazy EmptyHolder builds the empty singleton via the constructor instead of reading a mid-<clinit> static.
• isOptimized() is intentionally left in place — it now constant-folds to true and is DCE'd at its call site (DDSpanContext); simplifying it away is a trivial follow-up.

Validation

• internal-api TagMap suite green (TagMapTest, TagMapLedgerTest, TagMapFuzzTest); internal-api / dd-trace-api / dd-trace-core compile clean.
• ⏳ Draft pending a broader dd-trace-core integration run before marking ready.

Benchmark — overhead-neutral (parity)

TagMapAccessBenchmark (threading-correct, @Threads(8)) run on the pre-kill tree vs. this branch — the two differ by exactly these two commits (verified byte-identical pre-image). Throughput, 1 fork, 3 warmup + 5 measurement iters:

Benchmark	Pre-kill (ops/s)	Post-kill (ops/s)	Δ
insert	55.2M ±2.7M	53.9M ±6.2M	−2.3%
getObject	91.8M ±8.7M	93.3M ±10.2M	+1.6%
getEntry	91.7M ±18.2M	93.3M ±11.2M	+1.8%

All deltas are within run-to-run error (single fork) → no measurable change, as expected for a dead-code removal.

🤖 Generated with Claude Code

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⚠️ Warnings

🚦 1 Pipeline job failed

DataDog/apm-reliability/dd-trace-java | linux-java-spring-petclinic-sca-load-parallel     

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[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	13.97 s	[-0.7%; +0.7%] (no difference)
startup:insecure-bank:tracing:Agent	12.88 s	12.98 s	[-1.4%; -0.1%] (maybe better)
startup:petclinic:appsec:Agent	16.76 s	16.59 s	[+0.0%; +2.0%] (maybe worse)
startup:petclinic:iast:Agent	16.95 s	17.01 s	[-1.1%; +0.4%] (no difference)
startup:petclinic:profiling:Agent	16.83 s	16.93 s	[-1.6%; +0.4%] (no difference)
startup:petclinic:sca:Agent	16.87 s	16.79 s	[-0.5%; +1.3%] (no difference)
startup:petclinic:tracing:Agent	16.06 s	16.06 s	[-1.0%; +1.0%] (no difference)

Commit: 82f5311f · 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.