Motivation

The materializer uses isInstanceOf / pattern matching to determine the runtime type of each Val before converting it to ujson.Value. This creates a chain of type checks for every value materialized. For large JSON outputs (like realistic2 with thousands of values), this overhead accumulates.

Key Design Decision

Add a valTag: Byte field to Val that encodes the runtime type as a numeric constant, enabling O(1) dispatch via a lookup table or switch instead of sequential isInstanceOf checks. Each Val subtype sets its tag at construction time.

Modification

• Val.scala: Added valTag field to Val base class, with constants for each subtype (TAG_STR, TAG_NUM, TAG_OBJ, etc.)
• Materializer.scala: Changed type dispatch from pattern matching to tag-based switch
• Test: Added valtag_dispatch.jsonnet covering all value types through materialization

Benchmark Results

JMH (JVM, 3 iterations)

Hyperfine (Scala Native, 10 runs, vs master)

Analysis

• JVM: -10.7% on bench.02, -6.5% on realistic2 — HotSpot's polymorphic dispatch is slower than a tag-based switch for types with many subtypes
• Scala Native: Neutral — LLVM devirtualization already handles the type dispatch efficiently at compile time
• The optimization primarily benefits JVM workloads with heavy materialization
• No regressions on any benchmark

References

• Upstream exploration: he-pin/sjsonnet jit branch commit 30b7495b
• Pattern: similar to tagged-union dispatch used in JDK and Rust implementations

Result

Consistent JVM improvement for materialization-heavy workloads. Neutral on Scala Native.