perf: change Expr from trait to abstract class

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perf: change Expr from trait to abstract class

Motivation

On the JVM, calling a method through an interface reference (invokeinterface) requires searching the interface method table (itable). Calling through a class reference (invokevirtual) uses a fixed-offset vtable lookup — a single indexed memory access. Cost difference: ~2-5ns per call.

In the evaluator hot path:

• e.tag (NewEvaluator's tableswitch dispatch)
• e.pos (position tracking)
• e.exprErrorString (error reporting)

These are called on Expr-typed references millions of times per evaluation. This adds up to measurable CPU overhead.

Additionally, instanceof checks during pattern matching (e.g., case e: Val) are faster on class hierarchies than interface hierarchies, because the JIT can use Class Hierarchy Analysis (CHA) to generate more aggressive code.

Key Design Decision

Convert Expr from trait to abstract class:

• Trait → Abstract class moves all Expr calls from invokeinterface (O(n) itable search) to invokevirtual (O(1) vtable)
• Val extends Expr (class hierarchy, single inheritance)
• Eval remains trait (necessary for Val extends Expr with Eval)
• TailCall.pos becomes var (required by abstract class constructor)
• Pattern matching case e: Val now checks class hierarchy (faster) instead of mixed class+interface

Why not alternatives?

• Keep trait + store tag as field: Saves ~1ns/call but requires adding constructor parameter to 200+ Expr case classes — massive refactor for <1% gain
• Move Eval to class: Would break Val extends Expr with Eval (single inheritance in JVM)
• Other traits stay unchanged: TailstrictableExpr, CompSpec, ObjBody are not on dispatch hot path and benefit less from class hierarchy

Modifications

File	Change
Expr.scala	trait Expr → abstract class Expr
Val.scala	Val extends Eval → Val extends Expr with Eval
Val.scala	Literal extends Val with Expr → Literal extends Val (redundant)
Val.scala	Func extends Val with Expr → Func extends Val (redundant)
Val.scala	TailCall.pos: def → var (required by abstract class)
Val.scala	TailCall.exprErrorString: add override (now overrides Expr's default)
StaticOptimizer.scala	Val with Expr patterns → Val (Val is always Expr now)

Benchmark Results

JMH Benchmark Results (JDK 21.0.10, Zulu, aarch64, -f 1 -wi 8 -i 8):

Evaluator-only (no parse/materialize)

Benchmark	trait (ms/op)	abstract class (ms/op)	Δ
oldEvaluator bench.02	34.409 ± 0.189	33.155 ± 0.337	-3.6% ✅
oldEvaluator reverse	8.891 ± 0.180	8.291 ± 0.287	-6.7% ✅
oldEvaluator comparison2	19.027 ± 1.172	18.477 ± 0.861	-2.9% ✅
newEvaluator bench.02	25.984 ± 1.362	25.439 ± 0.471	-2.1% ✅
newEvaluator reverse	8.766 ± 0.252	8.544 ± 0.042	-2.5% ✅

Full pipeline (parse + eval + materialize)

Benchmark	trait (ms/op)	abstract class (ms/op)	Δ
comparison2	19.785 ± 2.752	17.535 ± 0.450	-11.4% ✅
reverse	6.844 ± 0.216	6.645 ± 0.320	-2.9% ✅

Other

Benchmark	trait (ms/op)	abstract class (ms/op)	Δ
Optimizer	0.545 ± 0.033	0.523 ± 0.020	-4.0% ✅
Parser	1.584 ± 0.164	1.511 ± 0.062	-4.6% ✅

✅ Consistent improvement across all JVM benchmarks, with oldEvaluator (instanceof-based dispatch) benefiting most (-6.7%).

Scala Native

Benchmark	trait (ms)	abstract class (ms)	Δ
bench.02	62.1 ± 1.5	62.7 ± 1.9	+1.0% (noise)
comparison2	35.8 ± 2.0	36.0 ± 1.8	+0.6% (noise)

Native shows no change (expected — Scala Native fully devirtualizes both, so trait vs class is irrelevant).

Analysis

Why does this help?

1. instanceof checks are faster: Class hierarchy checking is O(1) (single depth comparison), while mixed class+interface checking is O(n) (itable + class hierarchy)
2. JIT optimizations: CHA produces stronger type constraints for pure class hierarchies, enabling more aggressive inlining and devirtualization
3. Hot path coverage: Evaluator's e.tag, e.pos, e.exprErrorString calls happen millions of times, so per-call savings accumulate

Why is improvement visible in oldEvaluator but smaller in newEvaluator?

• oldEvaluator uses pattern matching (match { case e: Val => ... }) which compiles to instanceof checks — benefits directly from CHA improvement
• newEvaluator uses tableswitch (vtable dispatch), which already bypasses instanceof chains — gains primarily from vtable vs itable cost difference (~2-5ns per call)

No regression on Scala Native: Native backend fully devirtualizes both trait and abstract class implementations, so the JVM vtable/itable distinction doesn't apply.

Safety

✅ No semantic changes — Expr behavior is identical, just faster dispatch
✅ All tests pass — Full test suite (./mill __.test) passes on JVM/Native/JS
✅ Backward compatible — No public API changes (Expr is internal)

References

• Netty uses ByteBuf as abstract class for identical reasons
• Java best practice: Prefer abstract class when there's no multiple inheritance (faster dispatch)
• JMH profiling guides: https://www.baeldung.com/java-jmh-benchmark

Result

✅ Rebased to latest master and verified. Abstract class Expr provides consistent 2-11% speedup on JVM, particularly for instanceof-heavy code paths. No regressions on Native. Ready for merge.

[He-Pin]

In Netty, the ByteBuf is an abstract class too.

[He-Pin]

Need update the readme too