Performance: replace the boxed ArrayDeque operand stack with a flat Object[] stack

[bertysentry]

Motivation

The AVM interpreter is ~4.4x slower than gawk on a tight arithmetic loop (measured on the PR #534 branch, but this is long-standing and unrelated to runtime typing):

BEGIN { s=0; for(i=0;i<5000000;i++) s=s+i; print s }

• gawk: ~1.5 s
• jawk: ~6.5 s

Problem

Every opcode pushes/pops operands through Deque<Object> operandStack = new ArrayDeque<Object>() (AVM.java). Each access pays:

• ArrayDeque.push/pop call overhead: internal head-index wraparound math, capacity checks, and null checks
• a NULL_OPERAND sentinel mapping on both push() and pop() (AVM.pop()/AVM.push())

A typical statement like s = s + i executes 4-6 opcodes, each with 1-2 stack operations, so this overhead multiplies across every executed tuple.

Proposal

Replace the deque with a plain Object[] and an explicit top-of-stack index owned by the AVM:

• push/pop/peek become single array accesses that the JIT can fully inline
• grow the array on demand (operand stack depth is bounded by expression nesting, so it stays small)
• the NULL_OPERAND sentinel can likely be dropped since the AVM controls all accesses and can allow null slots directly

The operand stack is fully encapsulated behind push()/pop() in AVM (with a couple of operandStack.clear() call sites), so this is a contained change.

Note

popArguments() and print/printf argument gathering also allocate an Object[] per call; worth revisiting in the same pass where they sit on hot paths.

Measurement

Use the project's existing JMH harness rather than wall-clock CLI timings:

mvn package -Pbenchmark -DskipTests
java -jar target/jawk-*-benchmarks.jar

src/jmh/java/io/jawk/backend/AVMExpressionBenchmark.java already exercises the interpreter's expression evaluation and is the right place to anchor before/after numbers (add a tight arithmetic-loop case there if not already covered). AwkScriptBenchmark gives the end-to-end view. The gawk-vs-jawk CLI numbers above are motivation only — JMH results are the acceptance criterion.

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