prometeu-runtime/discussion/workflow/plans/PLN-0033-vm-hot-path-ownership-and-string-copy-pressure.md

id	ticket	title	status	created	completed	tags
PLN-0033	perf-vm-allocation-and-copy-pressure	Plan - VM Hot Path Ownership and String Copy Pressure	done	2026-04-20	2026-04-20	perf runtime vm memory strings

Briefing

Implement the runtime-core portion of DEC-0018 by reducing repeated string payload copies and hot-path allocation pressure without changing guest-visible semantics.

Decisions de Origem

• DEC-0018 - VM Allocation and Copy Pressure Baseline

Alvo

Change the VM core so that hot paths over numeric values and already-materialized values converge toward zero allocation, while preserving the public semantics of string values and GET_GLOBAL.

Escopo

Included

• VM value representation and ownership audit for hot paths.
• GET_GLOBAL copy-pressure reduction without semantic drift.
• String-producing opcode hot-path changes, especially ADD.
• Unit and focused integration coverage for unchanged semantics.

Excluded

• Public ABI changes.
• Certification or host-facing telemetry policy changes.
• Broad speculative string interning or copy-on-write rollout unless directly required by a concrete implementation step.

Fora de Escopo

• Cartridge language changes.
• Host debugger protocol changes.
• General fault/log path rewrites beyond keeping them isolated from hot-path work.

Plano de Execucao

Step 1 - Audit current hot-path ownership

What: Identify the exact VM operations that still clone or allocate on already-materialized hot paths. How: Review Value, GET_GLOBAL, ADD, related stack helpers, and any nearby string coercion paths; classify each site as unavoidable first materialization versus avoidable repeated copy. File(s): crates/console/prometeu-bytecode/src/value.rs, crates/console/prometeu-vm/src/virtual_machine.rs

Step 2 - Rework internal representation/ownership for expensive payloads

What: Introduce the smallest internal representation change needed to avoid repeated payload copies for expensive values. How: Prefer internal ownership changes that preserve guest-visible semantics. If strings or other expensive payloads need to move behind handles or equivalent internal storage, keep the public value meaning unchanged and constrain the change to runtime-core internals. File(s): crates/console/prometeu-bytecode/src/value.rs, crates/console/prometeu-vm/src/heap.rs, crates/console/prometeu-vm/src/object.rs, crates/console/prometeu-vm/src/virtual_machine.rs

Step 3 - Fix GET_GLOBAL without semantic change

What: Remove avoidable repeated copy pressure from GET_GLOBAL. How: Keep the observable contract identical while changing retrieval/storage internals so already-materialized expensive payloads are not recopied on each hot-path access. File(s): crates/console/prometeu-vm/src/virtual_machine.rs

Step 4 - Optimize string-producing opcode paths

What: Reduce repeated allocation/copy pressure in string-producing operations, starting with ADD. How: Separate true new-string creation from repeated cloning of existing payloads. Ensure any operation that semantically creates a new string still does so correctly, while avoiding extra transient allocations around existing materialized values. File(s): crates/console/prometeu-vm/src/virtual_machine.rs, related helper modules if introduced

Step 5 - Preserve semantic coverage

What: Prove semantics did not change while ownership internals did. How: Add/update tests for constants, dynamic strings, globals, arithmetic/string ADD, and edge cases where strings cross stack/global boundaries. File(s): crates/console/prometeu-vm/src/virtual_machine.rs, VM test modules

Criterios de Aceite

• GET_GLOBAL keeps its public semantics unchanged.
• Hot-path access to already-materialized expensive values no longer performs avoidable repeated payload copies.
• Numeric hot paths remain allocation-free.
• String operations that truly create a new string remain correct and deterministic.
• VM tests cover constant-pool strings, runtime-created strings, and globals under the new ownership model.

Tests / Validacao

Unit Tests

• Value/ownership tests for expensive payload movement or referencing.
• VM opcode tests for GET_GLOBAL, SET_GLOBAL, and string ADD.

Integration Tests

• Small bytecode programs that read globals repeatedly and mix numeric/string paths.

Manual Verification

• Review allocation-sensitive hotspots in the final implementation to confirm repeated copies were removed rather than merely relocated.

Riscos

• Internal representation changes may ripple into GC/root traversal.
• A partial fix may hide copies in helper methods rather than eliminate them.
• Over-optimizing strings could accidentally change guest-visible behavior if semantic boundaries are not defended by tests.

Dependencies

• DEC-0018
• Existing VM heap/object/root traversal contracts