hakorune/docs/reference/architecture/phi-and-ssa.md

PHI and SSA in Nyash

Overview

• Nyash lowers high-level control flow (If/Loop/Match) to MIR and backends that rely on SSA form.
• We prioritize IR hygiene and observability while keeping runtime cost at zero.

Design points

• PHI hygiene: no empty PHIs; PHIs at block head only.
• JoinResult hint: when both branches assign the same variable, we emit a MIR hint for diagnostics.
• Loop carriers: loops may expose a carrier observation (≤ N variables, where N is unconstrained by design; smokes emphasize common cases).

Normalization

• If: may optionally wrap into LoopForm under a conservative gate (dev only). Semantics remain unchanged.
• Match: scrutinee evaluated once, guard fused; normalized to nested If‑chain in macro/core pass.

Testing

• LLVM smokes: fixed small cases ensure no empty PHIs and head placement.
• MIR smokes: trace scope|join|loop to validate shaping without peeking into IR details.

Roadmap

• Remove text-level sanitization once finalize‑PHI is trustworthy across Loop/If/Match.
• Expand goldens to cover nested joins and multi‑carrier loops while keeping CI light.