hakorune/docs/papers/phi_off_harness.md

PHI‑Off Edge‑Copy + Harness PHI Synthesis (Draft)

Problem

• Frontend/Builder PHI construction intertwines with optimization order and dominance; fragile in multi‑backend settings.

Contribution (Nyash)

• Keep builders PHI‑off using Edge‑Copy policy (pred→copy→merge), and synthesize PHIs late in the harness.
• Constrain PHI placement: always at block head (grouped) via phi_at_block_head/ensure_phi.
• Make PHI wiring observable by JSONL traces + checker.

Method

• IR discipline: JSON v0 Bridge emits no PHIs; records edge‑end values per block.
• Harness (finalize_phis) resolves (decl_b, src_vid) pairs to the nearest predecessor on CFG paths and wires incoming at block heads.
• Self‑carry handling: prefer a non‑self initial source if present.
• Helpers: phi_at_block_head, ensure_phi, wire_incomings.

Code References

• Bridge edge‑copy: src/runner/json_v0_bridge/lowering/*
• Harness PHI: src/llvm_py/phi_wiring/wiring.py, src/llvm_py/llvm_builder.py
• Head placement helper: phi_at_block_head()

Evaluation Plan

• Correctness: run curated smokes with complex mixes (loops, ifs, returns, exceptions) and diff traces.
• Robustness: mutate block orders; verify PHIs remain grouped at heads and traces stable.
• Cost: count PHIs vs. classic SSA on samples (optional).

Reproduce

• cargo build --release
• bash tools/test/smoke/bridge/try_result_mode.sh
• Optional trace: NYASH_LLVM_TRACE_PHI=1 NYASH_LLVM_TRACE_OUT=tmp/phi.jsonl ...