32 lines
1.5 KiB
Markdown
32 lines
1.5 KiB
Markdown
# PHI‑Off Edge‑Copy + Harness PHI Synthesis (Draft)
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Problem
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- Frontend/Builder PHI construction intertwines with optimization order and dominance; fragile in multi‑backend settings.
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Contribution (Nyash)
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- Keep builders PHI‑off using Edge‑Copy policy (pred→copy→merge), and synthesize PHIs late in the harness.
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- Constrain PHI placement: always at block head (grouped) via `phi_at_block_head`/`ensure_phi`.
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- Make PHI wiring observable by JSONL traces + checker.
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Method
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- IR discipline: JSON v0 Bridge emits no PHIs; records edge‑end values per block.
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- Harness (`finalize_phis`) resolves `(decl_b, src_vid)` pairs to the nearest predecessor on CFG paths and wires incoming at block heads.
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- Self‑carry handling: prefer a non‑self initial source if present.
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- Helpers: `phi_at_block_head`, `ensure_phi`, `wire_incomings`.
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Code References
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- Bridge edge‑copy: `src/runner/json_v0_bridge/lowering/*`
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- Harness PHI: `src/llvm_py/phi_wiring/wiring.py`, `src/llvm_py/llvm_builder.py`
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- Head placement helper: `phi_at_block_head()`
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Evaluation Plan
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- Correctness: run curated smokes with complex mixes (loops, ifs, returns, exceptions) and diff traces.
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- Robustness: mutate block orders; verify PHIs remain grouped at heads and traces stable.
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- Cost: count PHIs vs. classic SSA on samples (optional).
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Reproduce
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- `cargo build --release`
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- `bash tools/test/smoke/bridge/try_result_mode.sh`
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- Optional trace: `NYASH_LLVM_TRACE_PHI=1 NYASH_LLVM_TRACE_OUT=tmp/phi.jsonl ...`
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