30 lines
1.7 KiB
Markdown
30 lines
1.7 KiB
Markdown
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## MIR PHI Policy (Phase‑15)
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Status
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- Default: PHI‑off (edge‑copy mode). Builders/Bridge do not emit PHI; merges are realized via per‑predecessor Copy into the merge destination.
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- Dev‑only: PHI‑on is experimental for targeted tests (enable with `NYASH_MIR_NO_PHI=0`).
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- LLVM: PHI synthesis is delegated to the LLVM/llvmlite path (AOT/EXE). PyVM serves as the semantic reference.
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Rationale
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- Simplify MIR builders and JSON bridge by removing PHI placement decisions from the core path.
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- Centralize SSA join formation at a single backend (LLVM harness), reducing maintenance and divergence.
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- Keep PyVM parity by treating merges as value routing; short‑circuit semantics remain unchanged.
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Operational Rules (PHI‑off)
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- Edge‑copy only: predecessors write the merged destination via `Copy { dst=merged, src=pred_value }`.
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- Merge block: must not emit a self‑Copy for the same destination; merged value is already defined by predecessors.
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- Verifier: `verify_allow_no_phi()` is on by default; dominance and merge checks are relaxed in PHI‑off.
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- Developer Notes (PHI‑on dev mode)
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- Requires cargo feature `phi-legacy`. Build with `--features phi-legacy` and enable with `NYASH_MIR_NO_PHI=0`.
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Builders may place `Phi` at block heads with inputs covering all predecessors.
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- Use `NYASH_LLVM_TRACE_PHI=1` for wiring trace; prefer small, isolated tests.
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Backends
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- LLVM harness performs PHI synthesis based on predecessor copies and dominance.
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- Other backends (Cranelift/JIT) are secondary during Phase‑15; PHI synthesis there is not required.
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Acceptance
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- Default smokes/CI run with PHI‑off.
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- Parity checks compare PyVM vs. LLVM AOT outputs; differences are resolved on the LLVM side when they stem from PHI formation.
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