51 lines
2.5 KiB
Markdown
51 lines
2.5 KiB
Markdown
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MIR13 Mode (PHI-off by default)
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Overview
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- Goal: Stabilize execution by turning off PHI emission in the Bridge/Builder and letting the LLVM (llvmlite) layer synthesize PHIs as needed.
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- Default: MIR13 is ON by default (PHI-off). Use env flags to flip.
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Why
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- Fewer SSA obligations in the front-end (Bridge/Builder) reduces CFG corner cases (short‑circuit, nested if/loop merges).
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- Centralizes SSA invariants in a single place (llvmlite resolver/finalizer), improving correctness and maintenance.
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Flags and Behavior
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- NYASH_MIR_NO_PHI (default: 1)
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- 1: Bridge/Builder emit edge copies instead of PHIs at merges (MIR13).
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- 0: Bridge/Builder may emit PHIs (MIR14 experimental).
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- NYASH_VERIFY_ALLOW_NO_PHI (default: 1)
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- Relaxes verifier checks that assume PHIs at merges.
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- NYASH_LLVM_USE_HARNESS=1 (AOT via llvmlite harness)
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- Resolver/finalizer synthesize PHIs at block heads when needed.
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LLVM (llvmlite) Responsibilities
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- setup_phi_placeholders(): predeclare JSON‑provided PHIs and collect incoming metadata.
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- block_end_values: snapshot per block end to materialize predecessor values (dominance‑safe).
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- finalize_phis(): wire incoming edges for declared PHIs at block heads.
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- Resolver.resolve_i64():
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- single‑pred: take predecessor end value;
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- multi‑pred + declared PHI: reuse placeholder at block head;
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- multi‑pred + no PHI: synthesize a localization PHI at the current block head (MIR13 compatibility);
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- avoids reusing non‑dominating vmap values across blocks.
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Bridge/Builder (JSON v0) Behavior
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- If/Loop/Try are lowered without PHIs when MIR13 is ON; merges are performed with edge copies (merge_var_maps) and the final value is reconstituted by the LLVM layer if needed.
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- Helper split for readability (no behavior change):
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- lowering/{if_else.rs, loop_.rs, try_catch.rs, merge.rs}
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Testing
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- Curated LLVM (default = PHI‑off):
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- tools/smokes/curated_llvm.sh (use --phi-on to exercise MIR14)
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- PHI invariants/parity (AOT vs PyVM):
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- tools/pyvm_vs_llvmlite.sh (default compares exit code; use CMP_STRICT=1 for stdout+exit)
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- Bridge/PyVM:
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- tools/selfhost_stage2_bridge_smoke.sh
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How to Force PHI‑on (MIR14 experimental)
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- Set: NYASH_MIR_NO_PHI=0 and run tools/smokes/curated_llvm.sh --phi-on
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- Known: loop_if_phi may trip dominance issues in PHI‑on; MIR13 is the recommended default.
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Known Limitations (current)
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- No full tracing GC; object lifetime is managed via handle registries and Arc lifetimes.
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- PHI‑on path is still being refined for some control‑flow patterns.
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