42 lines
2.7 KiB
Markdown
42 lines
2.7 KiB
Markdown
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Phi Entry in Hako — Design Notes (SSA/CFG Parity)
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Purpose
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- Specify how to implement SSA φ (phi) on the Hakorune side cleanly, mirroring Rust/Core invariants while keeping the code small and testable.
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Rust/Core invariants to adopt (parity)
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- Placement: φ nodes are considered at the head of a block (grouped), applied once at block entry.
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- Selection: choose one incoming (value, pred) where pred == prev_bb (the block we arrived from).
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- Coverage: incoming pairs cover all reachable predecessors. Missing entries are a hard error in strict mode.
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- Execution: after φ application, the resulting dst registers are defined before any instruction in the block reads them.
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Hako design (Reader → IR → Runner)
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- Reader (JsonV1ReaderBox, extended):
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- Parse MIR JSON v1 into a minimal per-function IR: blocks (id, insts[]), and extract φ entries into a phi_table (block_id → [(dst, [(pred,val)])]).
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- Keep scanning light by using JsonFragBox helpers (read_int_from/after, seek_array_end, scan_string_end).
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- PhiTable (V1PhiTableBox):
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- API: apply_at_entry(regs, phi_table, prev_bb, block_id, policy) → writes dst from the matched incoming.
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- policy.strict (default ON): fail-fast when incoming is missing or source is undefined; policy.tolerate_void (dev) treats missing/undefined as Void/0.
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- Runner (NyVmDispatcherV1Box):
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- On block entry: apply φ via PhiTable; then run instructions (φ removed from the runtime loop).
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- Branch/jump update prev_bb and bb; compare/branch read the compare.dst as the condition value.
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Flags
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- HAKO_V1_PHI_STRICT=1 (default), HAKO_V1_PHI_TOLERATE_VOID=0 (dev-only safety).
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- HAKO_V1_DISPATCHER_FLOW=1 to run the IR-based flow; keep fallback to Mini-VM and Core for stability during bring-up.
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Testing plan
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- Canary 1: simple if (then/else with single incoming) → ret of φ.dst equals the selected value.
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- Canary 2: multi-incoming with (pred,val) pairs for both paths; ensure prev_bb select works for both branches.
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- Canary 3: nested branch (entry φ in deeper block).
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- Negative: missing incoming for reachable pred → strict fail; tolerate_void → rc stable with Void/0.
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Why this works in Hako
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- Although Hako doesn’t have first-class structs, the minimal IR and phi_table can be represented as arrays of tuples or MiniMap-backed strings with helper boxes.
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- JsonFragBox provides escape-aware scanning; Reader avoids brittle substring logic.
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- Runner remains small and composable: “read/apply/run” with φ isolated at entry.
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Migration plan
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- Phase 20.37: introduce Reader+PhiTable+entry-apply (flagged), keep fallback to Mini-VM/Core.
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- Phase 20.38+: expand coverage (binop/compare edges), flip v1 verify default to Hako when parity canaries are green.
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