99 lines
3.4 KiB
Markdown
99 lines
3.4 KiB
Markdown
# MIR 26-Instruction Diet (Agreed Final Set)
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Goal
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- Converge on a lean, proven instruction set guided by vm-stats and E2E.
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- Preserve hot paths, demote meta, fold type ops, reserve room for growth.
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Agreed Final Set (26)
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1) Const
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2) Copy
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3) Load
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4) Store
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5) BinOp
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6) UnaryOp
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7) Compare
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8) Jump
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9) Branch
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10) Phi
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11) Return
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12) Call
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13) BoxCall
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14) NewBox
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15) ArrayGet
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16) ArraySet
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17) RefNew
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18) RefGet
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19) RefSet
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20) Await
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21) Print
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22) ExternCall (keep minimal; prefer BoxCall)
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23) TypeOp (unify TypeCheck/Cast)
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24) WeakRef (unify WeakNew/WeakLoad)
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25) Barrier (unify BarrierRead/BarrierWrite)
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26) Reserve (future async/error instr)
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Hot/Core (keep)
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- Data: Const, Copy, Load, Store
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- ALU: BinOp, UnaryOp, Compare
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- Control: Jump, Branch, Phi, Return
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- Calls: Call, BoxCall
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- Objects: NewBox
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- Arrays: ArrayGet, ArraySet
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Likely Keep (usage-dependent)
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- Refs: RefNew, RefGet, RefSet (seen in language features; keep unless stats prove cold)
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- Async: Await (FutureNew/Set can be Box/APIs)
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Meta (demote to build-mode)
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- Debug, Nop, Safepoint
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Type Ops (fold)
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- TypeCheck, Cast → fold/verify-time or unify as a single TypeOp (optional).
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External (unify)
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- ExternCall → prefer BoxCall; keep ExternCall only where required.
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Extended/Reserve
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- Weak*: WeakNew, WeakLoad
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- Barriers: BarrierRead, BarrierWrite
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- 2 Reserve IDs for future async/error instrumentation
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Mapping Notes
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- HTTP E2E shows BoxCall/NewBox dominate (33–42%), then Const/NewBox, with Branch/Jump/Phi only in error flows.
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- FileBox path similarly heavy on BoxCall/NewBox/Const.
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- Implication: invest into BoxCall fast path and const/alloc optimization.
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Migration Strategy
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1) Introduce an experimental feature gate that switches TypeCheck/Cast to TypeOp or folds them.
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2) Demote Debug/Nop/Safepoint under non-release builds.
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3) Keep ExternCall available, route new external APIs via BoxCall when possible.
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4) Track Weak*/Barrier usage; unify under WeakRef/Barrier. Graduate dedicated ops only if vm-stats shows recurring use.
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Mapping from Current → Final
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- TypeCheck, Cast → TypeOp
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- WeakNew, WeakLoad → WeakRef
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- BarrierRead, BarrierWrite → Barrier
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- (Keep) ExternCall, but prefer BoxCall where possible（ExternCall は最小限に）
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- (Keep) Debug/Nop/Safepoint as meta under non-release builds（命令セット外のビルドモード制御に降格）
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Verification
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- Add MIR verifier rule: use-before-def across merges is rejected (guards phi misuse).
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- Add snapshot tests for classic if-merge returning phi.
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Appendix: Rationale
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- Smaller ISA simplifies VM fast paths and aids JIT/AOT later.
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- Data shows hot paths concentrated on calls, const, alloc; control sparse and localized.
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- Folding type ops reduces interpreter/VM dispatch; verification handles safety.
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---
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Feature Flags (Cargo)
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- Deprecated flags (unified by default):
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- `mir_typeop_poc`: no-op now. Builder always emits `TypeOp(Check/Cast)` instead of legacy `TypeCheck/Cast`。
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- `mir_refbarrier_unify_poc`: no-op now. Builder always emits `WeakRef/Barrier` instead of legacy `WeakNew/WeakLoad/BarrierRead/Write`。
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- Optimizer has a Pass 0 that normalizes any残存の旧命令 → 統合命令（安全ネット）。
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Status (2025-08-23)
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- Flags declared in `Cargo.toml` (off by default)
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- PoC design doc: `docs/development/proposals/mir-typeop-weakref-barrier-poc.md`
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- Next: land builder/VM mapping behind flags, add snapshot tests; no behavior change with flags off
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