431 lines
18 KiB
Markdown
431 lines
18 KiB
Markdown
# SX VM Opcode Extension Mechanism
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Mechanism in `hosts/ocaml/evaluator/` that lets language ports register
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specialized bytecode opcodes without modifying the SX VM core. Direct
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prerequisite for **erlang-on-sx Phase 9** (the BEAM analog) and a structural
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enabler for any future language port that wants performance-critical opcodes.
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Reference: `plans/erlang-on-sx.md` Phase 9, `plans/fed-sx-design.md` §17.5,
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`hosts/ocaml/lib/sx_vm.ml` (current VM).
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Status: **design** — implementation pending. Sister workstream to the
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`loops/erlang` loop, but lives in `hosts/`, not `lib/erlang/`.
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---
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## Goal
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Allow language ports to register custom bytecode opcodes in the SX VM, with:
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- **Zero overhead for core opcodes.** Existing 37 opcodes (per `sx_vm.ml`)
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must dispatch identically. No regression for any existing language port or
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the core SX runtime.
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- **One additional dispatch step for extension opcodes.** Acceptable cost; the
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win comes from avoiding the general CEK machinery.
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- **Per-extension state slot.** Erlang's process scheduler, Haskell's thunk
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cache, etc. need somewhere to hang state alongside the VM.
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- **Compiler awareness.** The bytecode compiler (`lib/compiler.sx`) must be
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able to emit extension opcodes by name, looked up against the registered
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set.
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- **JIT compatibility.** Existing JIT (lazy lambda compilation) continues to
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work for code paths using only core opcodes. Extension opcodes are
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interpreted in v1; JITing them is a follow-up.
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## Non-goals
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- **Hot opcode reload.** Adding/replacing opcodes mid-runtime is not in
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scope. Extensions are compile-time additions to the OCaml binary. (If
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needed, that's a separate project.)
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- **Per-instance opcode sets.** All running instances of the SX VM share
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the same opcode set determined at build time. Selective opcode loading
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per instance is out of scope.
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- **Opcode hot-swap or supersession.** Once registered, opcodes are stable
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for the lifetime of the binary.
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- **Language-port isolation at the dispatch layer.** Two language ports can
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see each other's opcodes (they share the dispatch table). Isolation is a
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build-time concern — don't compile in extensions you don't trust.
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---
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## Why now
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The Erlang-on-SX Phase 9 work needs this. Without it, Phase 9b-9g (the actual
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opcode implementations) have nowhere to plug in. The Erlang loop will hit
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this dependency as a Blocker; this design is what unblocks it.
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It also enables the **shared opcode pattern** discussed in `plans/fed-sx-
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design.md` §17.5: opcodes Erlang Phase 9 produces that other ports could
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plausibly use (pattern match, perform/handle, record access) get chiselled
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out to `lib/guest/vm/` when a second port has an actual second use. Without
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the extension mechanism, each port would have to fork the SX VM core or
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modify shared dispatch — neither acceptable.
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---
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## Architectural overview
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```
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┌──────────────────────────────────────────┐
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│ SX VM core (hosts/ocaml/lib/sx_vm.ml) │
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│ │
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│ ┌────────────────────────────────────┐ │
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│ │ Bytecode dispatch loop │ │
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│ │ │ │
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│ │ match op with │ │
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│ │ | 1 (OP_CONST) -> ... │ │
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│ │ | 2 (OP_NIL) -> ... │ │
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│ │ | ... │ │
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│ │ | 199 -> ... (last core opcode) │ │
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│ │ | op when op >= 200 -> │ │
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│ │ Extensions.dispatch op vm │ │ ◄── new
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│ │ frame │ │
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│ └────────────────────────────────────┘ │
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│ │
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│ ┌────────────────────────────────────┐ │
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│ │ Extension registry │ │
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│ │ opcode_id -> handler │ │ ◄── new
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│ │ opcode_name -> opcode_id │ │
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│ │ extension_state per extension │ │
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│ └────────────────────────────────────┘ │
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└──────────────────────────────────────────┘
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▲
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│ register at startup
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┌──────────────────┴──────────────────────┐
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│ Extension modules │
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│ hosts/ocaml/extensions/erlang.ml │
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│ hosts/ocaml/extensions/haskell.ml │
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│ hosts/ocaml/extensions/datalog.ml │
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│ hosts/ocaml/extensions/guest_vm.ml │ ◄── shared opcodes
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└─────────────────────────────────────────┘
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```
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### Opcode ID space partition
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Current SX VM uses opcode IDs in roughly the range 1-162 (per inspection of
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`sx_vm.ml`). We partition the 0-255 space:
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| Range | Use |
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|-------|-----|
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| 0 | reserved / NOP |
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| 1-127 | **core opcodes** — owned by the SX VM, locked schema |
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| 128-199 | **`lib/guest/vm/` shared opcodes** — chiselled-out shared opcodes |
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| 200-247 | **language-port opcodes** — registered by extensions |
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| 248-255 | reserved for future expansion / multi-byte opcodes |
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This gives ~50 slots for shared opcodes (Phase 1-2 of `lib/guest/vm/` will
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not exhaust this; we can renegotiate if it does), ~50 for any single language
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port's specialized opcodes, and clean separation that makes it obvious which
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opcodes are stable (core), shared (guest), or port-specific (extension).
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If we need more than 256 opcodes total, multi-byte opcodes (a leading 248-255
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byte plus a second byte) extend the space without breaking the schema.
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### Extension module signature
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```ocaml
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(* hosts/ocaml/lib/sx_vm_extension.ml *)
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(** A handler for an extension opcode. Reads operands from bytecode,
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manipulates the VM stack, updates the frame's instruction pointer.
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May raise exceptions (which propagate via the existing VM error path). *)
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type handler = vm -> frame -> unit
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(** State an extension carries alongside the VM. Opaque to the VM core;
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extensions cast as needed. *)
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type extension_state = ..
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module type EXTENSION = sig
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(** Stable name for this extension (e.g. "erlang", "guest_vm"). *)
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val name : string
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(** Initialize per-instance state. Called once when the VM starts and the
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extension is loaded. *)
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val init : unit -> extension_state
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(** Opcodes this extension provides. Each is (opcode_id, opcode_name, handler).
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opcode_id must be in the range allowed for this extension's tier
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(128-199 for guest, 200-247 for ports). Conflicts cause startup failure. *)
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val opcodes : extension_state -> (int * string * handler) list
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end
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```
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### Registration and dispatch
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```ocaml
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(* hosts/ocaml/lib/sx_vm_extensions.ml *)
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let extensions : (module EXTENSION) list ref = ref []
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let states : (string, extension_state) Hashtbl.t = Hashtbl.create 8
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let by_id : (int, handler) Hashtbl.t = Hashtbl.create 64
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let by_name : (string, int) Hashtbl.t = Hashtbl.create 64
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let register (m : (module EXTENSION)) =
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let module M = (val m) in
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let st = M.init () in
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Hashtbl.add states M.name st;
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List.iter (fun (id, name, h) ->
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if Hashtbl.mem by_id id then
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failwith (Printf.sprintf "Opcode %d (%s) already registered" id name);
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Hashtbl.add by_id id h;
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Hashtbl.add by_name name id
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) (M.opcodes st);
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extensions := m :: !extensions
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let dispatch op vm frame =
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match Hashtbl.find_opt by_id op with
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| Some handler -> handler vm frame
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| None -> raise (Invalid_opcode op)
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let id_of_name name = Hashtbl.find_opt by_name name
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let state_of_extension name = Hashtbl.find_opt states name
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```
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The dispatch path adds **one hashtable lookup per extension opcode**.
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Acceptable cost — and Erlang's specialized opcodes win >100× over going
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through the general CEK machine, so the overhead is negligible by comparison.
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### Bytecode compiler integration
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The compiler (`lib/compiler.sx`) needs to know extension opcode IDs to emit
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them. New SX primitive exposed to the compiler:
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```sx
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(extension-opcode-id "erlang.OP_PATTERN_TUPLE_2") ; → 200, or nil if not loaded
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```
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When the compiler wants to emit a specialized opcode, it queries by name. If
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the extension isn't loaded, the compiler falls back to the general path
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(emit a `CALL_PRIM` or general SX `case`). This means a language port's
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optimization is opt-in per build, and missing extensions degrade to slower
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correct execution rather than failure.
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Naming convention: `<extension-name>.OP_<NAME>`. So `erlang.OP_PATTERN_TUPLE_2`,
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`guest_vm.OP_PERFORM`, etc.
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### Per-extension state access
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Some opcodes need state beyond the VM stack (Erlang's scheduler, mailbox
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state, etc.). Extensions store state in their `init`-returned value, accessed
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via `state_of_extension`:
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```ocaml
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let op_spawn vm frame =
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let st = Sx_vm_extensions.state_of_extension "erlang"
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|> Option.get
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|> Obj.magic in (* extension casts to its known type *)
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let body = pop vm in
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let pid = Erlang_scheduler.spawn st body in
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push vm (pid_value pid);
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frame.ip <- frame.ip + 1
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```
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Shared scheduler state lives in the Erlang extension's state value. Other
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extensions don't see it.
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---
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## Phase plan
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Five sub-phases in dependency order. Each is testable in isolation.
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### Phase A — Opcode ID partition + dispatch fallthrough
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Smallest viable change to `sx_vm.ml`:
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- Add the `| op when op >= 128 -> Sx_vm_extensions.dispatch op vm frame`
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fallthrough case.
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- Document the partition in a comment at the top of the opcode list.
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**Tests:**
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- All existing SX VM tests pass unchanged (zero regression for core).
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- Calling `dispatch 200 ...` with no extension registered raises
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`Invalid_opcode 200`.
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**Effort:** small. ~50 lines + tests.
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### Phase B — Extension registry module
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`hosts/ocaml/lib/sx_vm_extensions.ml` per the sketch above. Pure plumbing, no
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opcodes yet.
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**Tests:**
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- Register a test extension with one opcode; dispatch finds it.
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- Duplicate opcode-id registration fails at startup.
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- `id_of_name` and `state_of_extension` lookups work.
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**Effort:** small. ~150 lines + tests.
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### Phase C — Compiler-side opcode lookup primitive
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Expose `extension-opcode-id` as an SX primitive in `hosts/ocaml/lib/`. The
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compiler in `lib/compiler.sx` can call it to emit extension opcodes by name.
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Does not require any extension to actually exist — the primitive returns
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`nil` for unknown names, and the compiler falls back.
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**Tests:**
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- Primitive returns nil for unknown name.
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- After registering a test extension, primitive returns the registered ID.
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**Effort:** small. Single primitive registration + compiler-side use docs.
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### Phase D — Test extension demonstrating end-to-end flow
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A dummy extension at `hosts/ocaml/extensions/test_ext.ml` registering one or
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two trivial opcodes (e.g. `OP_TEST_PUSH_42`, `OP_TEST_DOUBLE_TOS`). Wired
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into the build, available when running tests.
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Compiler test: write SX that triggers the test compiler-extension to emit
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`OP_TEST_PUSH_42`, then verify the VM executes it correctly via
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`bytecode-inspect` and `vm-trace`.
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**Tests:**
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- Bytecode emission via name lookup produces the right ID.
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- Execution produces the expected stack effect.
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- `bytecode-inspect` shows the opcode by name.
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- `vm-trace` correctly reports the extension opcode.
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**Effort:** small. ~100 lines including build wiring.
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### Phase E — JIT awareness (interpreted-only for v1)
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The JIT (lazy lambda compilation) currently compiles based on opcode ranges.
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Extension opcodes (≥128) should fall through to interpretation, not be
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JIT-compiled in v1.
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- Mark extension opcodes as "interpret only" in the JIT pre-analysis.
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- A lambda containing only core opcodes JIT-compiles as before.
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- A lambda containing any extension opcode runs interpreted.
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JITing extension opcodes is a follow-up project; v1 keeps the JIT scope
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unchanged and just makes it correctly route mixed bytecode.
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**Tests:**
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- Lambda with only core opcodes: JIT-compiled, fast path.
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- Lambda with extension opcode: interpreted, correct result.
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- Mixed lambda: interpreted, correct result.
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**Effort:** small-medium. Requires understanding the JIT's pre-analysis
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(per `project_jit_compilation.md` memory: "Lazy JIT implemented: lambda
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bodies compiled on first VM call, cached, failures sentinel-marked").
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Extension-opcode detection becomes another reason to mark a lambda
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"interpret-only."
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---
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## Acceptance criteria
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1. **Phase A-D pass their test suites.**
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2. **Zero regression on existing SX VM tests.** All language-port test
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suites currently passing on the architecture branch (Erlang 530+, Haskell
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285+, Datalog 276+, Smalltalk 625+, the SX core test suite, etc.) still
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pass.
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3. **Test extension demonstrates the flow end-to-end.** SX source compiles
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via the compiler with a registered extension opcode, executes through the
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VM via the dispatch fallthrough, returns correct result.
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4. **Documentation:** README in `hosts/ocaml/extensions/` explaining the
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pattern, with a worked example (the test extension is the canonical one).
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After acceptance, the Erlang-on-SX Phase 9 work in `lib/erlang/vm/` can use
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this mechanism. The Erlang loop's Blocker for 9a is resolved.
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---
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## Risk and mitigation
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**Risk: regression in core opcode dispatch.** A misplaced `match` arm could
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break something. *Mitigation:* run every existing language-port test suite
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before merging. The cost of this verification is real — probably an hour of
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machine time — but cheaper than discovering it after the fact.
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**Risk: opcode ID conflicts as more extensions land.** If Erlang Phase 9
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claims IDs 200-220 and Haskell wants 215-235, we have a problem.
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*Mitigation:* maintain a registry document at `hosts/ocaml/extensions/
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README.md` listing claimed ID ranges per extension. Convention: each
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extension claims a contiguous block at first registration; collisions caught
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at startup with a clear error.
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**Risk: extension state types leak through `Obj.magic`.** The extension state
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is type-erased in the registry. *Mitigation:* extensions cast in their own
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opcode handlers, never expose state to other extensions or the VM core.
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First-class modules / GADTs could add more type safety; deferred unless
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this becomes a concrete pain point.
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**Risk: extensions become a back door for kernel mutation.** An extension
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opcode handler has full access to the VM. *Mitigation:* extensions are
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build-time additions, not runtime; they're as trusted as the rest of the
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binary. Operators audit at build time, not runtime. Same trust model as
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any other compiled-in code.
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**Risk: shared `lib/guest/vm/` opcodes evolve under different language
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ports' needs.** *Mitigation:* the chiselling discipline (move to guest only
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on second use) ensures the shared opcodes are tested against at least two
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ports' actual usage before being considered stable.
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---
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## Open questions
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To be resolved during implementation, not blocking design approval:
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1. **Multi-byte opcode encoding.** If we need >256 opcodes total, the
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leading-byte 248-255 schema accommodates it. Do we need multi-byte at
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v1? Probably not — 200+ opcodes per port is more than any port should
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reasonably want.
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2. **Extension ordering matters?** If two extensions register opcodes that
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read the same VM state, ordering of registration could matter for
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initialization. Probably not in practice; flag if it bites.
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3. **Hot-reload of extensions.** Out of scope for v1 (per non-goals). If
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wanted later, the registry would need teardown + re-registration; the
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`gen_server` `code_change/3` model from Erlang Phase 7 is a precedent.
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4. **Cross-extension opcode composition.** Can `guest_vm.OP_PERFORM` invoke
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`erlang.OP_RECEIVE_SCAN`? In principle yes — handlers can do anything.
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The interface is clean; the question is whether we want any conventions
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to keep ergonomics tractable. Defer until composition appears in
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practice.
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---
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## Implementation roadmap and sequencing
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This is a sister workstream to `loops/erlang`. Probably best as a single
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focused session (not a continuous loop — the work is bounded, ~1-2 weeks
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of focused effort, not iterative).
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Recommended sequencing:
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1. **A + B + C land together** as a single PR — they're tightly coupled and
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easier to test as a unit. Branch: `loops/sx-vm-extensions` or similar.
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2. **D follows** in a second PR; demonstrates the end-to-end flow without
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committing to any real language port's opcode design.
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3. **E (JIT integration)** as a third PR, once the basic mechanism is
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battle-tested.
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4. **Extension scope check:** verify Erlang's Phase 9 sub-phases 9b-9g can
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actually use this mechanism. If gaps surface, they're addressable
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incrementally.
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5. **`hosts/ocaml/extensions/erlang.ml`** then becomes the *first real
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consumer* — written by whoever takes over from the Erlang loop's stub
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dispatcher. That's the integration moment that closes the loop.
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Estimated total effort: 1-2 weeks for one focused engineer with OCaml SX VM
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familiarity. Much less if the implementer already knows `sx_vm.ml`.
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---
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## Relationship to other plans
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- **`plans/erlang-on-sx.md` Phase 9:** unblocked by this work. Erlang loop
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develops opcodes against a stub dispatcher in `lib/erlang/vm/`; once this
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mechanism lands, swap stub for real registration via
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`hosts/ocaml/extensions/erlang.ml`.
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- **`plans/fed-sx-design.md` §17.5:** documents this as Layer-1 prerequisite.
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The shared-opcode discipline (lib/guest/vm/) is designed on top of this
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mechanism's `lib/guest/vm/` namespace allocation.
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- **Future language ports (Haskell, Datalog, Smalltalk perf phases):** will
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use the same mechanism. Each adds an extension module, claims an opcode
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range, registers handlers. The `lib/guest/vm/` opcodes get
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cross-referenced when the second port's needs justify chiselling.
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- **JIT roadmap (per `project_jit_architecture.md` memory):** extension
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opcodes are interpreted in v1. JITing them is a logical follow-up but
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a separate project.
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