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vm-ext: bootstrap loops/sx-vm-extensions plan + loop briefing

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plans/sx-vm-opcode-extension.md ports over from loops/erlang (f6a68656)
with the opcode partition adjusted to match real VM usage: 1-199 core
(current ceiling 175 = OP_DEC), 200-247 extensions, 248-255 reserved.

plans/agent-briefings/sx-vm-extensions-loop.md captures the per-fire
workflow and ground rules.
This commit is contained in:
giles
2026-05-14 22:29:15 +00:00
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# sx-vm-extensions loop agent
Role: drives `plans/sx-vm-opcode-extension.md` to completion. One phase per
fire (A → B → C → D → E). Bounded loop — after Phase E acceptance, the loop
is done.
```
description: sx-vm-extensions queue loop
subagent_type: general-purpose
run_in_background: true
isolation: worktree (already on loops/sx-vm-extensions)
```
## What this loop is for
Mechanism in `hosts/ocaml/lib/` that lets language ports register specialized
bytecode opcodes without modifying the SX VM core. Direct prerequisite for
**erlang-on-sx Phase 9** (the BEAM analog) and a structural enabler for any
future language port that wants performance-critical opcodes.
## The queue
Per `plans/sx-vm-opcode-extension.md`, in order:
- **Phase A** — Opcode ID partition + dispatch fallthrough in `sx_vm.ml`.
Add `Invalid_opcode of int` exception, `extension_dispatch_ref`, the
`| op when op >= 200 -> !extension_dispatch_ref op vm frame` arm, and a
partition comment near the opcode list.
- **Phase B** — Extension registry module (`sx_vm_extensions.ml`).
`register`, `dispatch`, `id_of_name`, `state_of_extension`. Wire dispatch
into Phase A's ref at module init.
- **Phase C** — Compiler-side opcode lookup primitive (`extension-opcode-id`).
- **Phase D** — Test extension at `hosts/ocaml/lib/extensions/test_ext.ml`,
end-to-end SX → bytecode → VM dispatch flow.
- **Phase E** — JIT awareness: extension opcodes mark a lambda as
interpret-only.
## Per-fire workflow (hard)
1. Read `plans/sx-vm-opcode-extension.md` — find the first un-ticked phase.
2. Implement the phase (only files in `hosts/ocaml/**` and the plan file).
3. Build via `sx_build target=ocaml`.
4. Run regression: every existing language-port conformance suite plus
the OCaml unit tests. The list lives at `lib/<lang>/conformance.sh` —
13 suites at last count (apl, common-lisp, datalog, erlang, forth, guest,
haskell, js, lua, ocaml, prolog, smalltalk, tcl).
5. If green, commit (short factual message — `vm-ext: phase A — dispatch
fallthrough` style).
6. Tick the `[ ]` for the completed phase in the plan, append one dated
line to the Progress log (newest first).
7. Stop. Wait for the next fire.
## Ground rules (hard)
- **Scope:** only `hosts/ocaml/**` and `plans/sx-vm-opcode-extension.md`.
Do **not** edit `lib/<lang>/**`, `spec/**`, `shared/**`, or any other
language port's tests.
- **One phase per fire.** Don't combine phases even if a phase looks small.
The point of the loop is incremental commits.
- **Commit locally only.** Do **not** push. Do **not** touch `main`.
- **Worktree:** you are on `loops/sx-vm-extensions` in
`/root/rose-ash-loops/sx-vm-extensions`.
- **OCaml SX VM gotchas:**
- `vm` and `frame` types are defined in `sx_vm.ml`, not `sx_types.ml`.
Forward refs (like the existing `jit_compile_ref` pattern) are how
sibling modules avoid circular dependency.
- Current core opcode ceiling is 175 (OP_DEC). The extension threshold
is 200, leaving 24 spare slots for future core opcodes.
- JIT compilation is lazy per-lambda. See `project_jit_compilation.md`
in memory for the cache + sentinel pattern.
- **SX edits:** `sx-tree` MCP tools only (none expected for this loop, but
if needed).
- **OCaml edits:** Edit/Write tools are fine — these aren't `.sx` files.
## Done condition
Phase E acceptance: all 13 (or however many exist at the time) language-port
conformance suites pass, OCaml unit tests pass, the test extension from
Phase D demonstrates end-to-end flow including JIT routing. Loop is
complete; mark and stop.
## After acceptance
Hand off to the Erlang loop: `hosts/ocaml/lib/extensions/erlang.ml` becomes
the first real consumer, written against this mechanism instead of the
Phase 9b stub dispatcher in `lib/erlang/vm/dispatcher.sx`.

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# SX VM Opcode Extension Mechanism
Mechanism in `hosts/ocaml/lib/` that lets language ports register specialized
bytecode opcodes without modifying the SX VM core. Direct prerequisite for
**erlang-on-sx Phase 9** (the BEAM analog) and a structural enabler for any
future language port that wants performance-critical opcodes.
Reference: `plans/erlang-on-sx.md` Phase 9, `plans/fed-sx-design.md` §17.5,
`hosts/ocaml/lib/sx_vm.ml` (current VM).
Status: **in progress** on `loops/sx-vm-extensions`.
---
## Goal
Allow language ports to register custom bytecode opcodes in the SX VM, with:
- **Zero overhead for core opcodes.** Existing opcodes (current ceiling 175,
see `sx_vm.ml`) must dispatch identically. No regression for any existing
language port or the core SX runtime.
- **One additional dispatch step for extension opcodes.** Acceptable cost; the
win comes from avoiding the general CEK machinery.
- **Per-extension state slot.** Erlang's process scheduler, Haskell's thunk
cache, etc. need somewhere to hang state alongside the VM.
- **Compiler awareness.** The bytecode compiler (`lib/compiler.sx`) must be
able to emit extension opcodes by name, looked up against the registered
set.
- **JIT compatibility.** Existing JIT (lazy lambda compilation) continues to
work for code paths using only core opcodes. Extension opcodes are
interpreted in v1; JITing them is a follow-up.
## Non-goals
- **Hot opcode reload.** Adding/replacing opcodes mid-runtime is not in
scope. Extensions are compile-time additions to the OCaml binary. (If
needed, that's a separate project.)
- **Per-instance opcode sets.** All running instances of the SX VM share
the same opcode set determined at build time. Selective opcode loading
per instance is out of scope.
- **Opcode hot-swap or supersession.** Once registered, opcodes are stable
for the lifetime of the binary.
- **Language-port isolation at the dispatch layer.** Two language ports can
see each other's opcodes (they share the dispatch table). Isolation is a
build-time concern — don't compile in extensions you don't trust.
---
## Why now
The Erlang-on-SX Phase 9 work needs this. Without it, Phase 9b-9g (the actual
opcode implementations) have nowhere to plug in. The Erlang loop hit this
dependency as a Blocker (`0abf05ed`); this design is what unblocks it.
It also enables the **shared opcode pattern** discussed in `plans/fed-sx-
design.md` §17.5: opcodes Erlang Phase 9 produces that other ports could
plausibly use (pattern match, perform/handle, record access) get chiselled
out to `lib/guest/vm/` when a second port has an actual second use. Without
the extension mechanism, each port would have to fork the SX VM core or
modify shared dispatch — neither acceptable.
---
## Architectural overview
```
┌──────────────────────────────────────────┐
│ SX VM core (hosts/ocaml/lib/sx_vm.ml) │
│ │
│ ┌────────────────────────────────────┐ │
│ │ Bytecode dispatch loop │ │
│ │ │ │
│ │ match op with │ │
│ │ | 1 (OP_CONST) -> ... │ │
│ │ | 2 (OP_NIL) -> ... │ │
│ │ | ... │ │
│ │ | 175 -> ... (last core opcode) │ │
│ │ | op when op >= 200 -> │ │
│ │ !extension_dispatch_ref op │ │ ◄── new
│ │ vm frame │ │
│ └────────────────────────────────────┘ │
│ │
│ ┌────────────────────────────────────┐ │
│ │ Extension registry │ │
│ │ opcode_id -> handler │ │ ◄── Phase B
│ │ opcode_name -> opcode_id │ │
│ │ extension_state per extension │ │
│ └────────────────────────────────────┘ │
└──────────────────────────────────────────┘
▲
│ register at startup
┌──────────────────┴──────────────────────┐
│ Extension modules │
│ hosts/ocaml/lib/extensions/erlang.ml │
│ hosts/ocaml/lib/extensions/haskell.ml │
│ hosts/ocaml/lib/extensions/datalog.ml │
│ hosts/ocaml/lib/extensions/guest_vm.ml │ ◄── shared opcodes
└─────────────────────────────────────────┘
```
### Opcode ID space partition
Current SX VM uses opcode IDs from 1 to 175 (per inspection of `sx_vm.ml`,
ceiling at OP_DEC = 175). We partition the 0-255 space:
| Range | Use |
|---------|------------------------------------------------------------------|
| 0 | reserved / NOP |
| 1-199 | **core opcodes** — owned by the SX VM, locked schema |
| 200-247 | **extension opcodes** — registered by extensions (ports + shared) |
| 248-255 | reserved for future expansion / multi-byte opcodes |
This gives the core 24 free slots above the current 175 ceiling for future
core additions, and 48 slots for extensions. Erlang Phase 9 expects to need
fewer than 30 specialized opcodes, so this is comfortable headroom.
The plan originally proposed a finer split (`128-199` for `lib/guest/vm/`
shared, `200-247` for ports). That distinction is preserved at the **naming
level** (`guest_vm.OP_X` vs `erlang.OP_Y`) and policed by the registry
(duplicate IDs fail at startup), without consuming separate ID ranges. The
chiselling discipline (move an opcode to `guest_vm` when a second port uses
it) operates at the source level.
If we need more than 256 opcodes total, multi-byte opcodes (a leading 248-255
byte plus a second byte) extend the space without breaking the schema.
### Extension module signature
```ocaml
(* hosts/ocaml/lib/sx_vm_extension.ml *)
(** A handler for an extension opcode. Reads operands from bytecode,
manipulates the VM stack, updates the frame's instruction pointer.
May raise exceptions (which propagate via the existing VM error path). *)
type handler = vm -> frame -> unit
(** State an extension carries alongside the VM. Opaque to the VM core;
extensions cast as needed. *)
type extension_state = ..
module type EXTENSION = sig
(** Stable name for this extension (e.g. "erlang", "guest_vm"). *)
val name : string
(** Initialize per-instance state. Called once when the VM starts and the
extension is loaded. *)
val init : unit -> extension_state
(** Opcodes this extension provides. Each is (opcode_id, opcode_name, handler).
opcode_id must be in 200-247. Conflicts cause startup failure. *)
val opcodes : extension_state -> (int * string * handler) list
end
```
### Registration and dispatch
```ocaml
(* hosts/ocaml/lib/sx_vm_extensions.ml *)
let extensions : (module EXTENSION) list ref = ref []
let states : (string, extension_state) Hashtbl.t = Hashtbl.create 8
let by_id : (int, handler) Hashtbl.t = Hashtbl.create 64
let by_name : (string, int) Hashtbl.t = Hashtbl.create 64
let register (m : (module EXTENSION)) =
let module M = (val m) in
let st = M.init () in
Hashtbl.add states M.name st;
List.iter (fun (id, name, h) ->
if Hashtbl.mem by_id id then
failwith (Printf.sprintf "Opcode %d (%s) already registered" id name);
Hashtbl.add by_id id h;
Hashtbl.add by_name name id
) (M.opcodes st);
extensions := m :: !extensions
let dispatch op vm frame =
match Hashtbl.find_opt by_id op with
| Some handler -> handler vm frame
| None -> raise (Invalid_opcode op)
let id_of_name name = Hashtbl.find_opt by_name name
let state_of_extension name = Hashtbl.find_opt states name
```
Phase B installs this dispatcher into `Sx_vm.extension_dispatch_ref` at
module init. Until then, the ref's default raises `Invalid_opcode op` for
any opcode ≥ 200, which is the Phase A test condition.
The dispatch path adds **one hashtable lookup per extension opcode**.
Acceptable cost — and Erlang's specialized opcodes win >100× over going
through the general CEK machine, so the overhead is negligible by comparison.
### Bytecode compiler integration
The compiler (`lib/compiler.sx`) needs to know extension opcode IDs to emit
them. New SX primitive exposed to the compiler:
```sx
(extension-opcode-id "erlang.OP_PATTERN_TUPLE_2") ; → 200, or nil if not loaded
```
When the compiler wants to emit a specialized opcode, it queries by name. If
the extension isn't loaded, the compiler falls back to the general path
(emit a `CALL_PRIM` or general SX `case`). This means a language port's
optimization is opt-in per build, and missing extensions degrade to slower
correct execution rather than failure.
Naming convention: `<extension-name>.OP_<NAME>`. So `erlang.OP_PATTERN_TUPLE_2`,
`guest_vm.OP_PERFORM`, etc.
### Per-extension state access
Some opcodes need state beyond the VM stack (Erlang's scheduler, mailbox
state, etc.). Extensions store state in their `init`-returned value, accessed
via `state_of_extension`:
```ocaml
let op_spawn vm frame =
let st = Sx_vm_extensions.state_of_extension "erlang"
|> Option.get
|> Obj.magic in (* extension casts to its known type *)
let body = pop vm in
let pid = Erlang_scheduler.spawn st body in
push vm (pid_value pid);
frame.ip <- frame.ip + 1
```
Shared scheduler state lives in the Erlang extension's state value. Other
extensions don't see it.
---
## Phase plan
Five sub-phases in dependency order. Each is testable in isolation.
### Phase A — Opcode ID partition + dispatch fallthrough
- [ ] Define `exception Invalid_opcode of int` in `sx_vm.ml`.
- [ ] Add `extension_dispatch_ref : (int -> vm -> frame -> unit) ref`
whose default handler raises `Invalid_opcode op`. Forward-declared in
the same style as the existing `jit_compile_ref`.
- [ ] Add `| op when op >= 200 -> !extension_dispatch_ref op vm frame` arm
in the dispatch loop, immediately before the catch-all.
- [ ] Document the partition in a comment near the top of the opcode list.
**Tests:**
- All existing OCaml VM/CEK tests pass unchanged (zero regression for core).
- Constructed bytecode using opcode 200 raises `Invalid_opcode 200` when no
extension is registered.
**Effort:** small. ~50 lines + tests.
### Phase B — Extension registry module
`hosts/ocaml/lib/sx_vm_extensions.ml` per the sketch above. Pure plumbing, no
opcodes yet. Phase B's module init installs the real `dispatch` into
`Sx_vm.extension_dispatch_ref`, replacing Phase A's stub.
- [ ] `Sx_vm_extension` interface module (handler type, EXTENSION sig).
- [ ] `Sx_vm_extensions` registry module (`register`, `dispatch`,
`id_of_name`, `state_of_extension`).
- [ ] Wire the registry's `dispatch` into `Sx_vm.extension_dispatch_ref` at
module init.
**Tests:**
- Register a test extension with one opcode; dispatch finds it.
- Duplicate opcode-id registration fails at startup.
- `id_of_name` and `state_of_extension` lookups work.
**Effort:** small. ~150 lines + tests.
### Phase C — Compiler-side opcode lookup primitive
Expose `extension-opcode-id` as an SX primitive in `hosts/ocaml/lib/`. The
compiler in `lib/compiler.sx` can call it to emit extension opcodes by name.
Does not require any extension to actually exist — the primitive returns
`nil` for unknown names, and the compiler falls back.
- [ ] Register `extension-opcode-id` in `sx_primitives.ml`.
- [ ] Returns `Integer id` when registered, `Nil` otherwise.
**Tests:**
- Primitive returns nil for unknown name.
- After registering a test extension, primitive returns the registered ID.
**Effort:** small. Single primitive registration + compiler-side use docs.
### Phase D — Test extension demonstrating end-to-end flow
A dummy extension at `hosts/ocaml/lib/extensions/test_ext.ml` registering
one or two trivial opcodes (e.g. `OP_TEST_PUSH_42`, `OP_TEST_DOUBLE_TOS`).
Wired into the build, available when running tests.
Compiler test: write SX that triggers the test compiler-extension to emit
`OP_TEST_PUSH_42`, then verify the VM executes it correctly via
`bytecode-inspect` and `vm-trace`.
- [ ] `test_ext.ml` registers two opcodes.
- [ ] Wired into the build (extensions registered at startup).
- [ ] Bytecode emission via name lookup produces the right ID.
- [ ] `bytecode-inspect` shows the opcode by name.
**Tests:**
- Bytecode emission via name lookup produces the right ID.
- Execution produces the expected stack effect.
- `bytecode-inspect` shows the opcode by name.
- `vm-trace` correctly reports the extension opcode.
**Effort:** small. ~100 lines including build wiring.
### Phase E — JIT awareness (interpreted-only for v1)
The JIT (lazy lambda compilation) currently compiles based on opcode ranges.
Extension opcodes (≥200) should fall through to interpretation, not be
JIT-compiled in v1.
- [ ] Mark extension opcodes as "interpret only" in the JIT pre-analysis.
- [ ] Lambda containing only core opcodes JIT-compiles as before.
- [ ] Lambda containing any extension opcode runs interpreted.
JITing extension opcodes is a follow-up project; v1 keeps the JIT scope
unchanged and just makes it correctly route mixed bytecode.
**Tests:**
- Lambda with only core opcodes: JIT-compiled, fast path.
- Lambda with extension opcode: interpreted, correct result.
- Mixed lambda: interpreted, correct result.
**Effort:** small-medium. Requires understanding the JIT's pre-analysis
(per `project_jit_compilation.md` memory: "Lazy JIT implemented: lambda
bodies compiled on first VM call, cached, failures sentinel-marked").
Extension-opcode detection becomes another reason to mark a lambda
"interpret-only."
---
## Acceptance criteria
1. **Phase A-D pass their test suites.**
2. **Zero regression on existing SX VM tests.** All language-port test
suites currently passing on the architecture branch (Erlang 530+, Haskell
285+, Datalog 276+, Smalltalk 625+, the SX core test suite, etc.) still
pass.
3. **Test extension demonstrates the flow end-to-end.** SX source compiles
via the compiler with a registered extension opcode, executes through the
VM via the dispatch fallthrough, returns correct result.
4. **Documentation:** README in `hosts/ocaml/lib/extensions/` explaining the
pattern, with a worked example (the test extension is the canonical one).
After acceptance, the Erlang-on-SX Phase 9 work in `lib/erlang/vm/` can use
this mechanism. The Erlang loop's Blocker for 9a is resolved.
---
## Risk and mitigation
**Risk: regression in core opcode dispatch.** A misplaced `match` arm could
break something. *Mitigation:* run every existing language-port conformance
suite before merging.
**Risk: opcode ID conflicts as more extensions land.** If Erlang Phase 9
claims IDs 200-220 and Haskell wants 215-235, we have a problem.
*Mitigation:* maintain a registry document at `hosts/ocaml/lib/extensions/
README.md` listing claimed ID ranges per extension. Convention: each
extension claims a contiguous block at first registration; collisions caught
at startup with a clear error.
**Risk: extension state types leak through `Obj.magic`.** The extension state
is type-erased in the registry. *Mitigation:* extensions cast in their own
opcode handlers, never expose state to other extensions or the VM core.
First-class modules / GADTs could add more type safety; deferred unless
this becomes a concrete pain point.
**Risk: extensions become a back door for kernel mutation.** An extension
opcode handler has full access to the VM. *Mitigation:* extensions are
build-time additions, not runtime; they're as trusted as the rest of the
binary. Operators audit at build time, not runtime. Same trust model as
any other compiled-in code.
**Risk: shared `lib/guest/vm/` opcodes evolve under different language
ports' needs.** *Mitigation:* the chiselling discipline (move to guest only
on second use) ensures the shared opcodes are tested against at least two
ports' actual usage before being considered stable.
---
## Open questions
To be resolved during implementation, not blocking design approval:
1. **Multi-byte opcode encoding.** If we need >256 opcodes total, the
leading-byte 248-255 schema accommodates it. Do we need multi-byte at
v1? Probably not — 48 extension opcodes is more than any single port
should reasonably want.
2. **Extension ordering matters?** If two extensions register opcodes that
read the same VM state, ordering of registration could matter for
initialization. Probably not in practice; flag if it bites.
3. **Hot-reload of extensions.** Out of scope for v1 (per non-goals). If
wanted later, the registry would need teardown + re-registration; the
`gen_server` `code_change/3` model from Erlang Phase 7 is a precedent.
4. **Cross-extension opcode composition.** Can `guest_vm.OP_PERFORM` invoke
`erlang.OP_RECEIVE_SCAN`? In principle yes — handlers can do anything.
The interface is clean; the question is whether we want any conventions
to keep ergonomics tractable. Defer until composition appears in
practice.
---
## Implementation roadmap and sequencing
This is a sister workstream to `loops/erlang`. Driven by Erlang Phase 9.
Single bounded loop on `loops/sx-vm-extensions`, ~1-2 weeks.
Recommended sequencing (one phase per loop fire):
1. **Phase A** — dispatch fallthrough. Smallest viable change to `sx_vm.ml`.
2. **Phase B** — extension registry module.
3. **Phase C** — compiler-side opcode lookup primitive.
4. **Phase D** — test extension demonstrating end-to-end flow.
5. **Phase E** — JIT awareness (interpret-only routing).
After acceptance:
- **`hosts/ocaml/lib/extensions/erlang.ml`** becomes the *first real
consumer* — written by whoever takes over from the Erlang loop's stub
dispatcher in `lib/erlang/vm/dispatcher.sx`. That's the integration
moment that closes the loop.
Estimated total effort: 1-2 weeks for one focused engineer with OCaml SX VM
familiarity.
---
## Relationship to other plans
- **`plans/erlang-on-sx.md` Phase 9:** unblocked by this work. Erlang loop
develops opcodes against a stub dispatcher in `lib/erlang/vm/`; once this
mechanism lands, swap stub for real registration via
`hosts/ocaml/lib/extensions/erlang.ml`.
- **`plans/fed-sx-design.md` §17.5:** documents this as Layer-1 prerequisite.
The shared-opcode discipline (lib/guest/vm/) is designed on top of this
mechanism's namespace allocation.
- **Future language ports (Haskell, Datalog, Smalltalk perf phases):** will
use the same mechanism. Each adds an extension module, claims an opcode
range, registers handlers. The `lib/guest/vm/` opcodes get
cross-referenced when the second port's needs justify chiselling.
- **JIT roadmap (per `project_jit_architecture.md` memory):** extension
opcodes are interpreted in v1. JITing them is a logical follow-up but
a separate project.
---
## Progress log
Newest first.