rose-ash/plans/jit-bytecode-correctness.md

# JIT bytecode correctness — enable the JIT in serving mode

> Kickoff handed over from the **host-on-sx** loop (2026-06-19). This is the
> highest-leverage perf win on the platform.

## Why this matters

Every SX-on-SX subsystem runs **interpreted on the tree-walking CEK**: the
Smalltalk runtime (→ content-on-sx rendering), and the guest languages
(Datalog, Prolog, APL, Scheme, Haskell, Erlang, Maude). The lazy JIT
(`register_jit_hook` → bytecode VM) would speed all of them up ~10–60×. It is
currently **only installed in `--http` page-server mode**, not the epoch /
`http-listen` serving mode — because it **miscompiles** these workloads.

Concrete impact: the host serves a blog post (`content/html`, interpreted
Smalltalk) in **~2 seconds per request**. With a correct JIT it should be tens
of ms. Same slowdown applies to every guest-language-backed service.

## Concrete repro (from the host loop)

In `hosts/ocaml/bin/sx_server.ml`, the persistent server mode (`make_server_env`,
~line 4871) does **not** call `register_jit_hook env` — only the `--http` mode
(~line 4034) does. To reproduce the miscompile:

1. Add `register_jit_hook env;` right after `let env = make_server_env () in` in
   the persistent server-mode branch (~4871).
2. Rebuild: `eval $(opam env --switch=5.2.0); dune build bin/sx_server.exe`.
3. Run a Smalltalk/content-heavy suite, e.g. the host-on-sx conformance
   (`bash /root/rose-ash-loops/host/lib/host/conformance.sh`, or any
   content-on-sx suite). **With the hook ON, tests FAIL** — host-on-sx dropped to
   `router 3/6, feed 4/11, relations 9/16, blog 4/11`. With the hook OFF: all green.

So the JIT produces **wrong results** (the known "compiled compiler helpers loop
on complex nested ASTs" — see memory `project_jit_bytecode_bug`).

## Goal

Make the JIT compile the Smalltalk-on-SX evaluator + guest-language evaluators
**correctly**, so `register_jit_hook` can be enabled in serving mode with
conformance **fully green**. Then enable it there.

## Suggested approach

- Minimal repro to bisect: render a `lib/content` doc via `content/html` with JIT
  ON vs OFF, diff the output, find the first divergence.
- Localize with the VM debugging tools (see CLAUDE.md): `(vm-trace ...)`,
  `(bytecode-inspect ...)`, `(prim-check ...)`, `(deps-check ...)`.
- Likely suspects: nested closures / TCO, dict construction, `st-send` dispatch
  patterns, recursion through the Smalltalk method interpreter.

## Pointers

- `register_jit_hook` — `sx_server.ml` ~1493; JIT VM-suspend/resolve path ~1497–1514.
- `hosts/ocaml/lib/sx_vm.ml` — the bytecode VM + compiler.
- `plans/jit-cache-architecture.md`, `plans/jit-perf-regression.md`, `restore-jit-perf.sh`.
- Memory: `project_jit_bytecode_bug.md` (plan ref `plans/reflective-rolling-treehouse.md`).
- The shared `sx_server.exe` binary is used by ALL loops — coordinate before
  changing VM semantics that could affect sibling conformance runs.

---

## Resolution (2026-06-19, loop loops/sx-vm-extensions)

JIT is now enabled in the persistent (epoch) serving mode (`register_jit_hook`
in `sx_server.ml`'s server-mode branch). Smalltalk conformance is **847/847 —
identical to the no-JIT baseline** (no failures, no double-counted rows).
Datalog conformance (a non-continuation guest) is **356/356** under JIT.

Five distinct root causes were found and fixed (not one "miscompile"):

1. **Serving mode never loaded `lib/compiler.sx`.** The JIT then used the
   native `Sx_compiler.compile` stub, which emits arity-0 bytecode with every
   parameter compiled as `GLOBAL_GET` → "VM undefined: <param>" on the first
   call of essentially every function. `http`/`cli`/`site` modes already load
   `compiler.sx`; the epoch serving branch now does too (before the hook).
   *Fix: `sx_server.ml` server-mode branch loads `lib/compiler.sx`.*

2. **`compile-cond`/`compile-case-clauses`/`compile-guard-clauses` only treated
   the keyword `:else` and `true` as the catch-all** — not the bare symbol
   `else` that the CEK's `is-else-clause?` accepts. They emitted
   `GLOBAL_GET "else"` → runtime "VM undefined: else".
   *Fix: `lib/compiler.sx` — add the symbol-`else` case to all three.*

3. **`OP_DIV` produced a float for non-divisible Integer/Integer** (`1/2` → 0.5)
   instead of the exact `Rational` the `/` primitive returns → diverged from CEK
   and broke equality vs rational results.
   *Fix: `sx_vm.ml` — delegate non-divisible int/int to the `/` primitive.*

4. **`OP_EQ` / `_fast_eq` lacked `Rational`/`ListRef` cases** that the real `=`
   primitive's `safe_eq` has → `(= 1/2 1/2)` was false under JIT.
   *Fix: `OP_EQ` delegates non-trivial types to the `=` primitive;
   `_fast_eq` (also used by `prim_call "="`) gained rational + ListRef cases.*

5. **Continuation-based control flow can't run in the stack VM.** Smalltalk's
   non-local return (`^expr`), block escape, and exception unwinding use
   `call/cc`; a JIT-compiled frame between a `call/cc` capture and its `(k v)`
   invocation cannot transfer control and (via the hook's re-run-on-failure)
   double-executes side effects.
   *Fix: a general, data-driven exclusion set — `Sx_types.jit_excluded`,
   populated from SX via the new `jit-exclude!` primitive, consulted in
   `jit_compile_lambda` so it covers BOTH JIT entry points (CEK hook + in-VM
   tiered path). `lib/smalltalk/eval.sx` self-declares its continuation-using
   dispatch core interpret-only; pure helpers (parsing, lookup, formatting,
   arithmetic) still JIT.* One SUnit suite-runner test helper
   (`pharo-test-class`) miscompiles under JIT on a specific iteration and is
   excluded in the test prelude (`tests/tokenize.sx`).

### Known residual / follow-up
- The hook still **re-runs a failed VM execution via CEK** (always yields the
  correct result, but can duplicate side effects if a JIT'd function fails
  mid-run after a side effect). `run_tests`'s hook instead propagates non-IO /
  non-"VM undefined" exceptions. Adopting that propagate-don't-rerun semantics
  in the serving hook would remove the double-execution class entirely, but it
  surfaces genuine mid-run miscompiles as errors — so it must land together
  with fixing/excluding any function that miscompiles mid-run (e.g.
  `pharo-test-class`). Deferred to avoid changing shared VM/CEK semantics under
  this loop.
- Other continuation-heavy guests (Scheme, Erlang use `call/cc`) will need
  their own `jit-exclude!` declarations for their dispatch cores; the mechanism
  is in place. Non-continuation guests (Datalog/Prolog/Haskell/APL) JIT as-is.
- A debug aid was added to the serving hook: `SX_JIT_DENY=name,...` /
  `SX_JIT_ONLY=name,...` env vars to bisect which named lambda the VM
  mishandles (hook-path only).