The plan for an x86 redefinition emitter, written before the work

HANDOFF-x86-redef.md

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+# Handoff — a redefinition emitter in `lib/x86.ml`
+
+Branch `dev-loop`, worktree `agent-a884e1e2fcffe5052`, starting from `b1cc67b`.
+Item 1 of `HANDOFF-x86-rt.md` §6. Written **early**, before the work, because the session's budget is short;
+each section is updated as the work lands, and a section marked *not done* is honestly not done.
+
+## The baseline this must not regress
+
+Measured at the start of this session, on `b1cc67b`:
+
+- `spike/x86/survey.sh` — (filled in below once the run finishes)
+- `dune test --root .` — check the **exit code**, not the output. It prints two `ld` / `clang` failures from
+  inside the `dev-robust` fixture and still exits 0. Grepping the output will mislead you.
+
+## Why this cannot be skipped by leaning on LLVM
+
+`x86.ml`'s header licenses its own calling convention on the grounds that a dev build is compiled entirely by it
+and a release build entirely by LLVM, *and the two never meet in one process*. The conventions agree on scalars
+and disagree on **every aggregate** (here: every aggregate by pointer with a hidden `sret` in the first integer
+register; LLVM classifies by eightbyte). An `Emit.redefinition` module dlopened into an `--x86` host is correct
+until the first redefined function takes or returns a struct. So an `--x86` host must get `--x86` redefinition
+modules. **Do not write an aggregate classifier.**
+
+## The crux: PIC references to the host's symbols
+
+`program` emits a whole executable; every symbol it names is either its own or the runtime's, and a
+rip-relative `PC32` reference is right for all of them. A redefinition module is a `.so` and almost every symbol
+it names belongs to the **host executable**:
+
+- the cells `flan.cell.<n>` (data)
+- the globals `flan.<g>` (data)
+- the sibling function bodies, when taken by address
+
+A `PC32` relocation against an undefined symbol in a `-shared` link fails with
+`relocation R_X86_64_PC32 against undefined symbol ... can not be used when making a shared object`.
+Data references have to go through the GOT (`sym@GOTPCREL`, load the address, then dereference).
+Calls are fine as `call sym` — they get a PLT entry.
+
+This is exactly what `llc -relocation-model=pic` does for `Emit.redefinition`'s
+`@"flan.cell.x" = external global ptr`. **Get the spelling from `llc`, not from recall**: compile a three-line
+`.ll` with an `external global` and a call through a loaded `ptr`, read the asm, and confirm end to end with
+`as` → `ld -shared` → `readelf -r` (want `R_X86_64_GOTPCREL`, not `PC32`).
+
+## Plan
+
+1. **Additive addressing.** A new `loc` constructor `Lgot of string * int` — a symbol reached through the GOT —
+   handled in `shift`, `lmem` (load the GOT slot into `scratch`, then `Reg (scratch, d)` — the `Lp` shape) and
+   `addr_into`. `Lg` keeps its exact meaning, so the whole-program path emits byte-identical output and the
+   survey is protected structurally rather than by re-measurement. A `shared : bool` on `fnctx` (default false)
+   is what selects between them.
+2. **Every non-local PC-relative site.** Not just loads — the `lea` sites too. From the grep:
+   - `lib/x86.ml:944` / `:956` — `FnAddr (Flanfn n)` / the cell read for `Fnval`
+   - `lib/x86.ml:959` — `lea` of an FFI/extern symbol
+   - `lib/x86.ml:1463` / `:1574` — `Tast.Global` / `Tast.Pglobal`
+   - `lib/x86.ml:1766` — `` `Cell `` in `call_flan`
+   Locals stay `PC32` and must: string literals (`:922`, `:1129`) and lifted-clause addresses (`:1157`) are
+   defined in the module.
+3. **`X86.redefinition`.** What it must *not* emit, each of which `program` does and each of which is wrong in
+   a module: no `main`/`emit_main`; **no `.init_array` and no `init_sym`** — re-running the globals initialiser
+   wipes the live state reloading exists to preserve; no `flan_dev_reg_enable` ctor; no `emit_globals_data`
+   (host globals are undefined references); no `emit_cells` (the cells are undefined references). What it
+   **must** emit: `.hidden` on each target body — default visibility in a `.so` is interposable, so a plain
+   reference would resolve to the host's copy and the module would install the very body it is replacing
+   (`emit.ml:2072` says this is load-bearing) — and a `flan_reload_install` function, unquoted and not hidden,
+   which is what `reload_host.c:72` and `vendor/agent/flan_agent.c:1128` `dlsym`.
+4. **Scope cut, deliberate.** Build the **known-name** path only: redefining functions the host already has,
+   referring to globals the host already has. **Refuse by name** the rest of `Emit.redefinition`'s surface —
+   new functions (`flan_dev_cell` + `Emit.cellptr` slot), new globals (`flan_dev_global` + slot + init
+   constant), `consts`, and the transient `call` thunk. That is this file's existing idiom (`call_native`,
+   `check_no_transfer`): a refusal with the reasoning written down beats a half-built path nothing can run.
+   Redefining an existing `defn` is `C-c C-c`, is the demo, and is what `p8-cell.flan` can test.
+5. **Assembling it.** `Build.shared` is `llc` + `ld -shared`. The x86 counterpart is `as` + `ld -shared` on the
+   same output; `-shared` is the part that matters.
+6. **Verify by running.** Item 15: a disassembly that reads correctly beside a wrong answer is the normal
+   outcome of hand-encoding. The harness to copy is `test/test_reload.ml` + `test/reload_host.c` — build the
+   host with `{ dev = true; x86 = true }` (`Build.executable` accepts both; the `--x86` refusal only names
+   wasm, `--debug` and `--sanitize`), build the module with `X86.redefinition`, and check the printed answers
+   change. `spike/x86/cells.sh` is the smaller check that already exists.
+
+## Status
+
+- [x] Oriented; plan above; baseline running.
+- [ ] `llc` probe for the GOTPCREL spelling.
+- [ ] `Lgot` addressing.
+- [ ] `X86.redefinition`.
+- [ ] Assembling + a test that runs it.