117 lines
8.2 KiB
Markdown
117 lines
8.2 KiB
Markdown
# Handoff — a redefinition emitter in `lib/x86.ml`
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Branch `dev-loop`, worktree `agent-a884e1e2fcffe5052`, from `b1cc67b`. Item 1 of `HANDOFF-x86-rt.md` §6.
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**It works and it is checked by running it.** An `--x86` host dlopens an `--x86` redefinition module, the
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module publishes a new body into the host's cell, and the host's un-rebuilt call site follows it.
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`test/test_reload.ml` is green on both backends.
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## Baseline, measured before and after
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| | before (`b1cc67b`) | after |
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|---|---|---|
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| `spike/x86/survey.sh` | 97 MATCH / 0 DIFFER / 0 refused | (see §"After", below) |
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| `spike/x86/cells.sh` | 4/4 ok | 4/4 ok |
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| `dune test --root .` | exit 0 | exit 0 |
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`dune test` prints two `ld` / `clang` failures from inside the `dev-robust` fixture and still exits 0. Check the
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**exit code**, not the output; grepping will mislead you.
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## Why this had to exist
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`x86.ml` licenses its own calling convention on the grounds that a dev build is compiled entirely by it and a
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release build entirely by LLVM, *and the two never meet in one process*. The conventions agree on every scalar
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and disagree on **every aggregate** (here: every aggregate by pointer with a hidden `sret` in the first integer
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register; LLVM classifies by eightbyte). An `Emit.redefinition` module dlopened into an `--x86` host is correct
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until the first redefined function takes or returns a struct. So an `--x86` host gets `--x86` modules.
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**There is still no aggregate classifier and there must not be one.**
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## The crux, and the answer
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`program` emits an executable; every symbol it names it defines, or the runtime does, and a pc-relative `PC32`
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reference is right for all of them. A redefinition module is a `.so` and almost everything it names belongs to
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the **host executable** — the cells `flan.cell.<n>`, the globals `flan.<g>`, the runtime's entry points. A
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`PC32` relocation against an undefined symbol cannot be used in a `-shared` link; `ld` refuses it.
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So a reference to a symbol the object does not define goes through the GOT. Measured, not recalled:
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- The spelling is `.long sym@GOTPCREL - 4`. Quoted symbols (`"flan.cell.x"@GOTPCREL`) are accepted.
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- **`- .` is wrong.** `@GOTPCREL` is already pc-relative; written `- . - 4` as the plain form needs, `as`
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produces an addend of **-8** and the load reads the wrong slot. `readelf -r` on the object is how you see it:
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want `flan.cell.x - 4`.
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- `llc -relocation-model=pic` emits `movq x@GOTPCREL(%rip), %rbx` for `@x = external global i64`, which is the
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same instruction bytes this backend already emits for a `Sym` operand. Only the relocation changes.
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## What was built
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| file | what |
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|---|---|
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| `lib/x86.ml` — `modrm_got`, `mem`'s new `Got` case | the `@GOTPCREL` field. `mem_op` and `store_int`'s byte case were the only two matches on `mem` |
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| `lib/x86.ml` — `loc`'s new `Lgot` case | a symbol the object does not define. `lmem` loads the GOT slot into `scratch` and answers `Reg (scratch, d)` — the `Lp` shape, so a field offset is arithmetic on a register and never on the relocation |
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| `lib/x86.ml` — `fnctx.ext` | `string -> bool`, true of a symbol this object does not define. **Default `false`**, so the whole-program path emits byte-identical output and the survey is protected structurally |
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| `lib/x86.ml` — `sym_loc`, `addr_sym`, `load_sym` | the three spellings of naming a symbol; each picks pc-relative or GOT off `ext` |
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| `lib/x86.ml` — `emit_fn ?ext ?hidden` | `.hidden` on a module's own bodies |
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| `lib/x86.ml` — `program`, the no-`main` case | was `unsupported "no main"`, now emits none, as `emit.ml` does. A program linked against a C host that brings its own entry point is legitimate — `reload_host.c` is exactly that, and the refusal made an `--x86` host for the reload tests impossible to build |
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| `lib/x86.ml` — `redefinition` | the emitter. No `main`, no `.init_array`, no `flan..init-globals`, no `flan_dev_reg_enable` ctor, no `.bss` for globals, no `.data` for cells. Bodies `.hidden`; one `flan_reload_install` that reads each cell's address out of the GOT and stores the new body's `lea` into it |
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| `lib/build.ml` — `shared_x86` | `as` + `ld -shared`, the same link `shared` does. No `-relocation-model=pic` to ask for: the emitter already writes every foreign reference through the GOT |
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| `test/test_reload.ml` | the x86 section: an `--x86` host, two `--x86` modules, the same transcript, plus the refusal check |
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## The one real bug, because it is the shape of the next one
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`call_flan`'s `` `Cell `` target loaded **one** level: `mov r11, cell@GOTPCREL(%rip)` then `call *%r11`. That
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reads as "call through the cell" in a disassembly and in fact *calls the cell*. The GOT slot holds the cell's
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**address**; reading what the cell holds is two loads, not one. Found by the segfault, not by reading — which is
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../DISCUSS.md item 15's claim, now with a sixth instance. `load_sym` is the fix and is the only helper that
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double-loads; `Lgot` gets it right for free because `lmem` already had the shape.
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## Scope, deliberately narrower than `Emit.redefinition`
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Only names the **host already has**. Refused by name, each with the reason in the message:
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- a function new to the session — needs `flan_dev_cell` and `Emit.cellptr`'s second spelling (a module-local
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slot resolved by string at install time)
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- a global new to the session — needs `flan_dev_global`, plus the init constant that travels with it
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- `consts` — republishing a `defconst`
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- `call` — the transient `flan_reload_call` thunk and the `@flan_reload_transient` marker the agent unloads on
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That is `C-c C-c` on an existing `defn`, which is the demo, and it is what `test/programs/reload.flan` and
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`reload-v2.flan` exercise (v3 and v4 are the new-name cases and are asserted to refuse).
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## What remains
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1. **The new-name path.** `flan_dev_cell` / `flan_dev_global` and `Emit.cellptr`'s spelling, so that a `C-c C-k`
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introducing a function or a `defvar` works on `--x86`. `test/test_reload.ml`'s v3/v4/v5 are the fixtures
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already written; the refusal check there is what to replace.
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2. **The transient thunk** — `flan_reload_call` plus `@flan_reload_transient`, which is what an *expression*
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evaluation (the break loop, `C-x C-e`) compiles to. Note `emit.ml`'s three conditions for emitting the
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marker, including `m.nstr = 0`: a module holding a string literal can never say it is transient.
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3. **Wiring it to `flan dev` / `flan reload`.** Today nothing reaches this: `--x86` is read only by
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`flan build`'s argument list in `bin/main.ml`, and the daemon builds host and module through
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`Build.executable` / `Build.shared` with no `x86` field set. When it is wired, the host's backend and the
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module's backend must be chosen together — that is the whole licence.
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4. **A redefined function that takes and returns a struct.** This is the case that motivated the whole lane —
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the two backends agree on every scalar and disagree on every aggregate — and it is exercised by *nothing*,
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on either side of the reload boundary. `reload.flan`'s `bump` is `-> i64` and `helper` is `i64 -> i64`. The
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argument that an `--x86` host plus `--x86` modules is same-convention-by-construction is sound, but it is an
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argument and not a measurement. Do not edit `reload.flan` for it — its transcript is shared with the LLVM
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path; add a fixture beside it.
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5. Items 2–7 of `HANDOFF-x86-rt.md` §6, unchanged.
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Also still true and still worth doing: **run `spike/x86/survey.sh` in CI**.
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## After, measured
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| | before (`b1cc67b`) | after |
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| `spike/x86/survey.sh` | 97 MATCH / 0 DIFFER / 0 refused | **97 / 0 / 0**, skips `28 does-not-compile / 6 no-main / 2 runs-forever` — identical |
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| `spike/x86/cells.sh` | 4/4 ok | **4/4 ok** |
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| `dune test --root .` | exit 0 | **exit 0**, run without a pipe |
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The skip breakdown is the line that matters beyond the MATCH count, because the no-`main` relaxation is the one
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change here not gated behind `ext` being false. It cannot in fact move it: `survey.sh:70` classifies `no-main`
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off the **LLVM** build failing at the link, before it ever runs the x86 one.
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And run `dune test --root .` **without a pipe**: piping to `tail` gives you `tail`'s exit status, which is the
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trap this file warns about at the top, and the `dev-robust` fixture puts `ld` and `clang` failure text in the
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output either way.
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