# Lower phase — L7 (the flip): status and plan (2026-10-05)

Branch **`ast-stage-c`**.  L7 = "flip: lower from the AST instead of
inline; delete the inline emit + `Fwd*` machinery".

## Why L7 is not incremental

The flip makes `Lower` the sole emitter.  The obstacle is that `QbeGen`
holds **one** session state, used by the inline emit *during* parse; a
per-unit `Lower` emit would be clobbered by the next unit's inline emit.
So the flip is a **big-bang**: either remove/suppress the grammar's 533
`QbeGen.*` call sites, or give `QbeGen` two states — neither is a
byte-verifiable step.

The pragmatic path is therefore a **session re-emit**: collect every
unit's AST and re-emit the whole session with `Lower` after parsing.
That is byte-comparable against the legacy session image and becomes the
flip once the inline emit is removed.

## Started (`d30abfd`, tag `v3-lower7-scaffold`)

- `Lower.LowerSession/LowerOneUnit/MaterializeGlobals`.
- Driver (`compiler.frm`) collects `GetUnit()` per file and calls
  `Lower.LowerSession` under `-lower`.
- `SymTab.Materialize` makes a unit's own VAR/CONST visible in
  `globScope`, so `GlobalRef` resolves `"<unit>_<name>"` at session end
  (the unit scopes are closed by then).
- Removed `Lower.Reset` from the `M2` root so the owned tables persist
  across a session.

Still scaffolding (inline emit untouched); suite **200/200**, FIXPOINT OK
(3,401,129 bytes).

## Progress (`ce2fc67`, tag `v3-lower7-session`)

- **`DefUnit` now builds an AST** (proc headings recorded as
  `NkProcDecl`); `LowerDecls` replays a bodyless proc as a heading.
- **Session re-emit works** for small sessions: `lower_ok_files` added,
  and `DBasicProg`, `DQualProg`, `TQual` (def-CONST), `TQualP`
  (def-PROC) byte-match.  Fixes: image named after the program module;
  `MaterializeGlobals` also materializes TYPE/CLASS names;
  `SymTab.ReenterModGlobal` keeps nested-module globals'
  `"<outer>_<name>"` symbols; `AST.Init` once per session under `-lower`.

Suite **204/204**; FIXPOINT OK (3,405,813 bytes).

## Remaining gaps

1. **AST-arena size (fundamental).**  `LowerSession` needs every unit's
   AST alive at once, but `AST.MaxNodes = 65536` is sized for one unit;
   the compiler's session overflows it (`MakeNode` → `NoNode`, later
   units dump `(nil)`).  Growing the arena past the 4 MiB `sessBuf` is
   not viable.  So the session re-emit **cannot** handle the compiler —
   the flip must instead emit **per unit** at each unit's END (AST valid,
   scope open), which is the two-state / inline-emit-removal problem.
2. **External procs.**  `storage_prog` differs: Lower emits
   `$malloc_6(l link, ...)` where the legacy emits `$malloc(...)` — the
   `ext` flag in `Lower.NoteProc` is hardcoded `FALSE` and `Mangled`'s
   `ProcLink` fails after scopes close.
3. Definition-module proc replay (done), nested globals (done).

## Per-unit emission attempted — blocked by parse-time state

Tried plan item 3 (Lower emits each unit at its END, with the legacy
output suppressed via `SetNoEmit`).  Small sessions and every `lower_ok`
single-file test byte-match, **but the compiler's session hangs/differs**:

- The grammar's inline `QbeGen` calls still run during parse; `noEmit`
  only suppresses the *output*, not the **state**.  So `nTemp` (stuck at
  211), `inBody` (stuck TRUE, so `main`'s header is dropped), `nLoc`,
  `funcDepth`, … are all corrupted before Lower's per-unit emit.
- `EndModule` had to respect `noEmit` too, or it closed the session.

Conclusion: per-unit Lower needs the grammar's parse-time `QbeGen`
**state** neutralised — which is exactly the flip.  Options:
1. **Remove/replace the grammar's 533 `QbeGen.*` calls.**  The blockers
   are the ~20 semantic checks that read `QbeGen` values (`IsImm`,
   `ConstInt`, `Fold`); they must be reworked to use `SymTab`/the AST.
2. **Two `QbeGen` states** — but a second 4 MiB `sessBuf` pushes the
   compiler image past its own buffer, so this does not self-host.

So the flip is a grammar rewrite of the emit actions, gated by the
fixpoint.  Reverted to the green session-re-emit state (`ce2fc67`).