# Lower phase — slice 0 (mechanism + scalar L0 subset)

Branch `ast-stage-c`.  Suite **180/180** (178 + 2 Lower comparisons);
self-hosting fixpoint **OK** (**3,090,949 bytes**; slice 13 was
3,042,040).  Plan of record: `docs/plan-lower.md`.

## What landed

The **mechanism** that lets the AST be lowered and byte-compared against
the legacy emit, plus the first subset.

- **`compiler/src/Lower.def/.mod`** — the walk.  `CanLower(unit)` gates a
  unit to the supported subset; `LowerUnit(unit)` re-emits it:
  `OpenModule`, `SetOutName(<mod>L)`, `SetModule(<mod>)`, declarations,
  `BeginBody`, body, `EndModule(<mod>)`.
- **`QbeGen.SetOutName`** (new, additive): overrides only the *image file
  name* used by `EndModule`, leaving the mangled symbol name unchanged —
  so a matching walk writes a byte-identical image to a different file.
- **`-lower`** driver flag: the grammar re-emits the program unit from the
  AST **inside the unit's `END` action, right after the legacy
  `EndModule` and before `SymTab.EndUnit`** (the module scope must still
  be open — `Lower` resolves names through `SymTab`).
- Grammar exports `SetLower`/`GetUnit`; `parser.frm`'s definition module
  re-exports them; `Lower` is linked like `AST`.

### L0 subset

A program unit with scalar `CONST`/`VAR` and a body of
`x := <integer expression>` (literals/idents, `+ - * DIV MOD`, unary `-`),
reproducing `Design`'s scalar emit (`LoadVar` / const `CopyOp`) and
`AssOrDecl`'s scalar store, including `Fold2` constant folding.

## Verified

`./M2 -lower tests/t_exit.mod` and `t_arith.mod` produce
`gen_ssa/<Mod>L.ssa` **byte-identical** to the legacy image (now asserted
by `lower_ok` in `run_tests.sh`).

## Finding

`SymTab.EndUnit` closes the module scope, so `Lower` cannot run *after*
`Parse` (names no longer resolve).  It must run at unit end, before
`EndUnit` — and after the legacy `EndModule` so re-opening the backend
does not clobber the legacy image buffer.

## Next (per `docs/plan-lower.md`)

Extend the subset one step at a time — L1 relations/booleans/short-circuit,
L2 control flow, L3 procedures, … — each gated by `lower_ok` + suite +
fixpoint, until a unit can be lowered from the AST instead of inline
(phase 3), then delete the inline emit and `Fwd*` machinery.