# Session summary — 2026-10-02 (b): Stage A + the Stage B/C finding

Suite **178/178**; self-hosting fixpoint **OK** (**2,900,782 bytes**).
Tags `v3-ast-stageA` (AST module + bug fix).  This session also
established that the two-phase refactor is a **single** job (see §3).

## Steps, tags and commits

| Step | Tag | Commit | What |
| --- | --- | --- | --- |
| AST module | `v3-ast-stageA` | `53943fa` | `AST.def/.mod`, self-test |
| Docs | `v3-ast-stageA` | `c91695c` | `docs/summary_ast-stageA.md` |
| Finding | — | `7cec94a` | Stage B folds into Stage C (handoff) |

## 1. Stage A — the AST module

`compiler/src/AST.def` + `AST.mod`: an **arena-backed** node store,
referenced by integer index (`Node`) like `SymTab.TypeIndex` — no
pointers in the interface, no GC, bounded memory.

- One flat `NodeRec` (`kind`, `op`, `ty`, `nch`, `child[0..7]`); every
  construct is a kind + up to 8 children.
- Kinds for units, declarations, type constructors, statements and
  expressions; operator/selector codes in `op`.
- Interned text pool (`AddTxt`/`Txt`/`TxtLen`).
- `MakeNode`/`MakeLeaf`/`MakeBin`/`MakeUn`; `Kind`/`Op`/`Ty`/`NChild`/
  `Child`/`AddChild`/`SetChild`; debug `Dump`.

**Additive**: nothing in the grammar calls it yet.  `tests/t_ast.mod`
builds `a := b + 2`, an unary node and a block, then walks it (exit 42);
added to `run_tests.sh`.

## 2. Bug fixed — array-of-record element stride

`QbeGen.ElemSize` derived an element's stride from its **class**, and
`ClRecord`/`ClClass`/`ClSet` fell through to **4**.  So

```modula2
TYPE R = RECORD kind, op : INTEGER; child : ARRAY [0..7] OF INTEGER END;
VAR a : ARRAY [0..3] OF R;
```

strided by 4 instead of 48: consecutive elements overlapped and
`a[i].op` / `a[i].child[0]` read neighbouring elements' fields.  A
single record and a 2-element array happened to work; 4 elements did
not (`rc=1`).  Fix: record/class/set elements use `SymTab.TypeSize`.
Repro `rec3.mod` now exits 42.

This blocked the AST self-test (its `NodeRec` holds
`child : ARRAY [0..7] OF Node`) and is a real user-facing bug.

## 3. Finding — Stage B is not separable from Stage C

Starting the two-phase-declarations step (option 2) showed it is **not**
an independent stepping stone:

- Bodies are lowered **during** `ProcDecl`, interleaved with parsing.
- To complete the symbol table before lowering a body, the body must
  not be lowered while parsed — so it must be stored.
- A stored body is statements + expressions, i.e. **Stage C's AST**.
- A "declarations-only" AST does not help: the *bodies* need deferring.

**Options 1 and 2 are the same job.**  The refactor must be done as one
long branch: convert `Block`/`Stat`/`Expr` to AST nodes, move the
`QbeGen` body calls into a lowering walk, then declarations; delete the
forward-variable text-patching (`FwdPatchAll`/`FixLoadClass`/
`FixStoreClass`).  Keep the old path behind a flag until the new walk
passes the suite; expect a red fixpoint between increments.

## 4. False alarm (recorded so it is not re-chased)

“Nested calls mis-order arguments” — **not a bug**.  `rc` is the exit
status modulo 256, so an encoding of `1234` observed `210` and `1274`
observed `250`.  An equality check (`P(1,2,3,4)` → `rc=42`) confirmed
the call and codegen are correct.

## Resume

```sh
cd .../m2compiler-V3
./bootstrap/fixpoint.sh                       # FIXPOINT OK
cd compiler && ./build.sh && ./run_tests.sh   # 178/178
```

## Files

`compiler/src/AST.def`, `compiler/src/AST.mod`,
`compiler/src/QbeGen.mod` (`ElemSize`), `compiler/tests/t_ast.mod`,
`compiler/run_tests.sh`, `docs/summary_ast-stageA.md`,
`docs/session-handoff-2026-10-01.md` (revised plan), this doc.