# Session summary — 2026-10-02: AST Stage A + array-of-record fix

Suite **178/178**; self-hosting fixpoint **OK** (**2,900,782 bytes**).
Tag `v3-ast-stageA`.

This is the first step of the **AST / two-phase frontend** refactor
(option 1): make it possible to build a syntax tree while parsing and
lower it in a later pass, so forward references (variables, procedures,
typed constants, `CONST` constructors) resolve naturally and the
text-patching machinery can be deleted.

## 1. Stage A — the AST module (`compiler/src/AST.def/.mod`)

An **arena-backed** node store, referenced by an integer index (`Node`),
mirroring `SymTab.TypeIndex` (no pointers in the interface, no GC,
bounded memory — self-hosting-friendly).

- `NodeRec` = `kind`, `op`, `ty`, `nch`, `child[0..MaxChild-1]` — one
  flat record array; every construct is expressed with a kind + up to
  8 children (the `armornick/CocoR` `AST.java` pattern, flattened).
- Node kinds for units, declarations, type constructors, statements and
  expressions (`NkUnit`, `NkVarDecl`, `NkProcDecl`, `NkArray`,
  `NkAssign`, `NkIf`, `NkWhile`, `NkBinExpr`, `NkSelector`, …).
- Operator and selector codes (`OpAdd`…`OpIn`, `SelField`/`SelIndex`/
  `SelDeref`) in the node's `op`.
- An **interned text pool** (`AddTxt`/`Txt`/`TxtLen`) for identifiers
  and literal spellings.
- Constructors `MakeNode`/`MakeLeaf`/`MakeBin`/`MakeUn`, accessors
  (`Kind`/`Op`/`Ty`/`NChild`/`Child`/`AddChild`/`SetChild`), and a
  debug `Dump`.

**Nothing in the grammar calls it yet** — this is purely additive; the
suite and fixpoint are unaffected.  `tests/t_ast.mod` builds a small
tree (`a := b + 2`, an unary node, a block) and walks it: exit 42.

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

`QbeGen.ElemSize` computed an array 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;
```

used stride 4 instead of 48: consecutive elements overlapped, and
`a[i].op` / `a[i].child[0]` read neighbouring elements' fields (and the
array descriptor's count).  A single record, and a 2-element array,
happened to work; 4 elements did not.

Fix: record/class/set elements use `SymTab.TypeSize(elemT)` (the inline
footprint).  Repro that now passes: `rec3.mod` (exit 42); previously
`rc=1`.

This is what blocked the AST self-test (which stores
`child : ARRAY [0..7] OF Node` in a record), and it is a real
user-facing bug for any array of records with inline array fields.

## 3. Two false alarms (recorded so they are not re-chased)

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

## 4. Next (Stage B, option 2)

Convert **declarations** to two-phase: parse all `CONST`/`TYPE`/`VAR`/
`PROCEDURE` headings into the symbol table before lowering any body.
That removes the forward-variable text-patching (`FwdPatchAll` /
`FixLoadClass` / `FixStoreClass`) without a full AST, and is the
stepping stone to Stage C (statements/expressions → AST → lowering).

Branch-first discipline: after every increment run `fixpoint.sh` **and**
`run_tests.sh`; revert the increment if either fails.

## 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`, this doc.