M2compiler-V2/docs/session_steps1-4.md

m2comp sessions, steps 1–4 — day wrap-up

Date: 2026-09-16. Built in one day, with E. directing grammar choices against Kowarsch (arXiv:1809.07080) and the Blaise pipeline as process reference.

Where we stand (all committed and tagged)

Step	Tag	Content	Suite
1	m2comp-step1	Coco/R lexer+parser, gm2 -fiso, LL(1)-clean	3/3
1.1	m2comp-step1.1	Bare subrange type, syntax showcase	4/4
2	m2comp-step2	SymTab semantic analysis (200–224, 230/232/233)	19/19
3	m2comp-step3	AST layer (Mössenböck recipe) + dump	25/25
4	m2comp-step4	MC64 tree-walking backend, .MC4 under mcint	41/41

Layout: all sources in m2comp/src/ (grammar, frames, FileIO, SymTab, AST, MGen, generated scanner/parser/driver), binary at m2comp/M2comp, tests in m2comp/tests/ (check + reject + AST dump + execution with ExitCode values), docs in m2comp/docs/ (per-step summaries, Kowarsch text, AST.pdf, oberon-master/ reference, this note).

Hard-won facts (don't rediscover)

• gm2 is multipass: FORWARD is rejected ("too many errors in pass 3"); CR's -m flag comments forwards out; m2c has none.
• Op-code ranges must be globally unique for tree-walk dispatch (Add 0–2, Mul 10–14, Rel 20–27); overlap silently misroutes.
• 51H/70H store orders differ; use 70H ([addr, value]).
• Bodies emitted inline need jump-over (Jmp/DefLabel).
• | head masks VM hangs (rc is head's); harness uses timeout
  • rc checks; diverging programs fail fast.
• showcase.mod diverges by construction (infinite regress); check-only. ok_proc fixed to terminate.
• Negative-vs-zero D5 quirk reproduces byte-identically under m2c — VM domain, avoided in tests.

Tomorrow: step 5 candidates (in priority order)

1. DEFINITION/IMPLEMENTATION separate compilation (multi-.MC4, depCount loader path) — biggest language gap.
2. Opaque types + procedure types.
3. WITH / CASE (grammar + SymTab already shaped for them).
4. LONGINT/LONGREAL quads (0C5–CC, 0x12).
5. Debug listings / OPDF-style debug info.

SymTab (ByNum, exports, bounds), AST (pnum/typ/aux/tag), MGen (scope stack, image assembly) are ready for all of these. Start with ./build.sh && ./run_tests.sh (41/41, ~2 min).