M2compiler-V1/docs/goal_step4.md

Step 4 goal — Composite types (saved, to implement tomorrow)

Scope (Phase 3 of the plan)

• ARRAY: bounds, indexing (incl. multi-dimensional), scaled address arithmetic, indexed loads/stores (0EH/1EH), open-array parameters.
• RECORD: field layout with offsets, .field selection, WITH.
• SET: add operators (+ union, - difference, * intersection); literals/IN/=/# already work since step 2.
• POINTER: NEW/DISPOSE, dereference, NIL handling.
• Strings: string variables as ARRAY OF CHAR.

Design deltas (vs step 3)

• SymTab: byte sizes on descriptors, field/element offsets, per-variable sizes (frame + globals); new queries (TypeSize, FieldOffset, …).
• MGen: scaled indexing, copy_block for composite assignment.
• Policy to fix at start: range checking (checked vs unchecked), NIL dereference (runtime trap, no static check).
• NEW semantics: recommended — VM ALLOCATE (0x40 05), already implemented by the interpreter. Alternatives (e.g. compiler-managed bump region) to be decided if ALLOCATE misbehaves under test.

Acceptance (same bar)

~15 run + ~10 rejection tests, all green, no cascades; full 49-test regression still passing (→ ~74 total).

Suggested first tests

Matrix multiply or sieve (arrays); nested records + WITH; set ops with ranges; linked list via NEW/DISPOSE (combines pointers with step-3 recursion).

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Session checkpoint (saved, not committed)

• Tags step1, step2, step2.1, step3 exist locally (step3 = c0f1924, suite 49/49 + mc64 24/24 green).
• Pushing (commits + tags) is done by the user (credentials).
• Uncommitted on disk: summary_step1/2/2.1/3.md (docs commit 0e07f06 was undone per user request), this goal_step4.md, plus pre-existing leftovers never touched (m2comp/coco/ deletions, gnu-m2-grammar.txt, chatgpt folder, instructions file, STAK/DEP.MC4).
• Next action: implement the step-4 goal above.