M2compiler-V3/docs/summary_two-phase-lower2.md

Lower phase — slice 2 (control flow)

Branch ast-stage-c. Suite 182/182; self-hosting fixpoint OK (3,123,487 bytes; L1 was 3,104,868). Plan: docs/plan-lower.md.

What landed

LowerStmt (sequence iteration + statement dispatch merged, so no FORWARD — gm2's whole-program pass rejects it) now emits:

• IF/ELSIF/ELSE — flattening the AST's nested-NkIf chain into the grammar's single shared lEnd, threading lElse, exactly as the grammar does.
• WHILE, REPEAT, LOOP + EXIT (via PushLoop/TopLoop).
• FOR without BY (cslew/add loop), with the same StoreVar/LoadVar/Op3 order.
• HALT.

CanStmt mirrors the same shapes; FOR with a BY clause stays out of the subset (needs constant evaluation).

Finding

FORWARD declarations in Lower.mod broke gm2's whole-program pass ("too many errors in pass 3") even though -c compiled the module. Merging the mutually-recursive bodies (CanBody↔CanStat, LowerBody↔LowerIf) into single dispatching procedures removed it.

Verified

gen_ssa/TLower2L.ssa — a program with IF/ELSIF/ELSE, WHILE, REPEAT, LOOP/EXIT, FOR and a conditional HALT — is byte-identical to the legacy image (lower_ok t_lower2.mod).

Next

L3: procedures/functions (headers, params, locals, RETURN, calls) — BeginFunc/FuncParam/EndFuncHeader/EndFunc/CallBegin… This is where LowerUnit starts walking declaration bodies, not just the program body.