M2compiler-V3/docs/summary_lower_l6.md

Lower phase — L6 status (2026-10-05)

Branch ast-stage-c. L6 = "classes/vtables, imports, nested modules".

Landed

Nested modules (f82f72b, tag v3-lower6-nested)

• CanDecl accepts a nested NkUnit (with no imports).
• LowerDecls emits the nested unit's declarations and, when it has a BEGIN body, BeginInit/body/EndInit (registering <nested>_init so the enclosing BeginBody calls it).
• ReenterGlobals re-registers a nested module's non-exported globals in the still-open enclosing module scope, so QbeGen.GlobalRef resolves their "<outer>_<name>" symbols after the nested scope has been popped.
• lower_ok t_nestedmod.mod byte-matches. Suite 194/194; FIXPOINT OK (3,328,857 bytes).

Blocked / remaining

Imports — blocked by the session-image model

-lower is invoked only from ProgModule's END (M2.atg), so Lower re-emits only the last unit. But a session's image is accumulated across all units (sessBuf): a program that IMPORTs a module pulls that module's procs/init/globals into the same gen_ssa/<Prog>.ssa. So re-emitting just the program cannot byte-match.

Supporting imports therefore needs a mechanism change: lower every unit (definition/implementation/program) and accumulate a session image (mirroring sessBuf), or otherwise compare per unit. That is the same mechanism the L7 flip needs, so it is the natural next structural step. Qualified names (M.x) additionally need SymTab.MaterializeAlias at lowering time (module scope is open — workable) and qualified-call resolution in LowerCall (currently CalleeName reads only the base ident).

Classes/vtables — large, single-unit, verifiable

Surface: NkClassDecl/NkFieldDecl, CLASS IMPLEMENTATION, method binding (obj.Method + ArmRecv), virtual dispatch, inheritance, NEW for classes, vtables (QbeGen.EmitVTables), THIS. Independent of imports, so it can be byte-verified with lower_ok on a self-contained class program.

Suggested order

1. Classes/vtables (verifiable now).
2. The per-unit lowering mechanism (unblocks imports and is the L7 prerequisite).