# iter loop-recur.3 — Codegen (Component 5) + run-to-value E2E

**Date:** 2026-05-17
**Started from:** eae73bf320440f6b031715e72c1f20b0ac86e024
**Status:** DONE
**Tasks completed:** 3 of 3

## Summary

The **terminal iteration** of the standalone-`loop`/`recur` milestone:
the iter-1 `lower_term` `CodegenError::Internal` stub for
`Term::Loop`/`Term::Recur` is replaced with real LLVM-IR lowering, so
a `sum_to`-class loop program builds and runs to the correct printed
value, a deep-`n` variant is safe by construction, and an infinite
loop compiles. Loop binders are loop-carried values lowered as
entry-block allocas (the mut.3 `pending_entry_allocas` mechanism)
registered in the existing `mut_var_allocas` map, so the existing
`Term::Var` load path resolves them with zero new Var code
(representation-sharing); a fresh `loop.header.<id>` block is reached
by an unconditional `br` from the pre-header, `recur` lowers each arg
to SSA *before* any store (simultaneous positional rebind), stores
into the binder allocas, back-edges `br` to the header, and sets the
single existing `block_terminated` field at its OWN new emit site so
the loop's exit value is the emergent product of the existing
`if`/`match` join (no separate loop-result phi/exit-block). A new
`loop_frames` codegen stack lets `recur` find its target; it is
saved/reset/restored at the single lambda-lowering boundary exactly
as mut.3's `mut_var_allocas` triple. `clang -O2` mem2reg promotes
the binder allocas to the phi nodes the spec's implementation-shape
describes. This is a **codegen-only iteration**: no schema/AST/serde
change, no typecheck change; hash pins (`loop_recur` + iter13a) and
the drift trio + `carve_out_inventory` stay green untouched
(confirmed, not modified — empirical proof none scans the codegen
region). `cargo test --workspace` 616 → 619 / 0 red (the 3 new e2e
tests). After this iter the milestone is structurally CLOSED;
milestone-close (audit/fieldtest) is the Boss's post-iter call.

## Per-task notes

- iter loop-recur.3.1: positive `sum_to` run-to-value E2E (RED →
  GREEN). RED observed verbatim: `ail build` on `loop_sum_to_run.ail`
  fails with the iter-1 stub `internal: Term::Loop/Term::Recur
  lowering lands in loop-recur iter 3` (parse + iter-1/iter-2
  typecheck pass; only codegen stubbed — confirmed via direct `ail
  build`). GREEN: `loop_frames` field + init, lambda-boundary
  save/reset/restore, the two real `Term::Loop`/`Term::Recur` arms
  replacing the single stub arm. `loop_sum_to_run.ail` → `55`;
  `cargo build -p ailang-codegen` Finished; full e2e suite 88/0.
  Plan's literal arms compiled as-written — no plan-vs-reality
  substitution needed (pre-grounding confirmed `mut_var_allocas:
  (String, Type)` and `lower_term -> (String /*ssa*/, String
  /*llvm_ty*/)` matched the plan's assumptions exactly).
- iter loop-recur.3.2: deep-`n` safe-by-construction +
  infinite-loop-compiles (test+fixture only; the codegen mechanism
  shipped in 3.1, so no separate RED). `loop_sum_to_deep.ail` runs
  1e6 iterations via the back-edge → `500000500000` with no stack
  growth (the spec clause-2 correctness claim made executable —
  mem2reg promoted the binder allocas so the loop is an iterative
  back-edge, not a stack-growing recursion). `loop_forever_build.ail`
  (`spin`'s loop has no non-`recur` exit) compiles `ail build` exit
  0 — `recur` sets `block_terminated`, propagating through
  `emit_fn`'s `if !self.block_terminated` fall-through guard so
  `spin` emits no `ret` and is a well-formed never-returning fn; the
  binary is never executed (spec "typechecks AND compiles, no
  termination claim"). Full e2e suite 90/0.
- iter loop-recur.3.3: regression gates — pure verification, ZERO
  source changes (confirmed `git diff --name-only HEAD` = only the
  3 T1/T2 source files). tail-app/tail-do/`verify_tail_positions`
  non-regression green (the parallel `block_terminated` SET site
  touched no existing SET/READ site — Boss call 4); `loop_recur` +
  iter13a hash pins + drift trio + `carve_out_inventory` green
  untouched (codegen-only iter, no schema change); full workspace
  619 passed = exactly the loop-recur.2 baseline (616) + the 3 new
  e2e tests, zero FAILED/error; `round_trip` green (the 3 new `.ail`
  fixtures auto-discovered, parse→print→parse idempotent — Roundtrip
  Invariant holds for runnable loop/recur programs).

## Boss design calls (mirrored from the plan header at iter close)

All four were settled in the plan header on
architectural-consistency / spec-pinned-invariant grounds (NOT
effort); each verified against actual read source before
implementation; none reopened, re-derived, or "improved" in any
phase:

1. **Loop binders → entry-block alloca + `mut_var_allocas` reuse,
   NOT hand-emitted `phi`.** The spec's "phi" wording lives only in
   the explicitly-*secondary* implementation-shape subsection. The
   established in-repo loop-carried-value mechanism is
   alloca-in-entry-block (mut.3 `pending_entry_allocas`) + `clang
   -O2` mem2reg, which produces the identical optimized binary. The
   linear-emit codegen architecture structurally cannot hand-emit a
   header `phi` whose back-edge predecessors are discovered *during*
   body lowering without a string-splice hack with no precedent (the
   `if` arm sidesteps this by opening its join block *last* — a loop
   header cannot be opened last). Rationale = architectural
   consistency + absence of a linear-emit `phi` precedent + identical
   -O2 output. Implemented verbatim; `loop_sum_to_run.ail`→55 and
   `loop_sum_to_deep.ail`→500000500000 are the operational proof
   mem2reg delivers the loop-carried-SSA intent.
2. **Binders ride the existing `mut_var_allocas` map + the existing
   `Term::Var` load path (representation-sharing).** A loop binder's
   codegen representation — a named alloca slot, loaded on each
   `Term::Var` use, stored on `recur` — is structurally identical to
   a mut-var's. The `Term::Loop` arm inserts each binder into
   `mut_var_allocas` (value type `(String, Type)`, identical to the
   `Term::Mut` precedent); the existing `lib.rs` `Term::Var` arm
   resolves them via its pre-existing `mut_var_allocas.get(name)` →
   `load` path **byte-unchanged** (zero edits to Var-resolution; no
   parallel map). Verified against real source before implementing.
   mut.3's shipped+E2E-green mut-var read path is the proof the load
   works for loop binders.
3. **The loop's exit value is the emergent product of the existing
   `if`/`match` join once `recur` sets `block_terminated` — no
   separate loop-result phi/exit-block.** The body is typically
   `(if c <exit> (recur …))`; the existing
   one-branch-terminated logic at `lib.rs:1606-1613` already drops
   the terminated (recur) branch and returns the non-recur branch's
   `(ssa,ty)` directly — *iff* `recur` sets `block_terminated`. The
   `Term::Loop` arm therefore returns `lower_term(body)`'s `(ssa,ty)`
   directly; no separate loop-result phi was built. An infinite loop
   (body = bare `recur`) compiles because `recur` sets
   `block_terminated`, propagating through the `emit_fn`
   `if !self.block_terminated` fall-through guard exactly as a
   tail-app-terminated function.
4. **Reuse the single `block_terminated` field; `recur` sets it at
   its OWN new emit site (a parallel SET call-site), zero edits to
   any existing SET or READ site.** A back-edge `br` IS a block
   terminator with exactly `block_terminated`'s semantics. `recur`'s
   own `self.block_terminated = true;` is a NEW call-site parallel
   to (not replacing) tail-app's SET at `lib.rs:2279`. The diff
   confirms tail-app's SET and every READ site
   (`:1124/1581/1594/1600-1613/...`) are byte-identical; the
   tail-app/`verify_tail_positions` non-regression gate (T3 Step 1)
   is the operational proof.

## Concerns

- DONE_WITH_CONCERNS (iter loop-recur.3.3): the plan's literal Task-3
  Step-1 command `cargo test --workspace tail` filters on the
  test-name substring `tail`, which matches **no** test in the
  workspace (the real tail-app guards are
  `ailang-check::lib::tail_call_in_{non_,}tail_position_is_*`,
  `ailang-prose::lib` `*_tail_*`, e2e
  `iter14e_print_list_recursion_emits_musttail`). Ran the gate via
  the real test names instead (2 + 3 + 1, all green); the
  authoritative non-regression gate is Step 3's full-workspace
  619/0 which subsumes them. Recurring `feedback_plan_pseudo_vs_reality`
  class (a literal verification command whose filter doesn't resolve
  to its evident target); resolved by preserving the gate's evident
  intent (tail-app/`verify_tail_positions` byte-unchanged proven),
  no behaviour change. Observation, not a correctness risk — the
  full-suite count `619 = 616 + 3` is independent confirmation
  nothing regressed.

## Known debt

- None. The iteration's invariants (loop/recur lowers to real IR;
  run-to-value 55; deep-`n` 500000500000 safe-by-construction;
  infinite loop compiles; tail-app + hash pins + drift trio
  byte-unchanged; round-trip on the 3 new fixtures) are fully pinned
  by the 3 new e2e tests + the unchanged regression suite.

## Files touched

- Codegen: `crates/ailang-codegen/src/lib.rs` (`loop_frames` field +
  init; the two real `Term::Loop`/`Term::Recur` arms replacing the
  iter-1 single stub arm), `crates/ailang-codegen/src/lambda.rs`
  (lambda-boundary `loop_frames` save/reset + restore, alongside the
  mut.3 triple)
- Tests: `crates/ail/tests/e2e.rs` (3 new test fns: 2 build+run, 1
  build-only)
- New fixtures: `examples/loop_sum_to_run.ail`,
  `examples/loop_sum_to_deep.ail`, `examples/loop_forever_build.ail`
- No edit / byte-frozen (confirmed, not modified): every existing
  `block_terminated` SET/READ site + tail-app/tail-do lowering +
  `verify_tail_positions`; the iter-2 typecheck (`synth`
  Loop/Recur arms, `verify_loop_body`, `loop_stack`, the four
  `Recur*` variants); the iter-1 `synth_with_extras`
  Loop→body-type / Recur→Unit pass-through; `hash_pin.rs`
  (`loop_recur` + iter13a pins) + the drift trio +
  `carve_out_inventory.rs` (codegen-only iter; the 3 new `.ail`
  fixtures are round-trip-auto-covered, NOT carve-outs — inventory
  stays 8)

## Stats

bench/orchestrator-stats/2026-05-17-iter-loop-recur.3.json
