# iter hs.1 — Heap-Str ABI: Static-Str layout migration

**Date:** 2026-05-12
**Started from:** 69bb5f9952fb51af7cdf4e36f77cff1b6cd73c40
**Status:** DONE
**Tasks completed:** 5 of 5

## Summary

First iter of the heap-Str ABI milestone. Pure codegen-side refactor:
language `Str` literals now emit as packed-struct LLVM globals
`<{ i64, i64, [N+1 x i8] }> <{ i64 -1, i64 N, [N+1 x i8] c"...\00" }>`
carrying a `UINT64_MAX` sentinel rc-header in the first slot, an
explicit byte length in the second, and the bytes plus trailing NUL
in the array tail. The IR-Str-pointer convention is now uniform: a
language `Str` value flowing through the IR is a `ptr` landing on the
`len`-field (offset +8 from the header, -8 from the bytes). Every
codegen site that hands a `Str` pointer to a `NUL`-terminated-bytes
C-API consumer emits a `getelementptr inbounds i8, ptr <v>, i64 8`
immediately before the call. The sentinel slot is hardcoded in the
global initialiser and not yet observed by any caller — `hs.2` will
add the `UINT64_MAX` short-circuit to `ailang_rc_inc` /
`ailang_rc_dec` in `runtime/rc.c` to exploit it. Format strings
(`%lld\n`, `%g\n`, `true\n`, `false\n`) are unchanged — they are
runtime-internal scaffolding, never flow as language `Str` values,
and remain in the raw `[N x i8]` shape via the original
`intern_string` / `all_strings` path. Acceptance verified
end-to-end: full `cargo test --workspace` green (no regressions
across 74 e2e fixtures), `bench/compile_check.py` exit 0,
`bench/cross_lang.py` exit 0 (byte-identical stdout across all
ail/c benchmark pairs).

## Per-task notes

- iter hs.1.1: introduced parallel `str_literals` interning table on
  `Emitter`, `intern_str_literal` fn, workspace-level
  `all_str_literals` aggregation, and a parallel global-emission
  loop emitting the packed-struct shape. Switched both
  `Literal::Str` arms (`emit_const_def`, `lower_term`) to call
  `intern_str_literal` and materialise a constexpr
  `getelementptr inbounds (<{ i64, i64, [N x i8] }>, ptr @g, i32 0, i32 1)`
  landing on the `len`-field. Two pinning tests added.
- iter hs.1.2: `io/print_str` arm now emits a `+8 GEP` to land on
  the bytes pointer before calling `@puts`. One pinning test added.
- iter hs.1.3: `eq__Str` arm of `try_emit_primitive_instance_body`
  now emits `+8 GEP`s on both operands before calling
  `@ail_str_eq`. One pinning test added.
- iter hs.1.4: `compare__Str` arm of `try_emit_primitive_instance_body`
  now emits `+8 GEP`s on both operands before calling
  `@ail_str_compare`. One pinning test added.
- iter hs.1.5: regenerated `crates/ail/tests/snapshots/hello.ll`
  via `UPDATE_SNAPSHOTS=1`. Full `cargo test --workspace` revealed
  a regression in `eq_demo` traced to a fourth codegen site not
  enumerated in the plan: `lower_eq`'s `"Str"` arm calls `@strcmp`
  inline (separate from the `eq__Str` instance-method intercept)
  to lower `==` on `Str` literals and Str-pattern desugaring in
  `classify_str`. Fixed inline with the same `+8 GEP` shape; one
  additional pinning test added.

## Concerns

- The plan claimed three codegen sites pass a `Str` pointer to a
  `NUL`-terminated-bytes C-API consumer (`@puts`, `@ail_str_eq`,
  `@ail_str_compare`); in fact a fourth site exists in
  `lower_eq`'s `"Str"` arm, calling `@strcmp` directly. This is
  the dispatch path for the surface-level `==` operator on
  `Str` and for `(pat-lit "...")` desugar against a `Str`
  scrutinee. The fourth site is required to meet the iter's
  stated acceptance criterion (byte-identical stdout); fixing it
  was discovered via the e2e regression in `eq_demo` rather than
  via the per-task IR-shape checks. The fix has the same shape
  as Tasks 2-4 and the corresponding pinning test
  (`lower_eq_str_calls_strcmp_with_bytes_pointer`) is added. The
  plan's "Files this plan creates or modifies" enumeration
  should list `crates/ailang-codegen/src/lib.rs:2520-2529` as a
  fifth modify site for any future re-execution.
- The plan's two test fixtures in Task 1 contained schema-syntax
  errors that prevented compilation: `ConstDef.body` (actual:
  `value`), `Term::App { f, args }` (actual: `callee, args, tail`),
  and `Term::App { Term::Var "io/print_str", ... }` for what is
  actually a `Term::Do` effect op. Tests were adapted to compile
  while preserving the substantive assertions verbatim. Same
  applies to the test in Task 2 (`Term::App` → `Term::Do` for the
  `io/print_str` invocation and added `effects: ["IO"]` on the
  main fn so the typecheck-side dispatch finds the effect).
  These were small per-test fixture adaptations, not changes to
  the asserted IR shape.

## Known debt

- The sentinel value `-1` (`UINT64_MAX`) is hardcoded in the
  global initialiser. `hs.2` adds the matching short-circuit
  check in `ailang_rc_inc` / `ailang_rc_dec` so static-Str
  pointers can flow through generic RC paths without
  ref-count traffic. Until `hs.2` lands, no caller actually
  observes the sentinel — static `Str` slabs are still
  invisible to the RC runtime by construction (the call sites
  for `inc` / `dec` against `Str` values are still gated by
  the same uniqueness-inference pass that elided them
  pre-iter).

## Files touched

- `crates/ailang-codegen/src/lib.rs` — Emitter field, intern fn,
  workspace aggregation, global emission, two `Literal::Str` arm
  switches, four C-API-callsite `+8 GEP` adjustments
  (`io/print_str`, `eq__Str`, `compare__Str`, `lower_eq` Str arm),
  six new IR-shape pinning tests in the test module.
- `crates/ail/tests/snapshots/hello.ll` — regenerated snapshot
  showing the new packed-struct global and the `+8 GEP` before
  `@puts`. No other snapshot under
  `crates/ail/tests/snapshots/` changed (other snapshots contain
  only format-string globals which kept the raw `[N x i8]`
  shape).

## Stats

`bench/orchestrator-stats/2026-05-12-iter-hs.1.json`
