sx/current/CHECKPOINT-FIBERS.md

# CHECKPOINT-FIBERS — Stream B1 (fibers + Io + M:1 scheduler)

Companion to [PLAN-FIBERS.md](PLAN-FIBERS.md). Update after every step (one step at a time,
per the cadence rule). New corpus category: `18xx` concurrency.

## Last completed step
**B1.0b (`abi(.naked)` real emission) — DONE. B1.0 complete.** Replaced the emit bail with
real LLVM `naked` emission:
- `emit_llvm` declaration pass: for `func.is_naked`, add the LLVM `naked` + `noinline` +
  `nounwind` attributes and **skip** the `frame-pointer=all` attribute (incompatible with a
  frameless function). Pass 2 now emits the `.naked` body normally — `naked` makes the
  backend emit it verbatim (the inline asm + its own `ret`) with no prologue/epilogue.
- IR shape (verified): `; Function Attrs: naked noinline nounwind` / `define internal i64
  @answer() #0 { entry: call void asm sideeffect "…ret…", ""() unreachable }` /
  `attributes #0 = { naked noinline nounwind }`. The caller invokes it as an ordinary
  `() -> i64` call (`.naked` is `call_conv == .default`).
- `examples/1800-concurrency-naked-asm.sx` — now GREEN, aarch64-pinned (`.build {"target":
  "macos"}`): runs end-to-end → **exit 42** on this host, ir-only on a mismatch; `.ir`
  snapshot captured.
- `examples/1801-concurrency-naked-generic.sx` (renamed from `-bail`) — the generic `.naked`
  now emits a correct naked `answer__i64` (exit 42), proving generic.zig produces a naked
  body, not a framed one. aarch64-pinned.
- `examples/1802-concurrency-naked-asm-x86.sx` — x86_64 cross sibling (`.build {"target":
  "x86_64-linux"}`, ir-only here): `.ir` locks `naked` + `movl $42, %eax` / `ret`.
- Unit test `emit: abi(.naked) function gets the naked attribute (no frame-pointer)` in
  `emit_llvm.test.zig` (asserts `naked` present, `frame-pointer` absent).
- **B1.0c (review-hardening):** a param-bearing `.naked` fn emitted invalid LLVM (loud
  verifier error "cannot use argument of naked function") because the param-alloca loop
  wasn't gated. Fixed forward (this *enables* the B1.3 context-switch use case rather than
  rejecting it): gated the param-alloca loop on `fd.abi != .naked` in decl.zig (both paths) +
  generic.zig; a naked fn's args stay in registers (read by asm), declared-but-unused in
  LLVM. Locked by `examples/1803-concurrency-naked-asm-param.sx` (`add(a,b)` → x0+x1 → 42).
- `zig build && zig build test` green: **725 ran, 0 failed** + unit tests.

### Earlier — B1.0a (lock + review hardening)
Plumbed `Function.is_naked` (set from `fd.abi == .naked` at both decl sites + generic.zig +
pack.zig); `funcWantsImplicitCtx` skips `.naked` (no synthetic ctx, like `.c`); all
body-lowering paths bypass `lowerValueBody` for `.naked` (asm body + `unreachable` cap — no sx
return); `emit_llvm` Pass 2 bailed loudly (since flipped to real emission). Adversarial
review caught the generic/pack `is_naked` gap (a generic `.naked` silently shipped a framed
body); closed + locked. The review's `.naked`-lambda CRITICAL was a false positive
(unparseable — `isLambda` breaks on the `abi` keyword).

## Current state
Stream A (atomics) is feature-complete (✅). Stream B1: **B1.0 complete** — `abi(.naked)`
emits a real LLVM `naked` function end-to-end (decl, generic, pack paths), the substrate for
the fiber context-switch. No fibers/Io/scheduler code yet. Grounded floor facts:
- `context` is already an implicit `*Context` param (slot 0) + `push Context` is a stack
  `alloca` ⇒ **fiber-local for free**. Only shared root = `__sx_default_context` global
  (entry-point bind). B1.1 expected to be a **library convention** (spawn trampoline
  snapshots the spawner's ctx into slot 0), **likely zero compiler change** — probe first.
- Inline asm works end-to-end (lower→emit→JIT, aarch64 + x86_64) — the `.naked` body reuses it.
- **`.naked` with PARAMS works** (B1.0c, the B1.3 substrate): the param-alloca loop is gated
  on `fd.abi != .naked` in decl.zig (both paths) + generic.zig — a naked fn's args stay in
  ABI registers (read by the asm body), declared-but-unused in LLVM (verifier-legal).
  Example `1803-concurrency-naked-asm-param.sx` (`add(a,b)` reads x0/x1). **Unsupported (loud,
  not silent):** a `.naked` *variadic-pack* fn (pack.zig's param loop is intertwined with
  comptime-param/`#insert` handling, and a naked fn can't read a runtime-sized pack from
  registers anyway) → loud LLVM-verifier error for that nonsensical construct. Acceptable
  boundary; a sharper sx diagnostic for it is a candidate polish, not a blocker.

## Next step
**B1.1 (per-fiber `context` root) — probe-first.** Per PLAN-FIBERS.md "Phases → B1.1". Write
a probe confirming a spawn trampoline can pass a snapshotted `Context` as slot 0 with no
compiler change (grounded as likely zero-change); lock the behavior with an `18xx` example +
a checkpoint note on the convention. Only if the probe surfaces a real gap (a path re-reads
`__sx_default_context` mid-stack) does this become a compiler step.

## Known issues / capability gaps
- **Orthogonal (not a B1 blocker):** default VALUES for comptime params don't bind on
  generic-struct methods (free-fn defaults DO work) — inherited from Stream A. Only matters
  if a B2 lib type wants a defaulted comptime param; atomics/fibers require explicit, so
  unaffected.
- **Issue 0144 (open, independent):** calling an unrecognized bodiless `#builtin` silently
  returns 0 / exit 0 — a silent-fallback footgun in the generic builtin-call path. Filed;
  leave for its own fix session unless prioritized. Not a B1 blocker.
- **Deferred design gap (documented):** the B1.4 event-loop `Io` does not yet cooperate with
  a platform UI run loop (CFRunLoop/NSRunLoop/ALooper); pinning gives thread-affinity, not
  run-loop integration — a §6 app-target concern, out of B1 scope.

## Decisions (Stream B1 specifics; surface locked in design §4 / §4.6)
- **The async runtime is sx LIBRARY code.** The compiler provides only: the general
  primitives (inline asm ✅, `abi(.naked)` naked [B1.0], atomics ✅) + fiber-safe codegen
  (`context` already fiber-local — B1.1). Schedulers, fibers, channels, futures, `Io`
  vtables, `mmap` stacks are all sx.
- **`abi(.naked)` is the real spelling of the design's `callconv(.naked)`** — postfix slot,
  `name :: (sig) -> Ret abi(.naked) { asm { … }; }`. B1.0 = carry it into IR + emit LLVM
  `naked` + skip prologue/ctx (mirror the existing `.c` skip), NOT extend the enum (it's
  already there, just inert).
- **`.naked` ≠ `.c`:** a `.c` epilogue would restore SP from the wrong stack across a context
  switch (SP-in ≠ SP-out by design). `.naked` = no prologue/epilogue/frame; the asm emits its
  own `ret`. This is why the switch must be `.naked`.
- **Naming:** sx-facing name is **`naked`** (keyword `abi(.naked)`, field `is_naked`, the
  diagnostic), matching LLVM's `naked` attribute and the industry term (Zig/Rust/GCC/Clang).
  The ABI variant was renamed `.pure → .naked` (user direction): "pure" universally means
  *side-effect-free*, the opposite of a register-clobbering context switch.
- **B1.0 snapshot scope:** a `.naked` body is raw per-arch asm; LLVM's `naked` attr text is
  arch-invariant. **B1.0a** = one host example locked to the emit bail (host-independent —
  fires before instruction selection; no `.build` pin). **B1.0b** = pin aarch64 + add an
  x86_64 cross sibling (`.build` target-gated, ir-only on mismatch), like the asm corpus
  split. The `.ir` proves the `naked` attr + asm emitted, NOT register-save correctness
  (that's B1.3's stress harness).
- **B1.1 grounded as library-only (pending probe):** push frames are stack-`alloca`'d and
  the implicit ctx rides slot 0, so a spawn trampoline can pass a snapshotted ctx with no
  compiler change. The design doc's "never raw TLS" guards a non-problem (context is not
  TLS). Probe to confirm before sizing any compiler work.
- **Test keystones (design §10):** the **B1.3 switch-stress harness** gates the
  context-switch (the one piece the deterministic `Io` can't test — §8.1.1, §10.7); the
  **B1.4 deterministic-sim `Io`** (calibrated against blocking `Io` — §8.1.3) gates all
  scheduling tests. Both must exist + be calibrated before the async tests they gate are
  trusted. `18xx` asserts program-emitted ordering contracts, not raw interleaving.

## Log
- **carve** — wrote PLAN-FIBERS.md + CHECKPOINT-FIBERS.md. Grounded the B1 compiler floor:
  `ABI.naked` inert (type_resolver.zig:237), IR `Function` has no naked flag (inst.zig:605),
  attribute API pattern (emit_llvm.zig:1339 nounwind), `.c` ctx-skip precedent
  (decl.zig:515), `push Context` stack-alloca + slot-0 implicit ctx (stmt.zig:1263,
  lower.zig:259), `__sx_default_context` root (decl.zig:2667/2815), inline-asm corpus
  (1645/1651). Corrected the design's `callconv(.naked)` → real `abi(.naked)` spelling and
  the B1.0 snapshot story. B1.1 grounded as likely library-only. Baseline green (721/0).
- **B1.0a** — plumbed `Function.is_naked` (set from `fd.abi == .naked` at both decl sites);
  `funcWantsImplicitCtx` skips `.naked` (no implicit ctx, like `.c`); both body-lowering
  paths bypass `lowerValueBody` for `.naked` (asm body + `unreachable` cap — no sx return);
  `emit_llvm` Pass 2 bails loudly on `func.is_naked`. `examples/1800-concurrency-naked-asm.sx`
  locked to the bail (exit 1 + diagnostic). Suite green (722/0). (ABI variant later renamed
  `.pure → .naked` — see the Naming decision above — so all `is_*`/`abi(.*)`/example names
  here read `naked`.)
- **B1.0a review-hardening** — adversarial review found generic/pack Function-creation paths
  left `is_naked` false (silent framed body for a generic `.naked` instance — returned 42 but
  corrupted the stack). Fixed generic.zig + pack.zig (set `is_naked` + asm-only `unreachable`
  cap); locked by `examples/1801-concurrency-naked-generic-bail.sx`. The review's `.naked`-
  lambda CRITICAL was a false positive (unparseable — `isLambda` breaks on `abi`). Suite
  green (723/0).
- **B1.0b** — real `naked` emission: emit_llvm declaration pass adds LLVM `naked`/`noinline`/
  `nounwind` + skips `frame-pointer` for `func.is_naked`; Pass 2 emits the body verbatim (no
  prologue). `1800` green aarch64-pinned (exit 42 + `.ir`); renamed `1801` → `-generic`
  (generic `.naked` emits a naked body, exit 42); added x86_64 sibling `1802` (ir-only, `.ir`
  locks `naked` + `movl $42, %eax`). Unit test asserts `naked` present + `frame-pointer`
  absent. Suite green (724/0).
- **B1.0c** — review-hardening: param-bearing `.naked` emitted invalid LLVM (loud verifier
  error). Gated the param-alloca loop on `fd.abi != .naked` (decl.zig both paths + generic.zig)
  — naked args stay in registers, read by the asm body (the B1.3 context-switch shape).
  Locked by `examples/1803-concurrency-naked-asm-param.sx`. Pack `.naked` left unsupported
  (loud, nonsensical). **B1.0 complete.** Suite green (725/0). **Next: B1.1 (per-fiber
  context, probe-first).**