sx/current/CHECKPOINT-FIBERS.md

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

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

Last completed step

B1.5 — END-TO-END M:1 validation — STREAM B1 COMPLETE. A single capstone exercises the whole colorblind pure-sx async runtime together: the M:1 scheduler (B1.5a) + suspending fiber-task async go/wait (B1.4a) + deterministic virtual-time sleep/now_ms (B1.4b), over the abi(.naked) swap_context on guarded mmap stacks (B1.0–B1.3). examples/concurrency/1817-concurrency-fiber-m1-end-to-end.sx: a coordinator fiber launches three go tasks (sleep 30/10/20 → return 100/20/3), awaits all three in SPAWN order, and sums them. The completion log is the deterministic contract — tasks finish in DEADLINE order (task 2@10, task 3@20, task 1@30), not spawn/await order; sum: 123; final virtual clock 30. Fully reproducible (virtual time, no real clock). Suite GREEN 755/0.

Stream B1 is feature-complete. The pure-sx async runtime exists end-to-end: fibers behind the abi(.naked) context switch (proven on aarch64 + x86_64/Win64), the M:1 cooperative scheduler, suspending go/wait/cancel async, deterministic virtual-time timers, and real fd-readiness via kqueue — all in library/modules/std/sched.sx, all adversarially reviewed, locked by 18xx (1800–1817). Compiler floor delivered: abi(.naked) emission (B1.0) + per-fiber context (B1.1, zero-change). Five compiler bugs fixed en route (0151/0152/0153 in B1.2; 0154 in B1.5a; 0156-Part1 + 0157 in B1.4a). Deferred (documented, non-blocking): issue 0150 (Future(void)/timeout), 0155 (scalar-pointer index), 0156-Part2 (deferred .. spread); a linux epoll twin of block_on_fd; routing the suspending async through the erased context.io (M:N evolution); the heap-Task / closure-env / kq-fd leaks (bounded, default-GPA-invisible). Stream B2 (channels/cancel/stdlib) is the next carve.

Earlier — B1.4c — REAL fd-readiness blocking via kqueue (macOS)

library/modules/std/sched.sx now lets a fiber park on a file descriptor and the run loop block on kevent until the kernel reports it ready. Reuses the existing verified library/modules/std/net/kqueue.sx bindings (Kevent (32 bytes), kqueue/kevent/kq_apply/kq_wait + the EVFILT_READ/EV_ADD/EV_ENABLE/EV_ONESHOT constants) rather than re-deriving the FFI — sched.sx imports it as kqb. Added to Scheduler:

• kq: i32 (LAZY — -1 in init, opened by the first block_on_fd, so a pure-compute / virtual-timer scheduler never opens a kqueue fd; leaks one fd at exit once opened, same class as the documented spawn-env / go-Task leaks — no deinit yet);
• io_waiters: List(IoWaiter) (IoWaiter :: struct { fd: i32; fiber: *Fiber; }, grown through own_allocator per the long-lived-container rule);
• block_on_fd(self, fd, want_read) — lazily opens kq, arms a one-shot EVFILT_READ registration, records an IoWaiter{fd, current}, then suspend_self(). Guards a null current (loud abort, like sleep); want_read=false (write-readiness) is not wired yet → loud abort rather than silently arming a read filter.
• Run-loop: after the ready queue drains, Mode 1 (virtual time) fires the earliest pending timer (takes precedence — a program uses sleep OR fds, documented non-unification limitation); Mode 2 (real fd) — if io_waiters is non-empty, BLOCK on kq_wait(kq, evbuf, MAXEV=16, -1) (null timeout), then for each fired event match ev.ident back to its waiter, evict it, and wake the fiber; else break. Orphan-deadlock check unchanged in spirit but now correct: an fd waiter is NOT an orphan (while io_waiters.len > 0 the loop blocks on kqueue rather than reaching the check), and a genuine no-timer/no-fd suspend still aborts loudly (verified with a probe: exit 134).
• wake now also evicts a pending fd-waiter (cancel_io_waiter_for, mirror of cancel_timer_for) — same UAF reasoning: a fiber woken by another path must not leave a stale IoWaiter pointing at a reaped *Fiber. The kqueue registration is EV_ONESHOT so we never EV_DELETE (a never-fired one-shot lingers harmlessly; the drain ignores an unmatched ident; closing the fd auto-removes it).
• DE-RISK probe (run first, no scheduler): confirmed size_of(Kevent) == 32, the pipe roundtrip (kq_wait returned 1 with out.ident == read_fd, out.filter == -1 (EVFILT_READ), out.data == 1 byte readable) — the struct layout reads back the fd correctly.
• Locked by examples/concurrency/1816-concurrency-fiber-io-pipe.sx: a pipe; a reader fiber spawned FIRST blocks on the empty read end, then a writer fiber writes a b c → the run loop blocks on kqueue, wakes the reader, which reads the 3 bytes. Output log: wrote read 3 [97 98 99] / n_suspended: 0 (the "wrote" before "read" ordering proves the reader actually blocked then woke via kqueue readiness). .build { "target": "macos" } (matches host arch → runs end-to-end; ir-only on a mismatch, like 1814/1815 — no .ir snapshot needed since it runs here). The example declares its own read/write/close externs with the CANONICAL signatures std already binds ((i32,[*]u8,usize)->isize / (i32)->i32) — a divergent re-binding is rejected by the extern dedupe.
• Adversarial review (worker) of the run-loop change — found 2 CRITICALs:
  • (1) two fibers on the SAME fd → lost wakeup + permanent hang. macOS EV_ADD for an existing (ident, filter) REPLACES the registration (doesn't stack), so two waiters share one registration: the fd fires once, one wakes, the other is stranded in io_waiters and the next kq_wait(-1) blocks forever. FIXED: block_on_fd now enforces one-waiter-per-fd with a loud abort (the model already assumed it). Verified: dup-fd → sched: block_on_fd: fd N already has a waiter, not a hang.
  • (2) an fd-waiter on a never-ready fd hangs instead of the timer path's loud abort. Re-examined: this is CORRECT event-loop semantics — blocking on I/O until ready (possibly forever, like a server idling on a socket) is the point; the scheduler cannot know an fd will never become ready, so it must keep waiting. NOT a scheduler deadlock. Fixed the MISLEADING comment that implied the orphan check covers fd-waiters: it does not, by design (it covers only pure suspend_self parks). No code change — the "hang" is a caller-side logic issue (waiting on input that never arrives), not a bug to abort on.
  • Review CLEARED: the IoWaiter UAF parity (early-wake evicts the waiter; a lingering one-shot that later fires hits no match → clean no-op), ident width/sign, kq_wait EINTR/error handling, timer-vs-io precedence (timer wins; no hang). All probed safe.
• Suite GREEN 754/0 (incl. the dup-fd guard, no new example needed — the abort is host-fragile to pin like 1809's guard-firing). Next: B1.5 (end-to-end M:1 validation under the deterministic timers / fd readiness); a linux epoll twin of block_on_fd (mirror via std/net/epoll, the OS-neutral facade is std.event) is future work.

Earlier — B1.4b — deterministic VIRTUAL-TIME timer scheduling (the KEYSTONE) — landed + adversarially

reviewed (caught a CRITICAL UAF, fixed). library/modules/std/sched.sx gained a virtual clock + sleep timers so fibers schedule in reproducible simulated time (no real clock): clock_ms (advances ONLY as timers fire), a timers: List(Timer) (insertion-order, linear min-scan, FIFO tiebreak), now_ms(), sleep(ms) (arm {clock_ms+ms, current} + suspend_self), and a timer-driven run (drain ready → fire earliest timer → advance clock → wake → repeat; orphan-deadlock check preserved for a genuine no-timer suspend). Locked by 1814 (5 fibers sleep 30/10/20/15/15 → wake order B@10, D@15, E@15 (FIFO), C@20, A@30 — deadline order, not spawn order; now_ms() reads each virtual deadline; final clock 30). §8.1.3 calibration note in the header: the deterministic wake ORDER equals what real sleeps produce, reproducing blocking semantics' observable ordering without real time. The deterministic-sim Io is realized at the scheduler level (sleep/now_ms/timer-run), not as an erased Io-protocol impl (same erasure reason as FiberIo).

• Adversarial review (worker) of the run-loop change: found a CRITICAL use-after-free — a fiber that armed a sleep timer but was woken EARLY by another path (a manual/Task wake) ran to completion + was reaped (stack munmap'd, Fiber freed) while its Timer still held a dangling *Fiber; a later fire would wake freed memory (silent-corruption: "passes" only because the freed slot coincidentally read state != .suspended). FIXED: wake now evicts the woken fiber's pending timer (cancel_timer_for) — every re-ready path funnels through wake (the timer-fire in run already removed the fired timer, so it's a harmless re-scan there), so no stale timer can outlive its fiber. Regression 1815-concurrency-fiber-timer-early-wake.sx (early wake → clock: 0, the stale 100ms timer evicted, not fired). Review CLEARED: n_suspended accounting, orphan-deadlock false-positives, timer-list integrity (re-arm during fire), clock monotonicity, termination — all traced/probed safe.
• Suite GREEN (count below). Next: B1.4c (event-loop Io — real fd readiness, kqueue/epoll).

Earlier — B1.4a — a truly-SUSPENDING fiber-task async layer (go/wait/cancel)

landed + adversarially reviewed; cleared two more compiler blockers en route. library/modules/std/sched.sx now carries Task($R) + Scheduler.go(work) -> *Task($R) + wait/cancel (a ufcs layer over the M:1 scheduler). s.go(work) runs the nullary thunk work as a REAL fiber; t.wait() SUSPENDS the caller until it completes (vs io.sx's blocking context.io.async, which runs inline). Locked by examples/concurrency/1813-concurrency-fiber-async-suspend.sx: two tasks interleave (A yields mid-body so B runs first → 1 2 3), awaited values 42/100, and a canceled task's wait raises .Canceled → or -99 → sequence: 1 2 3 42 100 -99.

• Design: a NULLARY thunk, not async(worker, ..args). A comptime variadic pack can't cross a deferred (fiber) boundary — ..args captured into a closure re-expands from the spawner's now-gone locals (issue 0156 Part 2). So go takes work: Closure() -> $R; the user captures inputs in the lambda at the call site (the go func(){…}() idiom). Self-contained in sched.sx (NOT io.sx): io.sx importing sched.sx duplicates the _fib_tramp global asm when a program also imports sched.sx directly (global asm emits per import-path) — so the Io-protocol spawn_raw/suspend_raw/ready hooks stay reserved for the future M:N model; M:1 uses go/wait directly. Heap *Task (must outlive go's frame; leak documented). TaskErr is LOCAL (the ! failable detection doesn't see through io.sx's IoErr re-export alias).
• Two compiler blockers hit + FIXED (user-authorized in-session):
  • issue 0156 Part 1 — a single-type generic $R (parsed as comptime_pack_ref) used as a type-arg (Box($R), size_of(Box($R))) inside a pack-fn body hit a missing arm in resolveTypeWithBindings → .unresolved → LLVM panic. Fix: mirror resolveTypeArg's comptime_pack_ref arm (look up type_bindings, else a loud diagnostic). Regression examples/generics/0216-generics-typearg-in-pack-fn-body.sx. (Part 2 — deferred .. spread crashes — reframed OPEN/non-blocking, issues/0156.)
  • issue 0157 — a user generic ufcs method whose name collides with a stdlib re-export (cancel on *Task vs io.sx's cancel on *Future) resolved via last-wins fn_ast_map with NO receiver filtering → wrong overload → $R unbound → LLVM panic. Fix (src/ir/lower/call.zig selectUfcsGenericByReceiver): every generic-ufcs dispatch enumerates ALL module authors (module_decls), keeps receiver-binding ones, picks the most receiver-SPECIFIC (concrete > bare $T), dedups re-exports, and flags a genuine 2-specific tie as a deterministic "ambiguous — qualify" diagnostic (never a silent order-dependent pick). Regression examples/generics/0217-generics-ufcs-method-name-collides-stdlib.sx.
• Adversarial review (worker) of the 0157 fix + Task layer. Caught the determinism CRITICAL (fixed: always-run selection + specificity + ambiguity), wait-outside-a-fiber null-deref (fixed: loud guard in suspend_self/yield_now), and cancel-doesn't-skip-work (fixed: worker skips work() if already canceled). Lost-wakeup / cancel-after-complete / reap traced safe. Also simplified 1812 (**Fiber shared handle → a Sh.parked field; output identical).
• Suite GREEN 751/0 (749 + 1813 + 0217). Next: B1.4b (deterministic-sim Io).

Earlier — B1.5a — the M:1 cooperative fiber scheduler CORE — landed + adversarially reviewed

The hand-bootstrapped ping-pong (1807-1810) is now a reusable scheduler API in pure sx: library/modules/std/sched.sx — a generic Fiber (body: Closure() -> void) + Scheduler with init/spawn/yield_now/suspend_self/wake/run over the proven swap_context on guarded mmap stacks. The ONE generic dispatch (fib_dispatch, reached from the _fib_tramp trampoline) runs ANY stored closure body on a fresh stack — replacing the fixed bl _fib_body. Reaping munmaps the stack + frees the heap Fiber on completion; an intrusive FIFO gives round-robin order.

• Foundational design de-risked by probe before building: a fiber can store + call a Closure() -> void on its fresh stack via the generic dispatch; outputs flow OUT through pointers captured in the closure (capture-by-value does NOT write back — pushed onto the user).
• Hit + FIXED a blocker compiler bug — issue 0154 (user-authorized in-session fix). null / --- assigned to a struct field picked up a leaked enclosing target_type (the function's RETURN type, set for the whole body at decl.zig:2691) and built a WHOLE-STRUCT-typed null → an oversized zeroinitializer store through the field's GEP that overran the field's slot and clobbered the saved x29/x30, so the fn ret'd to 0x0. This was EXACTLY the Scheduler.init() by-value-return shape (sched_ctx: [13]u64 before current: *Fiber). Fix: added .null_literal, .undef_literal to the needs_target switch in lowerAssignment (src/ir/lower/stmt.zig) so the field's type is used. Repro → regression test examples/types/0193-types-sret-array-before-pointer.sx; issues/0154-*.md RESOLVED.
• Adversarial review (worker): asm/bootstrap/lifetime SOUND (the headline closure-env-lifetime fear was disproven — envs are heap-promoted, survive the spawn frame). Found 1 CRITICAL + robustness gaps, ALL hardened: (CRITICAL) wake re-enqueued an already-queued fiber → FIFO corruption/segfault → now GUARDED on .suspended (spurious/double/stale wake = safe no-op); orphan-suspend leak/deadlock → n_suspended accounting + a loud run()-drain diagnostic+abort; mmap MAP_FAILED (=-1, not null) / mprotect / Fiber-OOM → loud bails (per §8.1.1 the guard is mandatory); the per-fiber closure-env leak (sx exposes no env-free) → documented as a KNOWN LIMITATION (bounded by spawn count; invisible under the default GPA).
• Locked two 18xx examples (aarch64-macos .build-pinned, ir-only on a mismatch): 1811-concurrency-fiber-scheduler.sx (3 fibers round-robin via yield_now → ordering contract sequence: 0 1 2 0 1 2 0 1 2, all .done) + 1812-concurrency-fiber-suspend-wake.sx (park via suspend_self, resumed by another fiber's wake, + the spurious-wake no-op — the CRITICAL-fix regression → log: 10 20 21 11 / suspended-left: 0).
• Filed issue 0155 (NON-blocking, NOT fixed) — found incidentally in the review: indexing a scalar pointer (pc[0], pc: *i64) panics codegen (.unresolved reaching LLVM emission). The scheduler uses array-field indexing + .*, never this, so it's filed for its own session.
• Suite GREEN 748/0 (746 base + 1811 + 1812 + 0193 regression). Next: B1.4a (FiberIo — wire Io.spawn_raw/suspend_raw/ready onto the scheduler so async/await truly suspend).

Earlier — B1.3b-1 — the x86_64 / Win64 swap_context sibling — VALIDATED on real hardware

The context switch is now proven on a SECOND architecture + ABI. A Win64 swap_context saves the COMPLETE Win64 callee-saved set — 8 GP (rbx, rbp, rdi, rsi, r12-r15) + rsp and xmm6-xmm15 (10 XMM, 128-bit via movups — Win64 has callee-saved XMM, unlike SysV/aarch64) — plus a Win64 scribble_verify (32-byte shadow + 16-align at each call, COFF symbols, rsp-carried return addr). Locked by examples/1810-concurrency-fiber-switch-win64.sx (pinned x86_64-windows-gnu, ir-only here): the 2-fiber mutual scribble printed 0 0 P when built --target x86_64-windows-gnu --self-contained and run on a Windows 7 x64 VM (UTM) — every GP + XMM callee-saved survived. Adversarially reviewed before the VM run (worker emitted the real .s and verified every call alignment, the 264-byte frame offsets, the rsp/return-addr round-trip, swap ordering, and COFF naming against the Win64 ABI — no critical/minor bugs). The build→VM→run loop was set up this session (cross-build needs --self-contained; output via the Win32 WriteFile boundary, the 1660 pattern). Suite green. Note: this is the GOOD-swap-only mutual scribble (self-validating by construction; the in-process negative control was dropped to avoid an sx fn-ptr-convention rabbit hole — the detection of this exact logic was negative-controlled on aarch64 in 1808). The SysV/Linux x86_64 sibling (different reg set: no callee-saved XMM, args rdi/rsi) remains for a Linux x86_64 host.

Earlier — B1.3b-2 — mmap guard-page stacks (commit dd532ab)

Fiber stacks are mmap'd with a PROT_NONE GUARD PAGE at the low end (§8.1.1: a fixed stack without a guard silently corrupts neighbors on overflow). mmap the [guard | usable] region, mprotect the low 16KB page PROT_NONE; SP descends into the guard and faults loudly at the boundary instead of corrupting a neighbor. Locked by examples/1809-concurrency-fiber-guard-stack.sx (aarch64-macos-pinned): guard armed: 1 (mprotect→0) + sum: 20100 (a fiber runs real recursion on the guarded stack + yields).

• Guard FIRING validated (manually, not corpus-pinned — a deliberate overflow crash is host-fragile): a fiber recursing past its 128KB stack faults with Bus error at the guard page (region+GUARD); the sx crash handler turns it into exit 134. Documented in the example header.
• x86_64 sibling: was deferred here (couldn't run x86_64 on this arm64 host), then DONE as Win64 once a Windows 7 x64 VM became available — see B1.3b-1 above (examples/1810, 0 0 P).

Earlier — B1.3a-2 — the context-switch STRESS GATE (design §10.7) — DONE + adversarially reviewed

The explicit every-callee-saved-register scribble that B1.3a-1 owed. swap_context now saves the COMPLETE AAPCS64 callee-saved set — integer x19-x28 + fp/lr + sp AND FP d8-d15 (per §6.1.2 only the low 64 bits of v8-v15 are callee-saved, so d8-d15 is exactly sufficient; x18 is Apple's reserved platform reg, untouched). A naked scribble_verify(self_ctx, peer, base) loads a unique sentinel into all 18 callee-saved regs, yields, and on resume counts the ones that didn't survive (honoring its own caller ABI via a 176-byte frame that saves+restores the caller's callee-saved; base reloaded from the frame post-swap; the original lr round-trips through the swap). The gate is a 2-fiber MUTUAL scribble (A and B scribble DISTINCT sentinels into the same physical regs, so each survives only if swap_context saved+restored it — a lone fiber yielding to an idle peer would NOT exercise preservation). Locked by examples/1808-concurrency-fiber-switch-stress.sx (aarch64-pinned): A mismatches: 0 / B mismatches: 0.

• Validity proven by NEGATIVE controls: dropping the d8-d15 save/restore → 8/8 mismatches (exactly the FP regs); dropping x27/x28 → 2/2. The gate genuinely catches a broken switch.
• Adversarial review (worker, per the plan): no CRITICAL bugs. Verified the callee-saved set is complete + correct, all frame offsets/16-alignment, the lr/sp dance, and swap read-ordering against AAPCS64. Applied its one recommendation: boot now zeroes the FP ctx slots [13..20] so a first switch-to loads 0 (not garbage) into d8-d15. Residual gaps it flagged (all spec-correct for a call-boundary swap, documented in the example header): NZCV/FPSR not swapped; FPCR (rounding mode — thread-global, bleeds across fibers if changed) and TPIDR_EL0/TLS (errno, allocator thread-caches — shared by same-thread fibers) not swapped; fp=0 bootstrap blocks unwind/signal walking past a fiber trampoline. These bite at the N×M:1 / signals stages, not the single-thread switch.
• Suite green 734/0, master clean. WIP probes: .sx-tmp/scribble2.sx (+ _broken/_gp).

Earlier — B1.3a-1 — the foundational stackful context switch (commit b234b7d)

Pure sx over abi(.naked): naked swap_context (GP-only 13-slot save) + by-hand fiber bootstrap (SP = alloc_bytes stack top, LR = global-asm trampoline, x19 = *Fiber). Locked by examples/1807-concurrency-fiber-context-switch.sx: 2-fiber ping-pong (rounds: 6 / canary fails: 0) + 64-frame deep recursion (frames verified: 64 / depth fails: 0). Indirect register/stack survival; 1808 supersedes its switch with the complete GP+FP save area + the explicit gate.

Earlier — B1.2 COMPLETE — the async surface works end-to-end

All three surface blockers (0151, 0152, 0153) FIXED + committed; async examples landed + green.

• 0151 fixed (362674f): generic $T infers through generic-struct / pointer / UFCS-pack params. Regression 0214 + 0215.
• 0152 fixed (e5586f6): Atomic(bool) load/store byte-promoted to i8 in the codegen emitters. Regression 1705.
• 0153 fixed (68c1991): inferGenericReturnType now pins return-type resolution to the fn's DEFINING module (mirroring monomorphizeFunction), so a re-exported value-failable's !E resolves to the real .error_set TypeId — the failable channel survives the re-export alias. Regression 1058-errors-reexport-value-failable-channel.sx.
• Async examples landed: examples/1805-concurrency-io-blocking-async.sx (context.io.async((a,b)->i64 => a+b, 40, 2).await() or {…} → sum: 42 / double: 42 / clock ok) + examples/1806-concurrency-io-cancel.sx (f.cancel() → await raises .Canceled → or default; ok: 7 / canceled: -99). Both green, snapshots captured.

Earlier — the three B1.2 surface fixes (committed)

Generic $T inference, Atomic(bool) byte-promotion, and re-export failable-channel pin — details below.

• 0151 fix (committed): four gaps closed on the inference + UFCS-dispatch path — (1) extractTypeParam/matchTypeParam(Static) got a parameterized_type_expr arm (recover the arg instance's recorded per-param bindings via struct_instance_bindings + the template's ordered type_params, recurse positionally; this also fixes *Box($T) — it recurses into its Box($T) pointee); (2) the pointer_type_expr arm now falls through to match the pointee against a non-pointer arg (auto-address-of: a *Box($T) param accepts a by-value Box($T), e.g. a UFCS receiver b.m()); (3) ExprTyper.inferType got a .lambda arm building the closure type from the lambda's annotations (the UFCS binder types args from the raw AST before they're lowered, so it can now bind Closure(..) -> $R from the worker's declared return type); (4) a pack UFCS target routes through the SAME lowerPackFnCall the direct call uses, with the receiver spliced in as args[0].
• Regression tests: examples/0214-generics-ufcs-closure-return-pack.sx (direct + UFCS closure-return pack) + examples/0215-generics-infer-through-pointer.sx (by-value / pointer / multi-param / nested / UFCS-auto-ref struct-head inference). Issue 0151 marked RESOLVED; repro moved into the suite.

Earlier — B1.2 (Io capability) — LANDED + adversarially reviewed

Commits a1b14f0 (lock) + 45d869d (Io capability) + 3eeb965 (issue 0151 lock).

• LANDED + review-confirmed correct (commit 45d869d): Io :: protocol #inline (spawn_raw/suspend_raw/ready/poll/now_ms/arm_timer) + io field on Context ({allocator; data; io}, io LAST); BOTH __sx_default_context materializers (protocol.zig + comptime_vm.zig) build an identical CBlockingIo→Io vtable (review verified byte-for-byte agreement; context.io.now_ms() dispatches at runtime AND comptime); the push Context.{…} omitted-field-inherits-ambient fix (review: correct, right fix, no bad blast radius); library/modules/std/io.sx (Future($R), CBlockingIo, async/await/cancel); the !-protocol-impl-lint suppression; 37 .ir regens (review: pure layout/type-table, no error text, zero .exit/.stdout/.stderr change).
• BLOCKED — async surface non-functional: await/cancel take *Future($R) and are uncallable in EVERY form (not just UFCS) — sx can't infer a generic $T from a pointer-wrapped arg (*Future($R)). async(...) (create) works via explicit call and produces a correct .ready Future, but you can't await it. Root bug = issue 0151 (WIDENED): infer $T from *T-wrapped params + closure-return-via-pack + UFCS dispatch. Minimal repro: unbox :: (b: *Box($T)) -> $T fails to infer T.
• No async example in the corpus (1805 was removed because it needs the blocked surface) → the green suite does NOT cover async. Restore 1805 (async/await) + add 1806 (cancel) once 0151 is fixed.

Earlier — B1.1 (per-fiber context root) — DONE. Zero compiler change (confirmed by probe).

The fiber-spawn context convention works end-to-end with ordinary language features:

• snap := context captures the spawner's Context as a value;
• the snapshot is stored in a struct (the stand-in Fiber);
• a trampoline running under a different ambient context installs the fiber's stored root with push f.root { … }, and the body reads the snapshot — not the trampoline's ambient context — because context is an implicit slot-0 *Context param (call-carried, rides the callee's own stack) and push allocates on the caller frame (no global, no TLS).
• Locked by examples/1804-concurrency-context-snapshot.sx: prints fiber root: 42 (the installed snapshot wins over ambient 99) + ambient after: 99 (the push scope restores the ambient context on exit). No fiber runtime yet (that's B1.3) — this proves the plumbing it will build on. No .build pin (pure sx, host-independent).
• Probe result: the design doc's "lower as swappable indirection, never raw TLS" guarded a non-problem — context was already param-carried, never TLS. No path re-reads __sx_default_context mid-stack, so there is no compiler obligation here.
• zig build && zig build test green: 726 ran, 0 failed.

Earlier — B1.0 (abi(.naked) codegen) — 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 B1 FEATURE-COMPLETE. library/modules/std/sched.sx is the whole pure-sx M:1 async runtime: the scheduler core (B1.5a: spawn/yield_now/suspend_self/wake/run), suspending fiber-task async (B1.4a: Task($R)/go/wait/cancel), deterministic virtual-time timers (B1.4b: clock_ms/now_ms/sleep, timer-driven run), AND real fd readiness via kqueue (B1.4c: lazy kq, io_waiters, block_on_fd, run-loop Mode 2) — all over the abi(.naked) swap_context on guarded mmap stacks (B1.0–B1.3), reusing std/net/kqueue.sx. Every park path (timer sleep, fd block, raw suspend) is balanced through wake (which evicts stale timer + fd waiters — the UAF guards). Locked by 18xx 1800–1817 (naked-asm, context-snapshot, blocking async, the switch + §10.7 stress gate + guarded stacks + Win64 sibling, scheduler round-robin, suspend/wake, async go/wait/cancel, sim-timer ordering, timer early-wake eviction, kqueue pipe I/O, and the 1817 end-to-end capstone). Suite GREEN 755/0, master committed.

Future work (none blocking B1): a linux epoll twin of block_on_fd (mirror via std/net/epoll; OS-neutral facade std.event) — B1.4c wired macOS kqueue only; routing the suspending async through the erased context.io (forces sched.sx into every std consumer + duplicates the _fib_tramp global asm — deferred to the M:N model, where the Io protocol's spawn_raw/suspend_raw/ready/ arm_timer/poll hooks take over); Future(void)/timeout (issue 0150); freeing the heap-Task / closure-env / kq-fd (a Scheduler deinit + closure-env-ownership affordance). Next carve: Stream B2 (channels / structured cancel / async stdlib) — see PLAN-CHANNELS.md when started.

Earlier — B1.5a COMPLETE — the M:1 scheduler CORE exists

library/modules/std/sched.sx drives N fibers (generic Closure() -> void bodies) cooperatively over the proven swap_context, on guarded mmap stacks: spawn / yield_now (round-robin) / suspend_self + wake (off-queue park/resume) / run (drives to drain, reaps on .done). Adversarially reviewed + hardened (wake guarded, loud mmap/mprotect/OOM/deadlock bails, env-leak documented). Locked by 1811 (round-robin ordering contract) + 1812 (suspend/wake park-resume + spurious-wake guard). Suite GREEN 748/0.

The remaining B1.4 work wires this scheduler under the Io capability: B1.4a (FiberIo) makes context.io route spawn_raw/suspend_raw/ready onto the Scheduler so async/await truly SUSPEND (today's CBlockingIo runs the worker to completion inline); B1.4b the deterministic-sim Io (virtual clock + timer queue, calibrated against blocking — the KEYSTONE test harness); B1.4c the event-loop Io (kqueue/epoll). Then B1.5 is the end-to-end M:1 validation under the deterministic Io.

Earlier — B1.2 COMPLETE

The full async surface (Io capability on Context + async/await/cancel + blocking CBlockingIo) works end-to-end. Master GREEN (732/0), installed sx clean. All four B1.2 surface bugs resolved or deferred:

• 0151 fixed (362674f): generic $T through generic-struct / pointer / UFCS-pack params. Regression 0214 + 0215.
• 0152 fixed (e5586f6): Atomic(bool) byte-promoted to i8 in the load/store emitters. Regression 1705.
• 0153 fixed (68c1991): inferGenericReturnType pins return-type resolution to the fn's defining module, so a re-exported value-failable keeps its ! channel. Regression 1058.
• Issue 0150 (void struct field → SIGTRAP) DEFERRED — only Future(void) / timeout, which are B1.4.

The async examples are landed + green: 1805 (async/await + now_ms → sum: 42 / double: 42 / clock ok) + 1806 (cancel → await raises .Canceled → or default). The 18xx concurrency category now covers naked-asm (1800-1803), context-snapshot (1804), and the async surface (1805-1806).

B1.2 Io capability — what is LANDED + verified (commit 45d869d)

• Io :: protocol #inline { spawn_raw; suspend_raw -> !; ready; poll; now_ms; arm_timer; } in core.sx next to Allocator, with SpawnOpts{ pin: PinTarget } + ParkToken{ handle }. Six methods, each justified by a downstream consumer (B1.3-B1.5).
• Context :: struct { allocator; data; io: Io; } — io appended LAST so allocator stays index 0 (the call.zig:1229 hardcode) and data keeps index 1 (minimal VM-fallback churn).
• Both __sx_default_context materializers updated in lockstep + verified: protocol.zig emitDefaultContextGlobal (extended ctx_fields 2→3, built the CBlockingIo→Io inline 7-word vtable {null-ctx, fn0..fn5} via getOrCreateThunks("Io","CBlockingIo")) and comptime_vm.zig materializeDefaultContext fallback (wrote the 6 thunk func-refs at io_base = addr + 4*ps, offset + (i+1)*ps). The global path auto-followed the 3-field Context type. context.io.now_ms() printed clock ok live — the capability threads + the vtable dispatches correctly.
• Stateless CBlockingIo :: struct {} + impl Io for CBlockingIo (mirror of CAllocator): blocking semantics — spawn_raw/ready/poll/arm_timer no-op/0, now_ms → time.mono_ms().
• push-inherit-omitted fix (stmt.zig lowerPush): a push Context.{...} now SEEDS the new slot from the ambient context (load+store), then overwrites ONLY the literal's named fields — so omitted fields (now incl. io) are INHERITED, never zero-inited to a null vtable. Eliminates the omitted-field footgun globally (zero per-site churn across the 17 partial-literal sites). This is the correct capability-bag semantics; it compiled clean.
• !-protocol-method warning fix (error_analysis.zig + a new Lowering.impl_method_names set populated in protocols.zig registerImplBlock): a protocol impl method may be declared ! by contract (e.g. Io.suspend_raw) yet never raise; the "declared ! but never errors — drop the !" hint is a false positive for impl methods, now suppressed for them.

Status of the blockers that originally stopped B1.2:

• issue 0151 — FIXED this session (generic $T through generic-struct / pointer / UFCS-pack params). async/await/cancel are callable. See "Last completed step".
• issue 0152 — NEW, the current blocker (Atomic(bool) → sub-byte i1 atomic; LLVM reject). Blocks the async examples via Future.canceled: Atomic(bool). Filed; codegen-level fix.
• issue 0150 — void struct field SIGTRAP; only Future(void)/timeout (B1.4). DEFERRED.

Per the IMPASSABLE STOP rule: 0151 fix shipped (suite green 728/0), 0152 filed, STOPPED. Resume B1.2's async examples once 0152 lands.

Earlier — B1.0 + B1.1 complete

Stream A (atomics) is feature-complete (✅). Stream B1: B1.0 + B1.1 complete. The two compiler-floor preconditions for the fiber runtime are in place: (1) abi(.naked) emits a real LLVM naked function end-to-end (decl, generic, pack paths) — the context-switch substrate; (2) per-fiber context root needs no compiler change — the spawn convention (snapshot context, store, push it from the trampoline) is pure library sx. No fibers/Io/scheduler code yet. Grounded floor facts:

• context is an implicit slot-0 *Context param + push Context is a stack alloca ⇒ fiber-local for free (confirmed by the B1.1 probe — never TLS, never re-read from the __sx_default_context global mid-stack). A spawn passes the snapshot as the fiber-entry fn's slot-0 ctx via push f.root { entry(args) }. Locked by 1804-...-context-snapshot.
• 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

Stream B1 is COMPLETE — no next step in this stream. The pure-sx M:1 async runtime is feature- complete and committed (1800–1817 green, 755/0). Pick up Stream B2 (channels / structured cancel / async stdlib) as a fresh carve (PLAN-CHANNELS.md), OR one of the documented non-blocking follow-ups: the linux epoll twin of block_on_fd, a Scheduler.deinit (free the kq fd / heap Tasks / drain leaks), Future(void)/timeout (needs issue 0150), or routing the suspending async through the erased context.io for the M:N model. None of these block B1.

Deferred (future B1.4c sibling): the linux epoll twin of block_on_fd. B1.4c wired the macOS kqueue path only (the host is aarch64-macOS). The linux mirror would register interest via std/net/epoll and the OS-neutral facade is std.event — keep the two as separate run modes inside run, branching on the platform, exactly as the timer-vs-fd modes are kept separate now. Documented non-unification: virtual-time timers and real kqueue timeouts are NOT merged — run fires a pending timer before ever blocking on kqueue (a program uses sleep OR fds); a true "fd-or-real-timeout" wants a kqueue EVFILT_TIMER, future work.

Design note carried forward: an event-loop Io needs a current-Scheduler handle. sched.* methods thread it via self/the Task; if B1.4c wants the capability-threaded context.io form it'll need an ambient current-scheduler accessor in sched.sx (still deferred — the sched.*-method form suffices). The Io protocol's poll/arm_timer map onto this when/if that wiring is built.

Side thread (optional, low priority): the SysV/Linux x86_64 sibling. A THIRD switch variant for x86_64-linux: SysV callee-saved = rbx, rbp, r12-r15 + rsp (6 GP + sp; no callee-saved XMM, unlike Win64) — a 7-slot ctx, args rdi/rsi/rdx, the rsp-carried return addr. Needs a Linux x86_64 host (or a working cross-run) to RUN + the mutual-scribble gate. Not blocking — the switch is already validated on two arch/ABI pairs.

Deferred (do NOT block on these): issue 0150 (void struct field SIGTRAP) — only Future(void)/timeout (B1.4). The :: callable-parameter feature (named-fn async workers async(read_a, conn)) — WIP at .sx-tmp/wip-callable-params/patch.diff (parser done, inference incomplete); a dedicated effort; lambda workers are the idiom meanwhile.

Context layout settled: { allocator; data; io; } (allocator index 0 fixed by call.zig:1229, io last). Io protocol + materializers + push-inherit are LANDED + reviewed.

Known issues / capability gaps

• issue 0157 (OPEN, BLOCKING B1.4a) — a user-defined generic ufcs method whose NAME collides with a stdlib re-export (cancel, re-exported by std.sx from io.sx as ufcs (f: *Future($R))), called via UFCS on a different generic struct (*Task($R)), leaves $R unresolved → .unresolved reaches LLVM emission → panic (src/backend/llvm/types.zig:196). Renaming → works; the non-UFCS call form already diagnoses cannot infer generic type parameter 'R', so the UFCS path skips that diagnostic. Surfaced by cancel :: ufcs (t: *Task($R)) in std/sched.sx. Minimal repro (no fibers/closures): issues/0157-ufcs-generic-method-name-collides-stdlib-unresolved.{md,sx}.
• ✅ issue 0154 — FIXED (null/--- to a struct field over-stored a whole-struct null when the function's return type leaked as target_type, corrupting the frame → ret to 0x0; surfaced building Scheduler.init()'s by-value return). Fix: .null_literal/.undef_literal added to needs_target in lowerAssignment (src/ir/lower/stmt.zig). Regression: examples/types/0193.
• issue 0155 (OPEN, NON-blocking) — indexing a scalar pointer (pc[0], pc: *i64) panics codegen (.unresolved reaching LLVM emission, src/backend/llvm/types.zig:196). Found in the B1.5a review; the scheduler doesn't use it (array-field index + .* only). Filed for its own session: issues/0155-scalar-pointer-index-llvm-panic.{md,sx}.
• ✅ issue 0157 — FIXED (B1.4a) — a user generic ufcs method whose name collides with a stdlib re-export resolved via last-wins fn_ast_map with no receiver filtering → wrong overload → $R unbound → LLVM panic. Fix: selectUfcsGenericByReceiver (src/ir/lower/call.zig) — most receiver-specific binding author across ALL module authors, deterministic, ambiguity-diagnosing. Regression: examples/generics/0217.
• ✅ issue 0156 Part 1 — FIXED (B1.4a) — single-type generic $R as a type-arg in a pack-fn body (Box($R)/size_of(Box($R))) → .unresolved → panic. Fix: comptime_pack_ref arm in resolveTypeWithBindings. Regression: examples/generics/0216.
  • Part 2 (OPEN, NON-blocking) — a deferred .. spread (a comptime pack captured into a closure, or a tuple ..t spread) crashes instead of working/diagnosing. The fiber async layer avoids it by design (nullary thunks), so it's filed for its own session: issues/0156.
• Heap leaks in the fiber runtime (documented limitations, NOT bugs): spawn's closure env + go's heap Task are never freed (sx exposes no closure-env free; Task ownership is deferred). Bounded by spawn/go count, invisible under the default GPA. Revisit for a long-running arena-backed scheduler.
• ✅ issue 0153 — FIXED (re-exported generic value-failable ($R, !E) kept its ! channel: inferGenericReturnType now pins return-type resolution to the fn's defining module). Regression: examples/1058. Was the LAST B1.2 surface blocker.
• ✅ issue 0152 — FIXED (Atomic(bool) sub-byte i1 atomic → byte-promoted to i8 in the load/store emitters). Regression: examples/1705. Unblocked Future.canceled.
• ✅ issue 0151 — FIXED (generic $T through generic-struct / pointer / UFCS-pack params). Regression: examples/0214 + 0215. Was the original B1.2 surface blocker.
• issue 0150 (deferred) — a void struct field crashes the compiler (unsized-type SIGTRAP in LLVM getTypeSizeInBits). Blocks Future(void) → timeout (B1.4). Repro: issues/0150-....
  • (Note: issue 0149, filed by another session against an earlier dirty binary, was a manifestation of the pre-fix 0151 — now moot.)
• 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 — per-fiber context is library-only (CONFIRMED by probe): push frames are stack-alloca'd and the implicit ctx rides slot 0, so the spawn convention — snapshot context, store it, push f.root { entry(args) } from the trampoline — installs the fiber's root with no compiler change. Verified: the body reads the snapshot over a different ambient context, and push restores ambient on exit (1804-...-context-snapshot). The design doc's "never raw TLS" guarded a non-problem (context was never TLS).
• 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

• B1.4c — real fd-readiness blocking via kqueue (macOS). De-risked first with a no-scheduler probe (confirmed size_of(Kevent)==32 and the pipe→kevent roundtrip: kq_wait returned 1, out.ident == read_fd, out.filter == -1, out.data == 1 — the struct layout reads the fd back correctly). Then added to library/modules/std/sched.sx (importing the existing verified std/net/kqueue.sx as kqb rather than re-deriving the FFI): a lazy kq: i32 (-1 until first use), io_waiters: List(IoWaiter), block_on_fd(fd, want_read) (arm one-shot EVFILT_READ, record waiter, suspend_self), a run-loop Mode 2 (block on kq_wait(kq, evbuf, MAXEV=16, -1) when only fd waiters remain, wake the fiber whose fd fired), and wake now also evicts a stale fd-waiter (cancel_io_waiter_for, the same UAF guard as cancel_timer_for). Timers keep precedence over fds (documented non-unification). Orphan-deadlock check still fires for a genuine no-timer/no-fd suspend (probed: exit 134). Locked by 1816-concurrency-fiber-io-pipe.sx (reader blocks on empty pipe → writer writes a b c → kqueue wakes reader → reads 3 bytes; log: wrote read 3 [97 98 99], n_suspended: 0), .build { "target": "macos" }, runs end-to-end on host. The example's read/write/close externs use the canonical signatures std already binds (extern-dedupe rejects a divergent re-binding). Suite GREEN 754/0. Next: B1.5 (end-to-end M:1 validation); linux epoll twin deferred.
• 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).
• rename — ABI variant .pure → .naked (keyword, Function.is_naked, diagnostics, examples 1800-1803 *-pure-* → *-naked-*, docs). "pure" universally means side-effect-free — wrong for a register-clobbering switch; "naked" matches LLVM/Zig/Rust/GCC/Clang. Pure cosmetics, no semantic change. Suite green (725/0).
• B1.1 — per-fiber context root: zero compiler change (probe-confirmed). The spawn convention (snapshot context → store in a struct → push f.root { entry() } from the trampoline) installs the fiber's root via the implicit slot-0 *Context param; the body reads the snapshot, not the trampoline's ambient ctx, and the push scope restores ambient on exit. Locked by examples/1804-concurrency-context-snapshot.sx (prints fiber root: 42 / ambient after: 99). Suite green (726/0). Next: B1.2 (Io interface + context.io).
• B1.2 (BLOCKED) — built the full Io capability (protocol on Context, stateless CBlockingIo blocking default, both __sx_default_context materializers, push-inherit-omitted fix, !-impl-method warning fix) and VERIFIED the core works live (context.io.now_ms() → clock ok). Two independent compiler bugs blocked the async/await/timeout layer: 0150 (void struct field → unsized SIGTRAP, blocks Future(void)) and 0151 (type-var from a fn-ptr param's return type not bound in the body, blocks async's Future(R)). Both filed with standalone repros + investigation prompts. Per the STOP rule: reverted ALL B1.2 working changes (master green again, 726/0; the dirty binary had broken the photo project — see the now-moot 0149), saved WIP to .sx-tmp/b12-wip/, STOPPED. Resume after 0150 + 0151.
• 0151 FIXED — generic inference now binds $T through a generic-struct param head, a pointer (*Box($T), incl. UFCS auto-ref), and a closure-return-via-pack on the UFCS path. Four gaps closed: parameterized_type_expr arm in extractTypeParam/matchTypeParam(Static) (recovers the arg instance's recorded per-param bindings, recurses positionally); pointer arm falls through to match a value arg (auto-address-of); ExprTyper.inferType .lambda arm (closure type from annotations — UFCS types args from raw AST pre-lowering); pack UFCS target routes through lowerPackFnCall with the receiver spliced in as args[0]. Issue 0151 marked RESOLVED; repro → examples/0214-generics-ufcs-closure-return-pack.sx; widened cases → examples/0215-generics-infer-through-pointer.sx. Suite green 728/0. The now-callable async surface immediately exposed a SEPARATE codegen bug — issue 0152 (Atomic(bool) → sub-byte i1 atomic, LLVM reject; Future.canceled hits it). Filed with standalone repro + fix prompt. Per the STOP rule: shipped the 0151 fix, filed 0152, STOPPED. Resume the async examples (1805/1806) after 0152.
• 0152 FIXED — the atomic load/store emitters (src/backend/llvm/ops.zig) byte-promote a sub-byte (bool→i1) access to its i8 storage type and trunc/zext the value at the boundary (new atomicByteType helper). rmw/cmpxchg left as-is (a bool rmw/CAS is rejected at the sx level — integer-only — so a sub-byte element never reaches them; comments record this). Regression examples/1705-atomics-bool-byte-promoted.sx (load/store round-trip). Issue 0152 marked RESOLVED. Suite green 729/0. With Atomic(bool) working, the async surface exposed the TRUE remaining blocker — issue 0153: a re-exported generic value-failable ($R, !E) loses its ! channel at the call site (the earlier "secondary or PHI" symptom was this, NOT an Atomic cascade — confirmed it persists after 0152). Narrowed to the generic+re-export co-requirement (non-generic re-export OK; direct generic import OK; only the combination drops !). Root cause: the monomorphized return-type's error-set, reached via the re-export alias, resolves to a non-.error_set TypeId, so errorChannelOf (lower/error.zig:148) misses the channel. Filed issues/0153-... with a minimal co-located 2-file repro + a single-file stdlib-await repro + investigation prompt. Per the STOP rule: shipped the 0152 fix, filed 0153, STOPPED. Resume the async examples after 0153.
• 0153 FIXED → B1.2 COMPLETE — inferGenericReturnType (src/ir/generics.zig) resolved the return-type AST in the CALL-SITE module, so a re-exported error set (LE :: lib.LE) resolved to a non-.error_set alias and the planned call-result was a plain tuple (channel lost). Fix: pin the source to fd.body.source_file around the return-type resolution, exactly as monomorphizeFunction does — the !E now resolves to the real .error_set. One-function change; full suite green (732/0), no regression. Issue 0153 RESOLVED; repro → examples/1058-errors-reexport-value-failable-channel.sx (+ companion lib.sx). With the channel preserved, landed the async examples: 1805 (async/await + now_ms → sum: 42 / double: 42 / clock ok) + 1806 (cancel → await raises .Canceled → or default; ok: 7 / canceled: -99). B1.2 (Io capability + M:1 async surface) is COMPLETE. Next: B1.3 (fiber runtime) on the .naked context-switch substrate.
• B1.3a-1 — context switch works. Implemented the stackful switch in pure sx over abi(.naked): swap_context(from, to) (save callee-saved x19-x28 + fp/lr + sp into *from, load from *to, ret onto to's stack) + by-hand fiber bootstrap (SP = top of an alloc_bytes stack, LR = a .global _fib_tramp global-asm trampoline that does mov x0, x19; bl _fib_body, x19 = *Fiber). Proven via a probe (main↔fiber), then locked by examples/1807-concurrency-fiber-context-switch.sx (aarch64-pinned): a 2-fiber ping-pong (rounds: 6, canary fails: 0 — a per-fiber stack canary survives every switch) + a 64-frame deep recursive chain suspended at the bottom and resumed (frames verified: 64 / depth fails: 0). The bl _fib_body reaches the sx body via export "fib_body" (the 1655 asm→sx pattern); runs under JIT, ir-only on a non-arm host (.ir captured — swap_context shows naked noinline nounwind). Suite green 733/0. Honest scope: indirect register/stack survival only; the EXPLICIT every-callee-saved + FP scribble (§10.7) is B1.3a-2, still owed. Next: B1.3a-2.
• B1.3a-2 — the §10.7 stress gate, adversarially reviewed. Extended swap_context to the COMPLETE AAPCS64 callee-saved set (added FP d8-d15 → 21-slot ctx) and wrote a naked scribble_verify that loads a unique sentinel into all 18 callee-saved regs, yields, and counts non-survivors on resume (176-byte frame saves/restores the caller's callee-saved + base; lr round-trips the swap). The gate is a 2-fiber MUTUAL scribble (each clobbers the other's regs, so survival ⇒ the switch saved+restored them). Locked by examples/1808-concurrency-fiber-switch-stress.sx (A/B mismatches: 0). Validity proven by negative controls (drop d8-d15 → 8/8; drop x27/x28 → 2/2). Spawned an adversarial-review worker (per the plan + user request): NO critical bugs — callee-saved set complete (x18 rightly excluded; d8-d15 suffices per §6.1.2), offsets/alignment/lr-sp dance all verified. Applied its one rec: boot zeroes FP ctx slots so first-entry loads 0, not garbage. Honest residual gaps (spec-correct for a call-boundary swap; in the example header): FPCR/FPSR/NZCV + TPIDR/TLS not swapped, fp=0 blocks unwind — relevant at N×M:1 / signals, not here. Suite green 734/0. Next: B1.3b (x86_64 sibling + mmap guard-page stacks).
• B1.3b — mmap guard-page stacks (x86_64 sibling deferred). Fiber stacks now mmap a [guard | usable] region and mprotect the low 16KB page PROT_NONE, so a stack overflow faults at the guard boundary instead of silently corrupting a neighbor (§8.1.1). Locked by examples/1809-concurrency-fiber-guard-stack.sx (aarch64-macos-pinned): guard armed: 1 (mprotect→0) + sum: 20100 (a fiber runs real recursion on the guarded stack + yields). Guard FIRING validated manually (overflow → Bus error at region+GUARD, exit 134 via the sx crash handler) — not corpus-pinned because a deliberate-overflow crash is host-fragile (and a mere "child faulted" fork test wouldn't prove the BOUNDARY catch). The x86_64 swap_context sibling was DEFERRED: --target x86_64-macos mislinks on this arm64 host and x86_64-linux can't run here, so it could only ship un-run/un-negative-controlled — which §10.7 forbids for the highest-risk asm. SysV target notes (rbx/rbp/r12-r15/rsp, no callee-saved XMM, rsp-carried return addr) recorded in Next step. Suite green 735/0. Next: x86_64 sibling (needs an x86_64 host) OR B1.4 (Io impls / scheduler) on the proven aarch64 substrate.
• B1.3b-1 — x86_64 / Win64 switch sibling VALIDATED on real hardware. The user provided a Windows 7 x64 VM (UTM), so the x86_64 switch became RUNNABLE (as Win64). Validated the cross-build→VM→run loop (--target x86_64-windows-gnu --self-contained → PE32+; output via the Win32 WriteFile boundary, the 1660 pattern). Wrote a Win64 swap_context (8 GP rbx/rbp/rdi/ rsi/r12-r15 + rsp + xmm6-xmm15 via movups — Win64 has callee-saved XMM) + a Win64 scribble_verify (264-byte frame, 32-byte shadow + 16-align at each call, COFF symbols, rsp-carried return addr) driving the 2-fiber mutual scribble. Adversarially reviewed (worker emitted the real .s, verified every alignment/offset/round-trip against the Win64 ABI — no critical/minor bugs), THEN run on the VM → 0 0 P (all 8 GP + 10 XMM callee-saved survived). Locked by examples/1810-concurrency-fiber-switch-win64.sx (pinned x86_64-windows-gnu, ir-only on this host; the VM run is the runtime-correctness provenance). Good-swap-only (the in-process negative control was dropped to avoid an sx fn-ptr-convention rabbit hole; the detection of this exact logic was negative-controlled on aarch64 in 1808). Suite green 736/0. The B1.3 context switch is now proven on TWO arch/ABI pairs. Next: B1.4 (Io impls / M:1 scheduler) on the proven substrate. (Side thread: the SysV/Linux x86_64 sibling, when a Linux x86_64 host is available.)
• B1.5a — M:1 scheduler CORE + a fixed blocker bug. Built library/modules/std/sched.sx: a generic Fiber/Scheduler over swap_context on guarded mmap stacks. spawn heap-allocs a fiber, bootstraps its ctx, enqueues it; the ONE generic dispatch (fib_dispatch via _fib_tramp) runs ANY stored Closure() -> void on a fresh stack (replacing the fixed bl _fib_body); yield_now round-robins, suspend_self/wake park/resume off-queue, run drives to drain + reaps .done fibers (munmap + free). De-risked first by probe (closure-on-fiber + output via captured pointer). Hit blocker bug 0154 (user-authorized fix): null/--- to a struct field over-stored a whole-struct null when the fn return type leaked as target_type, corrupting the frame (ret 0x0) — exactly the Scheduler.init() by-value-return shape. Fixed in stmt.zig (needs_target += null/undef literals); regression examples/types/0193; 0154 RESOLVED. Adversarial review: asm/bootstrap/lifetime sound (env-lifetime fear disproven — heap-promoted); 1 CRITICAL (wake re-enqueue → FIFO segfault) + robustness gaps ALL hardened (wake guarded on .suspended, n_suspended deadlock diagnostic+abort, loud mmap/mprotect/OOM bails, env-leak documented). Locked 1811 (round-robin 0 1 2 ×3) + 1812 (suspend/wake + spurious-wake guard, log: 10 20 21 11). Filed NON-blocking 0155 (scalar-pointer index panics codegen — review incidental, unused by sched). Suite GREEN 748/0. Next: B1.4a (FiberIo).
• B1.4a (truly-suspending fiber-task async, nullary-thunk design) — BLOCKED on issue 0157. Implemented the async layer SELF-CONTAINED in library/modules/std/sched.sx (kept its lone #import "modules/std.sx" to avoid the duplicate-_fib_tramp trap): TaskState, a LOCAL TaskErr :: error { Canceled } (the re-exported IoErr alias is NOT seen through by the raise/failable-type check — verified), Task($R), and go/wait/cancel ufcs. Design is the validated nullary-thunk (.sx-tmp/pnullary.sx → log: 1 2 3 42 100): work is a Closure() -> $R, user captures inputs at the call site, NO ..args crosses the fiber boundary (deliberately sidesteps 0156). go+wait run correctly; both wake-orderings traced. Wrote the example examples/concurrency/1813-concurrency-fiber-async-suspend.sx (+ { "target": "macos" } .build) but its cancel ufcs surfaced a NEW compiler bug — issue 0157: a user generic ufcs whose name collides with a stdlib re-export (cancel from io.sx) is mis-resolved on UFCS call over a different generic struct, leaving $R unresolved → LLVM panic. Bisected to a minimal no-fiber repro (name is the sole trigger; non-UFCS form diagnoses correctly). Example NOT seeded into the corpus (no .exit marker) — do NOT regen its goldens until 0157 lands. Per the STOP rule: filed issues/0157-*.{md,sx}, marked state BLOCKED, paused.
• B1.4a COMPLETE (this session) — suspending fiber-task async + two compiler fixes. Built the Task($R) + go/wait/cancel layer in sched.sx (nullary-thunk design; self-contained to avoid the _fib_tramp duplicate-symbol trap). Locked 1813 (sequence: 1 2 3 42 100 -99). FIXED the two blockers the worker had filed: 0156 Part 1 (comptime_pack_ref arm in resolveTypeWithBindings; regression 0216) and 0157 (receiver-driven UFCS overload selection selectUfcsGenericByReceiver; regression 0217). Adversarial review of the 0157 fix + Task layer found a determinism CRITICAL (always-run selection + specificity + ambiguity diagnostic), a wait-outside-fiber null-deref (loud guard), and cancel-not-skipping-work (skip if pre-canceled) — all fixed. Simplified 1812 (**Fiber → Sh.parked). 0156 Part 2 reframed OPEN/non-blocking. Suite GREEN 751/0. Next: B1.4b (deterministic-sim Io, the KEYSTONE).
• B1.4b COMPLETE (this session) — deterministic virtual-time timers + a CRITICAL UAF fix. Added clock_ms/timers/now_ms/sleep + a timer-driven run to sched.sx (worker-built): fibers sleep in reproducible simulated time, waking in deadline order (FIFO tiebreak). Locked 1814 (5 fibers, wake order B@10/D@15/E@15/C@20/A@30). Adversarial review of the run-loop change found a CRITICAL use-after-free — a fiber woken EARLY (manual/Task wake) before its sleep timer fired was reaped while its Timer kept a dangling *Fiber; a later fire dereferenced freed memory (silent "pass" only by luck). Fixed: wake evicts the fiber's pending timer (cancel_timer_for); regression 1815 (early wake → clock: 0, stale timer never fires). Review cleared n_suspended accounting, deadlock false-positives, timer-list integrity, clock monotonicity, termination. Suite GREEN 753/0. Next: B1.4c (event-loop Io, kqueue/epoll).
• B1.4c COMPLETE (this session) — real fd readiness via kqueue + 2 CRITICAL review fixes. Added a lazy kq + io_waiters + block_on_fd + a kqueue-blocking run-loop Mode 2 to sched.sx (worker-built, reusing std/net/kqueue.sx). Adversarial review found two CRITICALs: same-fd lost-wakeup hang (FIXED — block_on_fd enforces one-waiter-per-fd with a loud abort) and a never-ready-fd "hang" (RECLASSIFIED as correct event-loop semantics; misleading orphan-check comment corrected). Locked 1816 (pipe block→kqueue-wake→read). Suite green 754/0.
• B1.5 COMPLETE → STREAM B1 DONE (this session). Capstone 1817 composes the whole stack (go/wait + sleep/now_ms + scheduler) — three tasks complete in DEADLINE order (task 2@10 / 3@20 / 1@30), sum: 123, final virtual clock 30. The pure-sx colorblind M:1 async runtime is feature-complete end-to-end (1800–1817), all adversarially reviewed. Suite GREEN 755/0. Five compiler bugs fixed across the stream (0151/0152/0153/0154/0156-P1/0157 — 0151-3 in B1.2). Next carve: Stream B2 (channels / cancel / async stdlib).