atomics A.0.5: full ordering surface (comptime $o: Ordering)

Migrate Atomic methods from seq_cst-only to the explicit ordering surface now
that comptime value params work on generic-struct methods (workers 3c4305f /
d7a6857 / d95ba0a):

- atomic.sx: load/store take a comptime $o: Ordering (explicit, Rust-style; no
  default, matching design 4.6). a.load(.acquire) -> 'load atomic .. acquire'.
- call.zig: atomicOrderingFromNode resolves a comptime-bound ordering identifier
  via comptimeIntNamed (+ atomicOrderingFromTag); documents the sx-Ordering <->
  IR-AtomicOrdering declaration-order invariant. The per-op validity guard fires
  through the method path (a.load(.release) is a compile error).
- 1700 migrated to explicit orderings (output unchanged 7/42/43).

Suite green (715/0).

current/CHECKPOINT-ATOMICS.md

@@ -59,6 +59,22 @@ Full atomic load/store plumbing with LLVM emission deliberately bailing loudly;
 `.ir`; add `emit_llvm.test.zig` unit. Then adversarial review, then the comptime-enum worker
 + A.0.5 migration to the full ordering surface.
 
+## A.0.5 — full ordering surface (DONE)
+`Atomic($T).load($o: Ordering)` / `store(v, $o)` — ordering is a COMPTIME value param,
+explicit (Rust-style, no default; design §4.6). `a.load(.acquire)` emits `load atomic …
+acquire`; `a.store(v, .release)` emits `store atomic … release`; `a.load(.release)` is a
+compile error (per-op validity guard fires through the method path). Recognizer
+`atomicOrderingFromNode` now resolves a comptime-bound ordering identifier via
+`comptimeIntNamed` (+ `atomicOrderingFromTag`, with the sx-Ordering ↔ IR-AtomicOrdering
+declaration-order invariant documented). 1700 migrated to explicit orderings (output
+unchanged 7/42/43). Suite green (715/0).
+
+**Unblocked by three comptime-value-param commits (workers):** enum (3c4305f), tagged_union
+(d7a6857), generic-struct methods (d95ba0a). NOTE: default VALUES for comptime params on
+generic-struct methods are NOT bound (orthogonal gap — free-fn defaults work); atomics
+sidesteps it cleanly by requiring explicit ordering (matches the design). Candidate
+follow-up, not an atomics blocker.
+
 ## Known issues / capability gaps
 - **Comptime-constant ordering propagation MISSING (blocks the full surface).** A runtime
   `Ordering` method param can't reach LLVM (orderings are instruction attributes, not
@@ -98,3 +114,7 @@ Full atomic load/store plumbing with LLVM emission deliberately bailing loudly;
 - **A.0c** — guard hardening from the adversarial review: scalar-kind allowlist + per-op
   ordering validity (call.zig), val_ty align bail (ops.zig), + diagnostic examples
   1130/1131. Suite green (713/0). (comptime enum value params landed via worker 3c4305f.)
+- **A.0.5** — full ordering surface: `Atomic($T).load/store($o: Ordering)` comptime ordering
+  (explicit). Recognizer resolves comptime-bound ordering via `comptimeIntNamed`. 1700
+  migrated to explicit orderings (acquire/release/relaxed/seq_cst). Unblocked by
+  comptime-value-param workers (3c4305f/d7a6857/d95ba0a). Suite green (715/0).

examples/1700-atomics-load-store.sx

@@ -1,17 +1,17 @@
-// Atomic($T) load/store (seq_cst), single-thread.
+// Atomic($T) load/store with explicit memory orderings, single-thread.
-// Stream A (atomics) A.0 — the first net-new atomic codegen example.
+// Stream A (atomics) A.0 + A.0.5 — the ordering is a comptime `$o: Ordering`
-// Explicit orderings (a.load(.acquire)) arrive in A.0.5; see PLAN-ATOMICS.md.
+// param (explicit, Rust-style): a.load(.acquire) emits `load atomic … acquire`.
-// Atomics is an opt-in import (not in the universal prelude) — like `trace`.
+// An invalid combination (a.load(.release)) is a compile error (see 1131).
 #import "modules/std.sx";
 #import "modules/std/atomic.sx";
 
 main :: () {
     a := Atomic(i64).init(7);
-    print("init: {}\n", a.load());
+    print("init: {}\n", a.load(.seq_cst));
 
-    a.store(42);
+    a.store(42, .release);
-    print("after store: {}\n", a.load());
+    print("after store: {}\n", a.load(.acquire));
 
-    a.store(a.load() + 1);
+    a.store(a.load(.relaxed) + 1, .seq_cst);
-    print("incremented: {}\n", a.load());
+    print("incremented: {}\n", a.load(.seq_cst));
 }

library/modules/std/atomic.sx

@@ -23,11 +23,11 @@ Ordering :: enum {
 atomic_load  :: ($T: Type, ptr: *T, o: Ordering) -> T #builtin;
 atomic_store :: ($T: Type, ptr: *T, v: T, o: Ordering) #builtin;
 
-// NOTE (A.0): the methods bake a literal `.seq_cst` (strongest, conservative)
+// The ordering is a COMPTIME value param (`$o`): it must be known at compile
-// rather than taking an `o: Ordering` parameter. A runtime ordering param can't
+// time because LLVM atomic ordering is an instruction attribute, not a runtime
-// reach the intrinsic as the compile-time constant LLVM requires, and comptime
+// operand. It is explicit (Rust-style — no default), so the caller always states
-// enum value params don't exist yet — so explicit orderings (`a.load(.acquire)`)
+// the ordering: `a.load(.acquire)`, `a.store(v, .release)`. An invalid
-// land in A.0.5 once that capability does. See current/PLAN-ATOMICS.md.
+// combination (`a.load(.release)`) is a compile error.
 Atomic :: struct ($T: Type) {
     value: T;
 
@@ -35,11 +35,11 @@ Atomic :: struct ($T: Type) {
         return .{ value = v };
     }
 
-    load :: (self: *Atomic(T)) -> T {
+    load :: (self: *Atomic(T), $o: Ordering) -> T {
-        return atomic_load(T, @self.value, .seq_cst);
+        return atomic_load(T, @self.value, o);
     }
 
-    store :: (self: *Atomic(T), v: T) {
+    store :: (self: *Atomic(T), v: T, $o: Ordering) {
-        atomic_store(T, @self.value, v, .seq_cst);
+        atomic_store(T, @self.value, v, o);
     }
 }

src/ir/lower/call.zig

@@ -1678,12 +1678,28 @@ pub fn lowerRuntimeDispatchCall(
     return self.builder.constInt(0, .void);
 }
 
-/// Map a bare ordering enum literal (`.seq_cst`) to the IR `AtomicOrdering`.
+/// The five `Ordering` variants by declaration-order tag. INVARIANT: the sx
-/// Returns null for anything that is not one of the five constant literals —
+/// `Ordering` enum (library/modules/std/atomic.sx) and the IR `AtomicOrdering`
-/// the caller turns that into a loud "must be a constant ordering literal"
+/// enum (inst.zig) declare these variants in the SAME order, so a comptime-bound
-/// diagnostic (never a silent default).
+/// ordering's tag indexes this list. Keep all three in sync.
-fn atomicOrderingFromNode(node: *const Node) ?inst_mod.AtomicOrdering {
+fn atomicOrderingFromTag(tag: i64) ?inst_mod.AtomicOrdering {
-    if (node.data != .enum_literal) return null;
+    return switch (tag) {
+        0 => .relaxed,
+        1 => .acquire,
+        2 => .release,
+        3 => .acq_rel,
+        4 => .seq_cst,
+        else => null,
+    };
+}
+
+/// Resolve an ordering argument to the IR `AtomicOrdering`. Accepts a bare enum
+/// literal (`.seq_cst`) OR a comptime-bound identifier (a `$o: Ordering` param
+/// forwarded into the intrinsic — read its bound variant tag via
+/// `comptimeIntNamed`). Returns null for a non-constant ordering — the caller
+/// turns that into a loud diagnostic (never a silent default).
+fn atomicOrderingFromNode(self: *Lowering, node: *const Node) ?inst_mod.AtomicOrdering {
+    if (node.data == .enum_literal) {
         const n = node.data.enum_literal.name;
         if (std.mem.eql(u8, n, "relaxed")) return .relaxed;
         if (std.mem.eql(u8, n, "acquire")) return .acquire;
@@ -1691,6 +1707,11 @@ fn atomicOrderingFromNode(node: *const Node) ?inst_mod.AtomicOrdering {
         if (std.mem.eql(u8, n, "acq_rel")) return .acq_rel;
         if (std.mem.eql(u8, n, "seq_cst")) return .seq_cst;
         return null;
+    }
+    if (node.data == .identifier) {
+        if (self.comptimeIntNamed(node.data.identifier.name)) |tag| return atomicOrderingFromTag(tag);
+    }
+    return null;
 }
 
 /// Recognize the atomic `#builtin` intrinsics and lower them to dedicated atomic
@@ -1729,7 +1750,7 @@ pub fn tryLowerAtomicIntrinsic(self: *Lowering, name: []const u8, c: *const ast.
     }
 
     const ord_node = c.args[expected - 1];
-    const ordering = atomicOrderingFromNode(ord_node) orelse {
+    const ordering = atomicOrderingFromNode(self, ord_node) orelse {
         if (self.diagnostics) |d| d.addFmt(.err, ord_node.span, "atomic ordering must be a constant ordering literal (.relaxed / .acquire / .release / .acq_rel / .seq_cst)", .{});
         return Ref.none;
     };