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atomics A.1c: fix comptime signed fetch_min/max (was unsigned compare)

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Adversarial review CRITICAL: the comptime VM's atomic_rmw min/max arm called
@max/@min directly on Reg (=u64) values for SIGNED types, doing an UNSIGNED
compare — so comptime fetch_min/max on negatives diverged from the runtime LLVM
atomicrmw min/max (signed). Fix: reinterpret as i64 in the signed branch before
comparing, bitcast back (mirrors the unsigned branch + the emit-side signedness).

Closes the coverage gap that hid it: extend examples/1701 with signed min/max on
a negative at BOTH comptime (#run) and runtime — they now agree (3 / -5). Suite
green (716/0).
This commit is contained in:
agra
2026-06-20 10:32:50 +03:00
parent 05311646aa
commit 68ed732b79
3 changed files with 28 additions and 4 deletions
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12
src/ir/comptime_vm.zig
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@@ -699,13 +699,17 @@ pub const Vm = struct {
.@"or" => old | operand,
.xor => old ^ operand,
.min, .max => blk: {
// `Reg` is u64, so `@max`/`@min` on it is an UNSIGNED
// compare. For a signed type, reinterpret as i64 first so
// a negative value loses to a positive one — matching LLVM
// `atomicrmw min`/`max` (signed) and the emit side.
const want_max = a.kind == .max;
if (table.isUnsignedInt(vty)) {
const uo: u64 = @bitCast(old);
const up: u64 = @bitCast(operand);
break :blk @bitCast(if (want_max) @max(uo, up) else @min(uo, up));
break :blk if (want_max) @max(old, operand) else @min(old, operand);
}
break :blk if (want_max) @max(old, operand) else @min(old, operand);
const so: i64 = @bitCast(old);
const sp: i64 = @bitCast(operand);
break :blk @bitCast(if (want_max) @max(so, sp) else @min(so, sp));
},
};
try self.writeField(table, frame.get(a.ptr.index()), vty, new_val);