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feat(asm): read-write + place outputs

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Implements read-write (`+r` / `+{reg}`) `-> @place` outputs. LLVM has
no `+` constraint, so a read-write place lowers to:

  - an output `=` constraint (return slot, stored back through the
    place after the call), with the leading `+` rewritten to `=`; plus
  - a TIED input constraint (the decimal index of that output) appended
    after the regular inputs, seeded with the place's loaded value
    passed as a call arg.

Tied inputs are appended last so existing operand indices (%[name] ->
${N}) are undisturbed; asmOperandIndex stays correct. Lowering no longer
rejects `+` (indirect `*` still rejected). emitInlineAsm grows the
arg/param arrays by the rw count, loads each seed, and emits the tied
constraint.

Verified by running: increment-in-place (41 -> 42) and a mixed case
(rw place + regular input + value output) producing the textbook
"=r,=r,r,0" constraint with correct ${N} indices. 1650 flipped from
the rejection lock to a runnable aarch64-pinned example (ir-only
elsewhere). zig build test green (658 corpus, 446 unit).
This commit is contained in:
agra
2026-06-15 23:07:38 +03:00
parent 335ac52374
commit 4128416d48
6 changed files with 83 additions and 19 deletions
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10
src/ir/lower/expr.zig
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@@ -2345,12 +2345,10 @@ pub fn lowerAsmExpr(self: *Lowering, ae: *const ast.AsmExpr, span: ast.Span) Ref
.input => operand_ref = self.lowerExpr(op.payload),
.out_value => out_ty = self.resolveTypeWithBindings(op.payload),
.out_place => {
// Read-write (`+`) and indirect-memory (`*`) place outputs aren't
// implemented yet — reject loudly rather than miscompile (§II.11).
if (op.constraint.len > 0 and op.constraint[0] == '+') {
diags.addFmt(.err, span, "read-write (`+`) asm outputs are not yet implemented; use a write-only `=` output", .{});
return self.emitPlaceholder("inline_asm");
}
// Indirect-memory (`*`) place outputs aren't implemented yet —
// reject loudly rather than miscompile (§II.11). Read-write (`+`)
// outputs ARE implemented (emit ties an input to the output and
// seeds it with the place's loaded value; see `emitInlineAsm`).
if (std.mem.indexOfScalar(u8, op.constraint, '*') != null) {
diags.addFmt(.err, span, "indirect-memory (`*`) asm outputs are not yet implemented", .{});
return self.emitPlaceholder("inline_asm");