sx/current/CHECKPOINT-ASM.md

# sx Inline Assembly — Checkpoint (ASM stream)

Companion to `current/PLAN-ASM.md`; design in
[docs/inline-asm-design.md](../docs/inline-asm-design.md). Update after every
commit, one step at a time per the cadence rule (no commit may both add a test
and make it pass).

## Last completed step
**B.1** — operand-name validation (design §II.5 auto-naming rule). Extended
`lowerAsmExpr` with a `pinnedRegister(constraint)` helper (`"={eax}"`→`eax`,
`"+{rax}"`→`rax`, `"=r"`→null) and two checks: (1) **reject the echo form**
`[eax] "={eax}"` — a label identical to its own pinned register is redundant
(the operand is already auto-named after the register); (2) **reject duplicate
operand names** (ambiguous `%[name]` / result field). Locked with
`examples/1643-platform-asm-echo-name.sx` + `1644-platform-asm-duplicate-name.sx`.
`zig build test` green (652 corpus, 0 failed; 445 unit). Files:
`src/ir/lower/expr.zig`.

Prior: **B.0** — asm shape validation (compile-path diagnostics). Restructured the
`.asm_expr` lowering arm into `lowerAsmExpr` (`src/ir/lower/expr.zig`, mixed into
`Lowering` in `src/ir/lower.zig`): it validates BEFORE the not-yet-implemented
codegen bail, so the user sees the real problem first. Two checklist items now
enforced with named diagnostics: (1) **template must be a compile-time-known
string** (`"..."` / `#string`); (2) **no value outputs ⇒ must be `volatile`**
(mirrors Zig — a result-less asm could be deleted). Valid shapes still bail with
the "codegen not yet implemented" message. Result-type derivation + auto-naming
stay deferred to a later step (observable only once Phase C produces a real IR
op). Locked with `examples/1641-platform-asm-missing-volatile.sx` (volatile
error) + `1642-platform-asm-nop-volatile.sx` (volatile no-output accepted →
codegen bail). `zig build test` green (650 corpus, 0 failed; 445 unit). Files:
`src/ir/lower/expr.zig`, `src/ir/lower.zig`, `examples/164{1,2}-*`.

Prior: **A.1** — parse `asm { … }` + loud lowering bail (folded A.1+A.2 into one honest
lock commit, since the loud bail IS current correct behavior — cadence option
(a)). Added `AsmExpr`/`AsmOperand` to `src/ast.zig` + the `asm_expr` `Node.Data`
arm; `parseAsmExpr` in `src/parser.zig` (`parsePrimary` `.kw_asm` dispatch) —
parses the template, flat operand list (`[name]? "constraint" -> Type` value
output / `= expr` input), and `clobbers(.…)`; `volatile`/`clobbers` recognized
contextually via `isContextualWord`. The new `asm_expr` tag forced (and got)
arms in three exhaustive `Node.Data` switches: `src/sema.zig` `analyzeNode` +
`findNodeAtOffset`, `src/ir/semantic_diagnostics.zig` `checkBindingNames` (all
recurse into template + operand payloads). Lowering bails LOUD + named in
`src/ir/lower/expr.zig` ("inline assembly codegen is not yet implemented…") via
an explicit `.asm_expr` arm (not the generic `unknown_expr` else) returning
`emitPlaceholder`. `-> @place` write-through is rejected with a clear "Phase 2"
parse error. Locked with `examples/1640-platform-asm-parse.sx` (multi-output
`divmod`, named operands, register pins, clobbers — parses then bails; called
from `main`). `zig build test` green (648 corpus, 0 failed; 445 unit). Files:
`src/ast.zig`, `src/parser.zig`, `src/sema.zig`, `src/ir/semantic_diagnostics.zig`,
`src/ir/lower/expr.zig`, `examples/1640-*`.

Prior: **A.0** — `kw_asm` keyword (first compiler code). Added the `kw_asm` `Token.Tag`
variant + `.{ "asm", .kw_asm }` keyword-map entry in `src/token.zig`; `volatile` /
`clobbers` deliberately stay OUT of the global table (contextual). New exhaustive
`Tag` switch in `src/lsp/server.zig` `classifyToken` flagged the missing arm (the
intended coverage tripwire) — added `.kw_asm` to the keyword group. Lock test in
new `src/lexer.test.zig` (`asm`→`kw_asm`, `volatile`/`clobbers`→`identifier`),
wired into the `src/root.zig` barrel as `lexer_tests`. `zig build test` green (648
corpus, 0 failed; 445 unit, 0 failed — +1). Files: `src/token.zig`,
`src/lexer.test.zig`, `src/root.zig`, `src/lsp/server.zig`.

Prior: **0.2** — CLAUDE.md docs for `<name>.build`; **Phase 0 COMPLETE**.
**0.1** — corpus runner **ir-only branch** for cross-target examples. Replaced
0.0's loud placeholder bail: when `cfg.target` doesn't match the host (`ir_only`),
`sweepRoot` skips run/build/exec and verifies via `sx ir --target` only —
asserting `.exit` (ir cmd) + `.ir` (normalized stdout) + `.stderr`, never
`.stdout` (write skipped in update mode, assertion skipped in verify mode). An
`.ir` snapshot is **required** in ir-only mode — its absence is a loud failure
("needs an .ir snapshot for ir-only mode"). Locked with
`examples/1639-platform-target-cross.sx` (asm-free `main :: () -> i64 { return 0;
}`), `.build` `{ "target": "x86_64-linux" }`, + checked-in `.ir`. Verified both
guards fire: corrupting the `.ir` → IR mismatch; deleting it → the require-failure.
`zig build test` green (647 corpus, 0 failed; 444 unit). Files:
`src/corpus_run.test.zig`, `examples/1639-*`.

## Current state
Phase A underway: `asm { … }` lexes (A.0) and **parses** into `AsmExpr` (A.1);
lowering bails LOUD + named (no IR op / emit yet). Result-type derivation, the
operand auto-naming rule, and the validation checklist are **Phase B** (not yet
implemented — any asm reaching lowering errors out). The adopted **operand
auto-naming rule** (design §II.5, decided this session): name auto-derived from a
`{reg}` pin; explicit `[name]` only when it differs or for register-class (`=r`)
operands; echo form `[eax] "={eax}"` rejected. Parser stores `name: ?[]const u8`;
the rule is a Phase-B (typing) concern, so the parser needs no change for it.

Known orthogonal bug: **issue 0137** — `sx run` on a program with no `main`
segfaults (`src/target.zig:256-273`, unguarded JIT entry lookup). Pre-existing,
asm-independent; does NOT block the ASM stream (every example has a `main`).

Phase B–E feasibility already confirmed against the live tree
(`LLVMGetInlineAsm` / `LLVMBuildCall2` / `LLVMAppendModuleInlineAsm` in LLVM@19
`Core.h`; ERR-stream `extractvalue`→tuple in `emit_llvm.zig:726-927`; lib-less
`extern`, 60 sites; `--target` a global CLI flag).

## Next step
**C.0** (Phase C — IR op) — add `inline_asm: InlineAsm` to `Op` (`src/ir/inst.zig`,
next to `objc_msg_send`) + the `AsmOperand` IR struct (role/name/constraint/operand
as `StringId`/`Ref`), and an interp `bailDetail` arm (`src/ir/interp.zig`) — inline
asm can never be comptime-evaluated. Unit-test the IR shape. This is a `lock`
commit (no behavior change yet — `lowerAsmExpr` keeps bailing). Then C.1 wires
`lowerAsmExpr` to actually intern strings + lower input `Ref`s + build the op and
compute the result `TypeId`, at which point **result-type derivation becomes
observable** and the auto-naming/tuple typing (`expr_typer.zig`, deferred from B)
can be tested end-to-end. See `PLAN-ASM.md` Phase C + design §II.6.

Remaining deferred validation (do alongside C, once template scanning is worth
it): `%[name]` references in the template that name no operand. Needs effective-
name resolution (explicit `[name]` ∪ auto-derived register names).

## Log
- (init) Plan + design doc written; ASM stream opened.
- (0.0) Corpus runner target-gating: `<name>.build` JSON config (replaces `.aot`
  marker), `--target` threading, `hostMatchesTarget` execute-gate, loud
  cross-target placeholder bail. Migrated 1226/1227 `.aot`→`.build`; locked with
  1638 fixture + unit tests. `zig build test` green.
- (0.1) ir-only branch: cross-target examples verify via `sx ir --target` only
  (exit+ir+stderr, no stdout; `.ir` required). Locked with 1639 fixture; verified
  corrupt-.ir → mismatch and missing-.ir → loud failure. `zig build test` green.
- (0.2) docs: CLAUDE.md documents `<name>.build` JSON sidecar (aot + target +
  ir-only gating), replacing stale `.aot` marker prose. **Phase 0 COMPLETE.**
- (A.0) `kw_asm` keyword in token.zig (+ map entry); LSP `classifyToken` switch
  coverage; lock test in new `lexer.test.zig` (wired via root.zig). `volatile` /
  `clobbers` stay contextual identifiers. `zig build test` green (445 unit, +1).
- (A.1) parse `asm { … }` → `AsmExpr` + loud lowering bail; `asm_expr` arms in 3
  exhaustive `Node.Data` switches; `-> @place` rejected (Phase 2). Adopted operand
  auto-naming rule (design §II.5). Locked with 1640 fixture. Filed orthogonal
  issue 0137 (no-`main` JIT segfault). `zig build test` green (648 corpus, 445 unit).
- (B.0) asm shape validation in `lowerAsmExpr`: comptime-string template +
  no-output⇒volatile, with named diagnostics before the codegen bail. Locked with
  1641 (volatile error) + 1642 (volatile accepted). `zig build test` green (650
  corpus, 445 unit).
- (B.1) operand-name validation: `pinnedRegister` helper + reject echo form
  (`[eax] "={eax}"`) and duplicate names. Locked with 1643 + 1644. `zig build
  test` green (652 corpus, 445 unit).

## Known issues
- **0137** — `sx run` on a program with no `main` segfaults (unguarded JIT entry
  lookup, `src/target.zig:256-273`). Pre-existing, asm-independent. Filed
  `issues/0137-jit-run-no-main-segfault.md`. Does not block A.1.