feat(cmd): GOOS-aware headers, audit battery shapes and semicolon spacing

Assisted-by: GLM 5.3 Flash

cmd/gasm/asmhdr.go

@@ -27,7 +27,10 @@ import (
 // GOROOT assembly includes it, and a standalone assembler has no compiler
 // to have produced it, so gasm generates the equivalent itself: the package
 // the .s file lives in is parsed and type-checked here, with the target
-// architecture's own sizes, and the same defines are written out.
+// architecture's own sizes, and the same defines are written out.  The
+// type-checking GOOS is selected by the caller: a GOOS-specific file
+// (sys_darwin_arm64.s) needs its platform's defines, which a header from
+// the ambient GOOS silently omits.
 //
 // The emitter mirrors cmd/compile's dumpasmhdr exactly: constants come out
 // as "const_NAME", struct entries as "NAME__size" followed by the fields in
@@ -67,11 +70,15 @@ func goAsmHeaderResolved(asmDir string, dirs []string) bool {
 	return false
 }
 
-// generateGoAsmHeader type-checks the Go package in pkgDir for goarch,
-// writes its go_asm.h equivalent into dir, and returns dir.  The caller
-// owns the directory and its removal.
-func generateGoAsmHeader(pkgDir, goarch, dir string) (string, error) {
-	imp := newSourceImporter(goarch)
+// generateGoAsmHeader type-checks the Go package in pkgDir for goos and
+// goarch, writes its go_asm.h equivalent into dir, and returns dir.  An
+// empty goos means the ambient one.  The caller owns the directory and its
+// removal.
+func generateGoAsmHeader(pkgDir, goos, goarch, dir string) (string, error) {
+	if goos == "" {
+		goos = build.Default.GOOS
+	}
+	imp := newSourceImporter(goos, goarch)
 	if imp.sizes == nil {
 		return "", fmt.Errorf("go_asm.h: unknown GOARCH %q", goarch)
 	}
@@ -89,7 +96,7 @@ func generateGoAsmHeader(pkgDir, goarch, dir string) (string, error) {
 	}
 
 	var b strings.Builder
-	fmt.Fprintf(&b, "// generated by gasm from package %s (GOARCH %s)\n\n", bp.Name, goarch)
+	fmt.Fprintf(&b, "// generated by gasm from package %s (GOOS %s, GOARCH %s)\n\n", bp.Name, goos, goarch)
 	// Files in the build's own order and declarations in source order: the
 	// same walk the compiler's reader makes, so the header reads the same
 	// way the toolchain's does.  Order carries no meaning to the assembler
@@ -203,13 +210,14 @@ type sourceImporter struct {
 	pkgs  map[string]*types.Package
 }
 
-// newSourceImporter returns the importer for one target architecture.
+// newSourceImporter returns the importer for one target GOOS and GOARCH.
 // Cgo is disabled so the file set is deterministic and independent of the
 // host's C toolchain: cgo-tagged files drop out of the build exactly as
 // they do from a CGO_ENABLED=0 build, whose assembly is what gasm targets.
-func newSourceImporter(goarch string) *sourceImporter {
+func newSourceImporter(goos, goarch string) *sourceImporter {
 	ctxt := new(build.Context)
 	*ctxt = build.Default
+	ctxt.GOOS = goos
 	ctxt.GOARCH = goarch
 	ctxt.CgoEnabled = false
 	return &sourceImporter{
@@ -291,7 +299,7 @@ func (im *sourceImporter) checkPackage(bp *build.Package, files []*ast.File) (*t
 // type-check must not be re-checked once per file.
 type asmhdrCache struct {
 	root string
-	dirs map[string]string // "pkgDir\x00goarch" -> directory holding go_asm.h
+	dirs map[string]string // "pkgDir\x00goos\x00goarch" -> directory holding go_asm.h
 	errs map[string]error
 }
 
@@ -304,17 +312,22 @@ func newAsmhdrCache() (*asmhdrCache, error) {
 }
 
 // dirFor returns the directory holding the generated go_asm.h for pkgDir
-// and goarch, generating it on first use.
-func (c *asmhdrCache) dirFor(pkgDir, goarch string) (string, error) {
-	key := pkgDir + "\x00" + goarch
+// under goos and goarch, generating it on first use.  An empty goos means
+// the ambient one, resolved here so that one package cannot generate twice
+// under an explicit and an implicit spelling of the same GOOS.
+func (c *asmhdrCache) dirFor(pkgDir, goos, goarch string) (string, error) {
+	if goos == "" {
+		goos = build.Default.GOOS
+	}
+	key := pkgDir + "\x00" + goos + "\x00" + goarch
 	if dir, ok := c.dirs[key]; ok {
 		return dir, nil
 	}
 	if err, ok := c.errs[key]; ok {
 		return "", err
 	}
-	dir := filepath.Join(c.root, fmt.Sprintf("h%d_%s", len(c.dirs), goarch))
-	if _, err := generateGoAsmHeader(pkgDir, goarch, dir); err != nil {
+	dir := filepath.Join(c.root, fmt.Sprintf("h%d_%s_%s", len(c.dirs), goos, goarch))
+	if _, err := generateGoAsmHeader(pkgDir, goos, goarch, dir); err != nil {
 		c.errs[key] = err
 		return "", err
 	}
@@ -327,10 +340,10 @@ func (c *asmhdrCache) close() { os.RemoveAll(c.root) }
 
 // ensureGoAsmHeader prepares the include directory a file that includes
 // go_asm.h needs: the generated header for the package in path's directory,
-// for the file's target architecture.  It reports a usage error when the
-// architecture cannot be determined, and passes through the generator's
-// diagnostics, which name the package.
-func ensureGoAsmHeader(path string, target arch.Arch, cache *asmhdrCache) (string, func(), error) {
+// for the file's target GOOS and architecture.  It reports a usage error
+// when the architecture cannot be determined, and passes through the
+// generator's diagnostics, which name the package.
+func ensureGoAsmHeader(path string, target arch.Arch, goos string, cache *asmhdrCache) (string, func(), error) {
 	if path == "-" {
 		return "", nil, errors.New("cannot generate go_asm.h for standard input (no package directory)")
 	}
@@ -338,14 +351,14 @@ func ensureGoAsmHeader(path string, target arch.Arch, cache *asmhdrCache) (strin
 		return "", nil, errors.New("a file that includes go_asm.h needs a target architecture: name the file _<arch>.s or pass -GOARCH")
 	}
 	if cache != nil {
-		dir, err := cache.dirFor(filepath.Dir(path), goarchName(target))
+		dir, err := cache.dirFor(filepath.Dir(path), goos, goarchName(target))
 		return dir, func() {}, err
 	}
 	root, err := os.MkdirTemp("", "gasm-asmhdr")
 	if err != nil {
 		return "", nil, err
 	}
-	dir, err := generateGoAsmHeader(filepath.Dir(path), goarchName(target), root)
+	dir, err := generateGoAsmHeader(filepath.Dir(path), goos, goarchName(target), root)
 	if err != nil {
 		os.RemoveAll(root)
 		return "", nil, err

cmd/gasm/asmhdr_test.go

@@ -22,12 +22,13 @@ func writePkg(t *testing.T, files map[string]string) string {
 	return dir
 }
 
-// generateFor generates the header for dir and returns its text.
-func generateFor(t *testing.T, dir, goarch string) string {
+// generateFor generates the header for dir and returns its text.  An empty
+// goos means the ambient one.
+func generateFor(t *testing.T, dir, goos, goarch string) string {
 	t.Helper()
-	hdrDir, err := generateGoAsmHeader(dir, goarch, t.TempDir())
+	hdrDir, err := generateGoAsmHeader(dir, goos, goarch, t.TempDir())
 	if err != nil {
-		t.Fatalf("generateGoAsmHeader(%q, %s): %v", dir, goarch, err)
+		t.Fatalf("generateGoAsmHeader(%q, %s, %s): %v", dir, goos, goarch, err)
 	}
 	b, err := os.ReadFile(filepath.Join(hdrDir, "go_asm.h"))
 	if err != nil {
@@ -72,7 +73,7 @@ type aliased struct {
 
 type alias = aliased
 `})
-	hdr := generateFor(t, dir, "amd64")
+	hdr := generateFor(t, dir, "", "amd64")
 	want := []string{
 		"#define const_bufSize 1024",
 		// iota resolves through go/types, one define per name.
@@ -133,8 +134,8 @@ package perarch
 const flavour = 2
 `,
 	})
-	amd64 := generateFor(t, dir, "amd64")
-	arm64 := generateFor(t, dir, "arm64")
+	amd64 := generateFor(t, dir, "", "amd64")
+	arm64 := generateFor(t, dir, "", "arm64")
 	if !strings.Contains(amd64, "#define const_flavour 1\n") {
 		t.Errorf("amd64 header misses const_flavour 1:\n%s", amd64)
 	}
@@ -149,19 +150,90 @@ const flavour = 2
 	if !strings.Contains(amd64, "#define layout__size 16\n") || !strings.Contains(amd64, "#define layout_p 8\n") {
 		t.Errorf("amd64 layout wrong:\n%s", amd64)
 	}
-	w386 := generateFor(t, dir, "386")
+	w386 := generateFor(t, dir, "", "386")
 	if !strings.Contains(w386, "#define layout__size 8\n") || !strings.Contains(w386, "#define layout_p 4\n") {
 		t.Errorf("386 layout wrong:\n%s", w386)
 	}
 }
 
+// TestGenerateGoAsmHeaderGOOS pins the GOOS half of the target: only the
+// platform's own files type-check into the header, which is why
+// sys_darwin_arm64.s cannot assemble against a linux-generated one.
+func TestGenerateGoAsmHeaderGOOS(t *testing.T) {
+	dir := writePkg(t, map[string]string{
+		"common.go": `package goosaware
+
+type shared struct {
+	a int32
+}
+`,
+		"plat_darwin.go": `//go:build darwin
+
+package goosaware
+
+type platform struct {
+	trampoline_numer int64
+}
+`,
+		"plat_windows.go": `//go:build windows
+
+package goosaware
+
+type platform struct {
+	callbackArgs__size int32
+}
+`,
+	})
+	darwin := generateFor(t, dir, "darwin", "arm64")
+	if !strings.Contains(darwin, "#define platform__size 8\n") || !strings.Contains(darwin, "#define platform_trampoline_numer 0\n") {
+		t.Errorf("darwin header misses the darwin layout:\n%s", darwin)
+	}
+	if strings.Contains(darwin, "callbackArgs") {
+		t.Errorf("darwin header must not carry the windows layout:\n%s", darwin)
+	}
+	windows := generateFor(t, dir, "windows", "arm64")
+	if !strings.Contains(windows, "#define platform_callbackArgs__size 0\n") {
+		t.Errorf("windows header misses the windows layout:\n%s", windows)
+	}
+	if strings.Contains(windows, "trampoline_numer") {
+		t.Errorf("windows header must not carry the darwin layout:\n%s", windows)
+	}
+	// The ambient GOOS is neither of the two, so only shared's defines are
+	// emitted; the shared type keeps its layout there.
+	ambient := generateFor(t, dir, "", "arm64")
+	if !strings.Contains(ambient, "#define shared__size 4\n") {
+		t.Errorf("ambient header misses the shared layout:\n%s", ambient)
+	}
+	if strings.Contains(ambient, "#define platform_") {
+		t.Errorf("ambient header must not carry either platform layout:\n%s", ambient)
+	}
+}
+
+func TestGoosFromFilename(t *testing.T) {
+	for path, want := range map[string]string{
+		"/x/sys_darwin_arm64.s":     "darwin",
+		"/x/sys_windows_arm64.s":    "windows",
+		"/x/asm_linux_amd64.s":      "linux",
+		"/x/rt0_darwin_arm64.s":     "darwin",
+		"/x/vgetrandom_zos_s390x.s": "zos",
+		"/x/rt0_js_wasm.s":          "js",
+		"/x/memmove_amd64.s":        "",
+		"/x/vlop_arm.s":             "",
+		"/x/stubs.s":                "",
+	} {
+		if got := goosFromFilename(path); got != want {
+			t.Errorf("goosFromFilename(%q) = %q, want %q", path, got, want)
+		}
+	}
+}
+
 func TestGenerateGoAsmHeaderErrors(t *testing.T) {
 	t.Run("type error", func(t *testing.T) {
 		dir := writePkg(t, map[string]string{"bad.go": `package bad
 
 const x = undefinedIdent
 `})
-		_, err := generateGoAsmHeader(dir, "amd64", t.TempDir())
+		_, err := generateGoAsmHeader(dir, "", "amd64", t.TempDir())
 		if err == nil {
 			t.Fatal("generation must fail for a package that does not type-check")
 		}
@@ -174,7 +246,7 @@ const x = undefinedIdent
 	})
 	t.Run("no go files", func(t *testing.T) {
 		dir := t.TempDir()
-		_, err := generateGoAsmHeader(dir, "amd64", t.TempDir())
+		_, err := generateGoAsmHeader(dir, "", "amd64", t.TempDir())
 		if err == nil {
 			t.Fatal("generation must fail without Go files")
 		}
@@ -281,6 +353,45 @@ type header struct {
 	}
 }
 
+// TestRunCorpusAuditGOOS covers the filename-derived GOOS end to end: a
+// kernel whose name names darwin must have its header type-checked with
+// GOOS=darwin, so the darwin-only constant it offsets with is defined.  The
+// operand mirrors sys_darwin_arm64.s's trampoline, where a missing define
+// leaves an unexpanded symbol in the offset and fails.
+func TestRunCorpusAuditGOOS(t *testing.T) {
+	dir := t.TempDir()
+	write := func(name, src string) {
+		t.Helper()
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(src), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	write("pkg.go", "package corpus\n")
+	write("plat_darwin.go", "//go:build darwin\n\npackage corpus\n\nconst trampolineNumer = 8\n")
+	write("kern_darwin_arm64.s", "#include \"go_asm.h\"\n"+
+		"GLOBL timebase<>(SB), NOPTR, $16\n"+
+		"TEXT \xc2\xb7g(SB), NOSPLIT, $0-0\n"+
+		"\tMOVD\ttimebase<>+const_trampolineNumer(SB), R0\n"+
+		"\tRET\n")
+
+	stats, err := runCorpusAudit(dir, nil)
+	if err != nil {
+		t.Fatalf("runCorpusAudit: %v", err)
+	}
+	var arm *corpusTally
+	for i, tg := range stats.targets {
+		if tg.name == "arm64" {
+			arm = stats.tallies[i]
+		}
+	}
+	if arm == nil {
+		t.Fatal("no arm64 tally")
+	}
+	if arm.attempted != 1 || arm.assembled != 1 {
+		t.Errorf("arm64 = %d/%d, want 1/1; reasons: %v", arm.assembled, arm.attempted, arm.reasons)
+	}
+}
+
 // TestGenerateGoAsmHeaderRuntime pins the generator against the real thing:
 // the runtime package, whose header the toolchain's own -asmhdr output was
 // sampled from.  Skipped in short mode: it type-checks the whole package.
@@ -288,7 +399,7 @@ func TestGenerateGoAsmHeaderRuntime(t *testing.T) {
 	if testing.Short() {
 		t.Skip("type-checks the whole runtime package")
 	}
-	dir, err := generateGoAsmHeader("/usr/local/go/src/runtime", "amd64", t.TempDir())
+	dir, err := generateGoAsmHeader("/usr/local/go/src/runtime", "", "amd64", t.TempDir())
 	if err != nil {
 		t.Fatalf("generateGoAsmHeader(runtime): %v", err)
 	}

cmd/gasm/audit.go

@@ -5,6 +5,7 @@ package main
 
 import (
 	"fmt"
+	"maps"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -288,6 +289,11 @@ func probeShapes(a arch.Arch) []string {
 			"V1.B16, [V2.B16], V3.B16", "V1.B8, [V2.B16, V3.B16], V4.B8",
 			"$4, V1.B16, V2.B16, V3.B16", "$15, V1", "V1, V2, p2",
 			"R0, R1, $1, $4, p2",
+			// The landing-pad kind, the compiler's PCDATA
+			// bookkeeping and the four-operand bitfield
+			// insert/extract family, as the toolchain's own
+			// testdata spells them.
+			"C", "$1, $0", "$0, R1, $1, R2",
 		}
 	case arch.RISCV:
 		return []string{
@@ -307,6 +313,9 @@ func probeShapes(a arch.Arch) []string {
 			"X5, X6, p2", "R5, R6, p2",
 			"X5, E8, M8, TA, MA, X6", "$4, E32, M1, TA, MA, X1",
 			"(X5), X6, V1, V2",
+			// The CSR immediate forms the toolchain's testdata spells:
+			// immediate, CSR name, destination.
+			"$2, TIME, X5",
 			"",
 		}
 	case arch.LOONG64:
@@ -323,6 +332,12 @@ func probeShapes(a arch.Arch) []string {
 			"V1, V2, V3", "X1, X2, X3", "V1, V2", "X1, X2", "V1", "X1",
 			// The vector compare-to-flag forms land in an FCC register.
 			"V1, FCC0", "X1, FCC0",
+			// The compiler's bookkeeping pair and the raw spellings the
+			// toolchain's own testdata carries: JIRL rd, rj, offset (the
+			// form RET lowers to), the prefetch with a 32-bit address and
+			// hint, and the byte-shuffle quads.
+			"$1, $0", "R1, R5, 0", "0(R7), $5, $0", "(R7), $5, $0",
+			"V1, V2, V3, V4", "X1, X2, X3, X4",
 			"",
 		}
 	}
@@ -487,10 +502,47 @@ func otherPortFile(path string) bool {
 
 // goOSNames are the GOOS values go/build recognises in file names.
 var goOSNames = map[string]bool{
-	"aix": true, "darwin": true, "dragonfly": true, "freebsd": true,
-	"ios": true, "js": true, "linux": true, "netbsd": true,
+	"aix": true, "android": true, "darwin": true, "dragonfly": true,
+	"freebsd": true, "hurd": true, "illumos": true, "ios": true,
+	"js": true, "linux": true, "nacl": true, "netbsd": true,
 	"openbsd": true, "plan9": true, "solaris": true, "wasip1": true,
-	"windows": true,
+	"windows": true, "zos": true,
+}
+
+// resolveGOOS validates a -GOOS flag value, mirroring the architecture
+// check's surface: a usage error naming what the tool accepts.
+func resolveGOOS(name string) (string, error) {
+	lower := strings.ToLower(name)
+	if goOSNames[lower] {
+		return lower, nil
+	}
+	return "", &usageError{fmt.Errorf("unknown GOOS %q: want one of %s", name, strings.Join(slices.Sorted(maps.Keys(goOSNames)), ", "))}
+}
+
+// goosFromFilename returns the GOOS the file's name carries, by go/build's
+// goodOSArchFile rule: the GOOS segment sits last, or last before the
+// architecture segment (sys_darwin_arm64.s, vlop_arm.s carries none).  An
+// empty result means the name names no GOOS and the ambient one applies.
+func goosFromFilename(path string) string {
+	base := path
+	if i := strings.LastIndexByte(base, '/'); i >= 0 {
+		base = base[i+1:]
+	}
+	base = strings.TrimSuffix(base, ".s")
+	// go/build ignores everything before the first underscore, so a GOOS
+	// segment is only ever looked for from there on.
+	i := strings.IndexByte(base, '_')
+	if i < 0 {
+		return ""
+	}
+	segs := strings.Split(base[i:], "_")
+	if n := len(segs); n >= 2 && goOSNames[segs[n-2]] && slices.Contains(goPortSuffixes, segs[n-1]) {
+		return segs[n-2]
+	}
+	if goOSNames[segs[len(segs)-1]] {
+		return segs[len(segs)-1]
+	}
+	return ""
 }
 
 // otherGOOSFile reports whether the file's name names a GOOS other than the
@@ -543,6 +595,12 @@ func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 			return nil, err
 		}
 
+		// The GOOS the header generation type-checks under follows the
+		// file's name when the name carries one; the ambient GOOS is the
+		// honest guess otherwise (a build tag naming another GOOS is
+		// invisible to a file-name rule).
+		goos := goosFromFilename(path)
+
 		var wanted []int // indexes into targets
 		if a := arch.FromFilename(path); a != arch.Unknown {
 			for i, tg := range targets {
@@ -567,11 +625,12 @@ func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 
 		// A file that includes go_asm.h parses against a per-target header:
 		// the defines differ per architecture (internal/cpu's layout, for
-		// one), so the parse cannot be shared the way a header-free file's
-		// can.  A generation failure is a failure for every target, named
-		// for the package rather than a bare "include not found".  A header
-		// already resolvable in the package directory or the -I list is
-		// left alone.
+		// one) and per GOOS (sys_darwin_arm64.s's trampoline constants,
+		// for another), so the parse cannot be shared the way a
+		// header-free file's can.  A generation failure is a failure for
+		// every target, named for the package rather than a bare "include
+		// not found".  A header already resolvable in the package
+		// directory or the -I list is left alone.
 		if len(wanted) > 0 && needsGoAsmHeader(src) && !goAsmHeaderResolved(filepath.Dir(path), dirs) {
 			if hdr == nil {
 				if hdr, err = newAsmhdrCache(); err != nil {
@@ -583,7 +642,7 @@ func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 			for _, i := range wanted {
 				tg, t := targets[i], tallies[i]
 				t.attempted++
-				hdrDir, err := hdr.dirFor(pkgDir, goarchName(tg.a))
+				hdrDir, err := hdr.dirFor(pkgDir, goos, goarchName(tg.a))
 				if err != nil {
 					ok = false
 					t.fail(path, corpusReason(err))

cmd/gasm/main.go

@@ -486,7 +486,7 @@ hover, document symbols, diagnostics and semantic-token highlighting.
 }
 
 func cmdAsm(args []string) int {
-	fs := newCommand("asm", "gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-o out] <file>", `
+	fs := newCommand("asm", "gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-GOOS os] [-o out] <file>", `
 	Assemble FILE without the Go toolchain: every TEXT function is encoded to
 	machine code and printed as a hex dump.  Supported architectures: amd64
 	(including VEX/AVX2 and EVEX/AVX-512), arm64 (AArch64 integer, FP,
@@ -507,18 +507,22 @@ and the format version from go version).
 A file that includes go_asm.h gets that header generated automatically from
 the package it lives in (the .go files beside it, type-checked for the
 target architecture, the toolchain's own defines), so GOROOT assembly
-assembles without a compiler.  A package that has no Go files for the
-target or does not type-check is a hard error naming the package.
+assembles without a compiler.  -GOOS selects the type-checking GOOS for
+that header: a GOOS-specific file (sys_darwin_arm64.s) needs its platform's
+defines, which a header from the ambient GOOS silently omits.  A package
+that has no Go files for the target or does not type-check is a hard error
+naming the package.
 `)
 	out := fs.String("o", "", "write the output to this file")
 	format := fs.String("format", "raw", "output format: raw (concatenated image), elf or goobj (Go object)")
 	pkg := fs.String("p", "", "package path for --format goobj (qualifies the exported symbols)")
 	archName := fs.String("GOARCH", "", "target architecture: amd64, arm64, riscv64 or loong64 (overrides the file-name suffix)")
+	goosName := fs.String("GOOS", "", "operating system for go_asm.h generation: a GOOS go/build recognises (default: the host's)")
 	var dirs includeDirs
 	fs.Var(&dirs, "I", "directory to search for #include files (may be repeated)")
 	fs.Parse(args)
 	if fs.NArg() != 1 {
-		fmt.Fprintln(os.Stderr, "usage: gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-o out] <file>")
+		fmt.Fprintln(os.Stderr, "usage: gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-GOOS os] [-o out] <file>")
 		return 2
 	}
 	// The format is validated before anything else, so a bogus value exits 2
@@ -539,6 +543,15 @@ target or does not type-check is a hard error naming the package.
 		}
 		targetArch = a
 	}
+	goos := ""
+	if *goosName != "" {
+		g, err := resolveGOOS(*goosName)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "gasm asm: %v\n", err)
+			return 2
+		}
+		goos = g
+	}
 	src, err := readSource(path)
 	if err != nil {
 		fmt.Fprintln(os.Stderr, "gasm:", err)
@@ -553,7 +566,7 @@ target or does not type-check is a hard error naming the package.
 	// A go_asm.h that already resolves (placed by hand, or passed with -I)
 	// is left alone.
 	if needsGoAsmHeader(src) && !goAsmHeaderResolved(filepath.Dir(path), dirs) {
-		hdrDir, cleanup, err := ensureGoAsmHeader(path, targetArch, nil)
+		hdrDir, cleanup, err := ensureGoAsmHeader(path, targetArch, goos, nil)
 		if err != nil {
 			fmt.Fprintln(os.Stderr, "gasm asm:", err)
 			return 1

docs/CLI.md

@@ -141,7 +141,7 @@ gasm lint kernel_amd64.s
 ## asm
 
 ```text
-Usage: gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-o out] <file>
+Usage: gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-GOOS os] [-o out] <file>
 ```
 
 | Flag | Default | Effect |
@@ -150,6 +150,7 @@ Usage: gasm asm [--format raw|elf|goobj] [-I dir] [-p pkg] [-GOARCH arch] [-o ou
 | `-I` | empty | directory to search for `#include` files; may be repeated, searched in order after the source directory |
 | `-p` | empty | package path for `--format goobj`, qualifying the exported symbols |
 | `-GOARCH` | empty | target architecture: `amd64`, `arm64`, `riscv64` or `loong64`; overrides the file-name suffix |
+| `-GOOS` | empty | operating system for the generated `go_asm.h`: any GOOS `go/build` recognises in file names; default is the host's |
 | `-o` | empty | write the output to this file instead of a hex dump on stdout |
 
 Supported architectures: amd64 (VEX/AVX2 and EVEX/AVX-512 included), arm64,
@@ -163,6 +164,13 @@ system toolchain; `goobj` emits the Go toolchain's own object format, which
 installed: the object preamble is captured from `go tool asm` and the format
 version from `go version`. `raw` and `elf` need no toolchain at all.
 
+A file that includes `go_asm.h` gets that header generated from the Go
+files beside it, type-checked for the target. `-GOOS` selects the
+type-checking GOOS for that header, because a GOOS-specific file needs its
+platform's defines: `sys_darwin_arm64.s` fails against the ambient GOOS
+(`machTimebaseInfo_numer` is missing from a linux type-check) and assembles
+with `-GOOS darwin`.
+
 Assembly preprocessing matches the toolchain's: `#define` macros (object and
 parameterised) expand at the point of use, `#undef`, `#ifdef`, `#ifndef`,
 `#else` and `#endif` behave as in `go tool asm`, `;` separates statements,
@@ -390,7 +398,9 @@ With `--corpus` the audit changes shape: it assembles every `.s` file under
 DIR (default `GOROOT/src`) with the gasm encoder only, no toolchain probing.
 A file whose name carries a recognisable `_arch` suffix is attempted for that
 architecture; a file without one is attempted for all four, exactly as a
-`GOARCH` build would compile it. The report gives the headline number (files
+`GOARCH` build would compile it, and a name that names a GOOS
+(`sys_darwin_arm64.s`) type-checks its generated `go_asm.h` for that GOOS.
+The report gives the headline number (files
 that assemble for every target architecture), the per-architecture pass rates
 and the most common failure reasons with one representative file each, which
 drive the encodability backlog by frequency rather than by table order. A run

docs/man/gasm-asm.1

@@ -2,7 +2,7 @@
 .SH NAME
 gasm-asm \- assemble Plan 9 assembly without the Go toolchain
 .SH SYNOPSIS
-.B gasm asm [\-\-format raw|elf|goobj] [\-I dir] [\-p pkg] [\-GOARCH arch] [\-o out] <file>
+.B gasm asm [\-\-format raw|elf|goobj] [\-I dir] [\-p pkg] [\-GOARCH arch] [\-GOOS os] [\-o out] <file>
 .SH DESCRIPTION
 Assemble FILE without the Go toolchain: every TEXT function is encoded
 to machine code and printed as a hex dump.  Supported architectures:
@@ -42,6 +42,15 @@ need no toolchain at all.
 Framed functions receive the stack-split guard and the trailing
 morestack block, byte-identical to the toolchain's output, so split
 functions link too.
+.PP
+A file that includes go_asm.h gets that header generated from the Go
+files beside it, type-checked for the target.
+.B \-GOOS
+selects the type-checking GOOS for that header, because a GOOS-specific
+file needs its platform's defines: sys_darwin_arm64.s fails against the
+ambient GOOS (machTimebaseInfo_numer is missing from a linux type-check)
+and assembles with
+.BR "\-GOOS darwin" .
 .SH OPTIONS
 .TP
 .B \-\-format \fIraw|elf|goobj\fR
@@ -59,6 +68,10 @@ Target architecture: amd64, arm64, riscv64 or loong64; overrides the
 file-name suffix, which is how the suffix-less majority of GOROOT's
 files (cpu_x86.s, stub.s, ...) become assemblable.
 .TP
+.B \-GOOS \fIos\fR
+Operating system for the generated go_asm.h: any GOOS go/build
+recognises in file names; the default is the host's.
+.TP
 .B \-o \fIfile\fR
 Write the output to this file instead of a hex dump on stdout.
 .SH EXIT STATUS

format/format.go

@@ -241,6 +241,13 @@ func renderInstr(line []token.Token, width int) string {
 	if line[0].Kind != token.Ident {
 		return "\t" + mnem + " " + ops
 	}
+	// A statement separator belongs to the statement it ends: when the
+	// operands open with a ';', the alignment padding would land between
+	// the mnemonic and its own separator (REP  ; MOVSQ), so such a line
+	// renders with a single space whatever the function's width.
+	if strings.HasPrefix(ops, ";") {
+		return "\t" + mnem + " " + ops
+	}
 	if width < len(mnem) {
 		width = len(mnem)
 	}

format/format_test.go

@@ -364,6 +364,17 @@ func TestSemicolonSeparators(t *testing.T) {
 			in:   "TEXT ·f(SB), $0\nBYTE $1;\nRET\n",
 			want: "TEXT ·f(SB), $0\n\tBYTE $1;\n\tRET\n",
 		},
+		{
+			// The REP shape: a prefix-style zero-operand statement
+			// followed by the instruction it prefixes.  The separator
+			// belongs to the statement it ends, so the function's
+			// alignment width (MOVSQ is the widest mnemonic here) must
+			// not open a gap before it: one space after the mnemonic
+			// whatever the neighbours' lengths.
+			name: "after a prefix-style statement",
+			in:   "TEXT ·f(SB), $0\nMOVQ AX, BX\nREP; MOVSQ\nRET\n",
+			want: "TEXT ·f(SB), $0\n\tMOVQ AX, BX\n\tREP ; MOVSQ\n\tRET\n",
+		},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {