fix(arch): narrow file names by go/build's suffix rule

Assisted-by: GLM 5.3 Flash

arch/arch.go

@@ -24,20 +24,41 @@ const (
 	Unknown Arch = ""
 )
 
-// FromFilename guesses the target architecture from a source file name.  Go
-// assembly files conventionally carry a GOARCH suffix such as "_amd64.s",
-// "_arm64.s", "_riscv64.s" or "_loong64.s".  It returns Unknown when no suffix
-// matches.
+// FromFilename guesses the target architecture from a source file name, by
+// go/build's goodOSArchFile rule: directories are stripped, the name is cut
+// at its first dot, everything before the first underscore is ignored and a
+// trailing _test segment is dropped; the name is per-architecture only when
+// its final underscore segment is an architecture name (foo_amd64.s,
+// sys_darwin_arm64.s, foo_amd64_test.s).  Any other name narrows nothing:
+// not one whose last segment merely contains an architecture name
+// (x_loong64y.s), not one with no underscore at all (amd64.s, which
+// go/build also keeps generic), and not one differing in case
+// (foo_AMD64.s), because the segment match is case-sensitive as go/build's
+// is.  A name carrying an operating system alone (foo_linux.s) narrows by
+// GOOS rather than architecture, and one carrying an architecture gasm
+// does not assemble for (vlop_arm.s) reports Unknown so the caller's own
+// port filter decides.  It returns Unknown when the name narrows nothing.
 func FromFilename(name string) Arch {
-	lower := strings.ToLower(name)
-	switch {
-	case strings.Contains(lower, "_amd64"):
+	if i := strings.LastIndexByte(name, '/'); i >= 0 {
+		name = name[i+1:]
+	}
+	name, _, _ = strings.Cut(name, ".")
+	i := strings.Index(name, "_")
+	if i < 0 {
+		return Unknown
+	}
+	segs := strings.Split(name[i:], "_")
+	if last := len(segs) - 1; segs[last] == "test" {
+		segs = segs[:last]
+	}
+	switch segs[len(segs)-1] {
+	case "amd64":
 		return AMD64
-	case strings.Contains(lower, "_arm64"):
+	case "arm64":
 		return ARM64
-	case strings.Contains(lower, "_riscv64"), strings.Contains(lower, "_riscv"):
+	case "riscv64":
 		return RISCV
-	case strings.Contains(lower, "_loong64"), strings.Contains(lower, "_loong"):
+	case "loong64":
 		return LOONG64
 	default:
 		return Unknown

arch/arch_test.go

@@ -7,13 +7,40 @@ import "testing"
 
 func TestFromFilename(t *testing.T) {
 	cases := map[string]Arch{
+		// Positive: the final underscore segment is the architecture.
 		"avx2_amd64.s":     AMD64,
 		"foo_arm64.s":      ARM64,
-		"portable.s":       Unknown,
-		"decode_ARM64.S":   ARM64,
 		"kernels_amd64.s":  AMD64,
 		"kernel_riscv64.s": RISCV,
 		"kernel_loong64.s": LOONG64,
+		"decode_arm64.S":   ARM64, // the extension is cut with the first dot
+		// OS before arch: the _<os>_<arch> pair form.
+		"sys_darwin_arm64.s": ARM64,
+		"rt0_linux_amd64.s":  AMD64,
+		// A trailing _test segment is dropped before the suffix rule.
+		"foo_amd64_test.s": AMD64,
+		// Trailing junk: a final segment that merely contains an
+		// architecture name narrows nothing, exactly as go/build's
+		// segment rule says.
+		"x_loong64y.s":          Unknown,
+		"asm_amd64x.s":          Unknown, // a real GOROOT name
+		"asm_darwin_arm64_gc.s": Unknown, // the trailing _gc segment is junk
+		"x_loong.s":             Unknown,
+		"foo_amd64x_test.s":     Unknown,
+		// Case sensitivity: the segment match is exact, as go/build's is.
+		"foo_AMD64.s":    Unknown,
+		"decode_ARM64.S": Unknown,
+		// No underscore at all: generic whatever the stem says.
+		"amd64.s":    Unknown,
+		"portable.s": Unknown,
+		// A GOOS-only name narrows by operating system, not architecture.
+		"foo_linux.s": Unknown,
+		// An architecture gasm does not assemble for: named, but Unknown.
+		"vlop_arm.s":  Unknown,
+		"foo_riscv.s": Unknown,
+		// Directories are stripped first, whatever dots they carry.
+		"some/dir/kernel_loong64.s": LOONG64,
+		"/a.b/x_amd64.s":            AMD64,
 	}
 	for name, want := range cases {
 		if got := FromFilename(name); got != want {