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