feat(cmd): add gofmt-style -l and -d modes to gasm fmt

cmd/gasm/main.go

@@ -56,6 +56,8 @@ func main() {
 		os.Exit(cmdLint(os.Args[2:]))
 	case "asm":
 		os.Exit(cmdAsm(os.Args[2:]))
+	case "dis":
+		os.Exit(cmdDis(os.Args[2:]))
 	case "verify":
 		os.Exit(cmdVerify(os.Args[2:]))
 	case "debug":
@@ -132,6 +134,7 @@ func usage(w io.Writer) {
 		{"fmt", "canonicalise formatting (gofmt for assembly)"},
 		{"lint", "run static checks"},
 		{"asm", "assemble .s files to machine code (amd64, arm64, riscv64, loong64)"},
+		{"dis", "disassemble machine code (raw bytes or an assembled .s file)"},
 		{"verify", "JIT-assemble and run dynamic checks (amd64, arm64, riscv64, loong64)"},
 		{"debug", "interactive source-level debugger (amd64, arm64, riscv64, loong64)"},
 		{"diff", "compare machine code of two .s files"},
@@ -268,7 +271,7 @@ standard input.
 }
 
 func cmdFmt(args []string) int {
-	fs := newCommand("fmt", "gasm fmt [-w] [path...]", `
+	fs := newCommand("fmt", "gasm fmt [-w|-l|-d] [path...]", `
 Canonicalise the formatting of Plan 9 assembly sources: indentation, operand
 spacing, per-function mnemonic alignment and blank-line layout (exactly one
 blank line before each label, TEXT and GLOBL block).  Formatting is
@@ -278,9 +281,19 @@ With no paths, or a directory path, every .s file below it is reformatted
 in place and the changed files are listed, the way go fmt does; "." and "_"
 directories are skipped.  Explicit file paths print to stdout unless -w is
 given.
+
+-l and -d rewrite nothing: -l prints the paths whose formatting differs
+from gasm's (empty output means everything is formatted, which is what a CI
+check wants), -d prints the diffs.  They are mutually exclusive.
 `)
 	write := fs.Bool("w", false, "write result to the source file")
+	list := fs.Bool("l", false, "list files whose formatting differs from gasm's")
+	diffMode := fs.Bool("d", false, "print diffs instead of rewriting files")
 	fs.Parse(args)
+	if *list && *diffMode {
+		fmt.Fprintln(os.Stderr, "gasm fmt: -l and -d are mutually exclusive")
+		return 2
+	}
 	// Like go fmt: with no arguments, or with a directory argument, every .s
 	// file below the directory is formatted in place and the names of the
 	// changed files are listed; explicit file arguments keep the -w / stdout
@@ -318,6 +331,16 @@ given.
 			continue
 		}
 		out := format.Source(src)
+		if *list || *diffMode {
+			if out != src {
+				if *list {
+					fmt.Println(path)
+				} else {
+					fmt.Print(unifiedDiff(path, strings.Split(src, "\n"), strings.Split(out, "\n")))
+				}
+			}
+			continue
+		}
 		if dirMode || *write {
 			if out != src {
 				if err := os.WriteFile(path, []byte(out), 0o644); err != nil {