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 {

cmd/gasm/unidiff.go

@@ -0,0 +1,140 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"fmt"
+	"slices"
+	"strings"
+)
+
+// unifiedDiff renders a unified diff with three lines of context between the
+// two line slices, in the form `gofmt -d` prints.  An empty result means the
+// inputs are identical.
+func unifiedDiff(name string, a, b []string) string {
+	if slices.Equal(a, b) {
+		return ""
+	}
+	var out strings.Builder
+	fmt.Fprintf(&out, "--- %s\n+++ %s\n", name, name)
+
+	// Longest common subsequence over the lines (assembly files are small
+	// enough for the quadratic table).
+	n, m := len(a), len(b)
+	lcs := make([][]int, n+1)
+	for i := range lcs {
+		lcs[i] = make([]int, m+1)
+	}
+	for i := n - 1; i >= 0; i-- {
+		for j := m - 1; j >= 0; j-- {
+			if a[i] == b[j] {
+				lcs[i][j] = lcs[i+1][j+1] + 1
+			} else if lcs[i+1][j] >= lcs[i][j+1] {
+				lcs[i][j] = lcs[i+1][j]
+			} else {
+				lcs[i][j] = lcs[i][j+1]
+			}
+		}
+	}
+
+	// Walk the LCS once, assigning every op its absolute position in both
+	// files (1-based, the position an insertion sits before).
+	type op struct {
+		kind          byte // ' ', '-' or '+'
+		aLine, bLine  int
+		text          string
+	}
+	var ops []op
+	aPos, bPos := 0, 0
+	emit := func(kind byte, text string) {
+		ops = append(ops, op{kind: kind, aLine: aPos + 1, bLine: bPos + 1, text: text})
+		switch kind {
+		case ' ':
+			aPos++
+			bPos++
+		case '-':
+			aPos++
+		case '+':
+			bPos++
+		}
+	}
+	i, j := 0, 0
+	for i < n && j < m {
+		switch {
+		case a[i] == b[j]:
+			emit(' ', a[i])
+			i++
+			j++
+		case lcs[i+1][j] >= lcs[i][j+1]:
+			emit('-', a[i])
+			i++
+		default:
+			emit('+', b[j])
+			j++
+		}
+	}
+	for ; i < n; i++ {
+		emit('-', a[i])
+	}
+	for ; j < m; j++ {
+		emit('+', b[j])
+	}
+
+	// Group the edits into hunks: consecutive changes separated by more than
+	// twice the context lines start a new hunk.
+	const context = 3
+	var changes []int
+	for k, o := range ops {
+		if o.kind != ' ' {
+			changes = append(changes, k)
+		}
+	}
+	for g := 0; g < len(changes); {
+		last := g
+		for last+1 < len(changes) && changes[last+1]-changes[last]-1 <= 2*context {
+			last++
+		}
+		lo := max(0, changes[g]-context)
+		hi := min(len(ops), changes[last]+1+context)
+		// The header numbers are the first line of each side actually shown:
+		// the first context, deletion or insertion line.  A hunk that shows
+		// no old lines is a pure insertion and reports the position it sits
+		// before (0 at the top of the file); the mirror rule holds for a
+		// pure deletion.
+		aStart := ops[lo].aLine - 1
+		bStart := ops[lo].bLine - 1
+		countA, countB := 0, 0
+		for _, o := range ops[lo:hi] {
+			switch o.kind {
+			case ' ':
+				countA++
+				countB++
+			case '-':
+				countA++
+			case '+':
+				countB++
+			}
+		}
+		for _, o := range ops[lo:hi] {
+			if o.kind != '+' {
+				aStart = o.aLine
+				break
+			}
+		}
+		for _, o := range ops[lo:hi] {
+			if o.kind != '-' {
+				bStart = o.bLine
+				break
+			}
+		}
+		fmt.Fprintf(&out, "@@ -%d,%d +%d,%d @@\n", aStart, countA, bStart, countB)
+		for _, o := range ops[lo:hi] {
+			out.WriteByte(o.kind)
+			out.WriteString(o.text)
+			out.WriteByte('\n')
+		}
+		g = last + 1
+	}
+	return out.String()
+}

cmd/gasm/unidiff_test.go

@@ -0,0 +1,94 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"slices"
+	"strings"
+	"testing"
+)
+
+func lines(ss ...string) []string { return ss }
+
+func TestUnifiedDiffIdentical(t *testing.T) {
+	if got := unifiedDiff("f", lines("a", "b"), lines("a", "b")); got != "" {
+		t.Errorf("identical inputs produced %q, want empty", got)
+	}
+}
+
+func TestUnifiedDiffSingleChange(t *testing.T) {
+	a := lines("1", "2", "3", "4", "5", "6", "7", "8")
+	b := lines("1", "2", "3!", "4", "5", "6", "7", "8")
+	want := "--- f\n+++ f\n" +
+		"@@ -1,6 +1,6 @@\n" +
+		" 1\n 2\n-3\n+3!\n 4\n 5\n 6\n"
+	if got := unifiedDiff("f", a, b); got != want {
+		t.Errorf("diff = %q, want %q", got, want)
+	}
+}
+
+func TestUnifiedDiffInsertAtStart(t *testing.T) {
+	got := unifiedDiff("f", lines("x"), lines("new", "x"))
+	// The single existing line is shown as trailing context, so the hunk
+	// covers it.
+	want := "--- f\n+++ f\n@@ -1,1 +1,2 @@\n+new\n x\n"
+	if got != want {
+		t.Errorf("diff = %q, want %q", got, want)
+	}
+}
+
+func TestUnifiedDiffDeleteAtEnd(t *testing.T) {
+	got := unifiedDiff("f", lines("x", "y"), lines("x"))
+	want := "--- f\n+++ f\n@@ -1,2 +1,1 @@\n x\n-y\n"
+	if got != want {
+		t.Errorf("diff = %q, want %q", got, want)
+	}
+}
+
+func TestUnifiedDiffTwoHunks(t *testing.T) {
+	var a, b []string
+	for i := 1; i <= 20; i++ {
+		a = append(a, itoa(i))
+		b = append(b, itoa(i))
+	}
+	b[1] = "2!"
+	b[17] = "18!"
+	got := unifiedDiff("f", a, b)
+	if !strings.Contains(got, "@@ -1,5 +1,5 @@\n 1\n-2\n+2!\n 3\n 4\n 5\n") {
+		t.Errorf("first hunk wrong:\n%s", got)
+	}
+	if !strings.Contains(got, "@@ -15,6 +15,6 @@\n 15\n 16\n 17\n-18\n+18!\n 19\n 20\n") {
+		t.Errorf("second hunk wrong:\n%s", got)
+	}
+}
+
+// TestUnifiedDiffAdjacentHunks merges changes separated by exactly twice the
+// context into one hunk.
+func TestUnifiedDiffAdjacentHunks(t *testing.T) {
+	a := lines("1", "2", "3", "4", "5", "6", "7", "8")
+	b := slices.Clone(a)
+	b[0] = "1!"
+	b[7] = "8!"
+	got := unifiedDiff("f", a, b)
+	want := "--- f\n+++ f\n" +
+		"@@ -1,8 +1,8 @@\n" +
+		"-1\n+1!\n 2\n 3\n 4\n 5\n 6\n 7\n-8\n+8!\n"
+	if got != want {
+		t.Errorf("diff = %q, want %q", got, want)
+	}
+}
+
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	var buf [4]byte
+	i := len(buf)
+	for n > 0 {
+		i--
+		buf[i] = byte('0' + n%10)
+		n /= 10
+	}
+	return string(buf[i:])
+}