feat(verify): add universal --fuzz differential testing driven by // func signatures

Assisted-by: Qwen 3.8 Max Preview

cmd/gasm/main.go

@@ -30,7 +30,7 @@ import (
 
 // version is the release version, stamped at build time via
 // -ldflags "-X main.version=…" (defaulting to the current release).
-var version = "0.25.0"
+var version = "0.26.0"
 
 func main() {
 	if len(os.Args) < 2 {
@@ -492,6 +492,8 @@ With -profile, the static basic-block structure is listed for each function.
 	abi := fs.Bool("abi", false, "run ABI-checking calls (sentinel registers + red zone)")
 	profile := fs.Bool("profile", false, "list basic-block structure per function")
 	groundTruth := fs.Bool("ground-truth", false, "compare machine code byte-for-byte against go tool asm")
+	fuzz := fs.Bool("fuzz", false, "differential fuzz: JIT both gasm and go-tool-asm versions, compare outputs")
+	fuzzN := fs.Int("n", 1000, "number of fuzz iterations per function")
 	fs.Parse(args)
 	if fs.NArg() != 1 {
 		fmt.Fprintln(os.Stderr, "usage: gasm verify [-smoke] [-abi] [-profile] <file.s>")
@@ -568,6 +570,43 @@ With -profile, the static basic-block structure is listed for each function.
 			rc = 1
 		}
 	}
+
+	// Differential fuzz: JIT both gasm and go-tool-asm, compare outputs.
+	// Each function runs in a subprocess so a crash (partial functions like
+	// decoders that fault on malformed input) doesn't kill the whole run.
+	if *fuzz {
+		gt, err := verify.GroundTruth(path)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "gasm verify: fuzz: %v\n", err)
+			return 1
+		}
+		src, err := readSource(path)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "gasm verify: %v\n", err)
+			return 1
+		}
+		sigs := verify.ExtractSignatures(src)
+		fuzzed := 0
+		for _, name := range names {
+			sig, ok := sigs[name]
+			if !ok {
+				fmt.Printf("  %s: SKIP (no // func signature)\n", name)
+				continue
+			}
+			goCode, ok := gt[name]
+			if !ok {
+				fmt.Printf("  %s: SKIP (not in go tool asm output)\n", name)
+				continue
+			}
+			res := k.FuzzFunc(name, sig, goCode, *fuzzN, int64(fuzzed*7+42))
+			fmt.Printf("  %s\n", res)
+			if !res.OK() {
+				rc = 1
+			}
+			fuzzed++
+		}
+		fmt.Printf("fuzz: %d functions tested, %d iterations each\n", fuzzed, *fuzzN)
+	}
 	for _, name := range names {
 		fl, _ := k.Func(name)
 		flags := ""