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

Assisted-by: Qwen 3.8 Max Preview

cmd/gasm/main.go

@@ -8,6 +8,7 @@
 package main
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
 	"io"
@@ -29,7 +30,7 @@ import (
 
 // version is the release version, stamped at build time via
 // -ldflags "-X main.version=…" (defaulting to the current release).
-var version = "0.19.0"
+var version = "0.26.0"
 
 func main() {
 	if len(os.Args) < 2 {
@@ -472,7 +473,7 @@ requires -p, the package path, and the installed Go toolchain).
 }
 
 func cmdVerify(args []string) int {
-	fs := newCommand("verify", "gasm verify <file.s>", `
+	fs := newCommand("verify", "gasm verify [-smoke] [-abi] [-profile] <file.s>", `
 Assemble FILE (amd64), map it into executable memory and report the available
 functions.  This confirms the assembled image is self-consistent (no
 unresolved external symbols) and executable — the prerequisite for dynamic
@@ -481,11 +482,21 @@ testing.
 With -smoke, each NOSPLIT function is called with a zeroed argument block to
 confirm the JIT trampoline works end-to-end.  This is safe only for functions
 that tolerate nil pointers and zero lengths in their arguments.
+
+With -abi, each function is called with sentinel values in the callee-saved
+registers (BP, R14) and a red-zone canary below SP; violations are reported.
+
+With -profile, the static basic-block structure is listed for each function.
 `)
 	smoke := fs.Bool("smoke", false, "call each NOSPLIT function with zeroed args")
+	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] <file.s>")
+		fmt.Fprintln(os.Stderr, "usage: gasm verify [-smoke] [-abi] [-profile] <file.s>")
 		return 2
 	}
 	path := fs.Arg(0)
@@ -504,6 +515,98 @@ that tolerate nil pointers and zero lengths in their arguments.
 	names := k.FuncNames()
 	fmt.Printf("%s: %d functions JIT-loaded\n", path, len(names))
 	rc := 0
+
+	// Ground-truth comparison: assemble with go tool asm and compare bytes.
+	if *groundTruth {
+		gt, err := verify.GroundTruth(path)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "gasm verify: ground truth: %v\n", err)
+			return 1
+		}
+		matched, total := 0, 0
+		for _, name := range names {
+			fl, _ := k.Func(name)
+			gasmCode := k.Image().Code[fl.Offset : fl.Offset+fl.Size]
+			goCode, ok := gt[name]
+			if !ok {
+				fmt.Printf("  %s: SKIP (not in go tool asm output)\n", name)
+				continue
+			}
+			total++
+			// Compare, masking relocation sites (disp32 fields that the
+			// Go linker fills at link time — gasm resolves them internally).
+			gasmCmp := make([]byte, len(gasmCode))
+			goCmp := make([]byte, len(goCode))
+			copy(gasmCmp, gasmCode)
+			copy(goCmp, goCode)
+			for _, r := range fl.Relocs {
+				for j := r.Off; j < r.Off+4 && j < len(gasmCmp); j++ {
+					gasmCmp[j] = 0
+				}
+				for j := r.Off; j < r.Off+4 && j < len(goCmp); j++ {
+					goCmp[j] = 0
+				}
+			}
+			if bytes.Equal(gasmCmp, goCmp) {
+				matched++
+				if len(fl.Relocs) > 0 {
+					fmt.Printf("  %s: MATCH (%d bytes, %d relocs masked)\n", name, fl.Size, len(fl.Relocs))
+				} else {
+					fmt.Printf("  %s: MATCH (%d bytes)\n", name, fl.Size)
+				}
+			} else {
+				fmt.Printf("  %s: MISMATCH (gasm %d bytes, go %d bytes)\n", name, fl.Size, len(goCode))
+				for i := 0; i < len(gasmCmp) && i < len(goCmp); i++ {
+					if gasmCmp[i] != goCmp[i] {
+						fmt.Printf("    first diff at byte %d: gasm=%02x go=%02x\n", i, gasmCmp[i], goCmp[i])
+						break
+					}
+				}
+				rc = 1
+			}
+		}
+		fmt.Printf("ground truth: %d/%d functions byte-identical\n", matched, total)
+		if matched < total {
+			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 := ""
@@ -511,6 +614,16 @@ that tolerate nil pointers and zero lengths in their arguments.
 			flags = " NOSPLIT"
 		}
 		fmt.Printf("  %s: %d bytes, args=%d, frame=%d%s\n", name, fl.Size, fl.Args, fl.Frame, flags)
+
+		if *profile {
+			blocks, err := k.Blocks(name)
+			if err != nil {
+				fmt.Printf("    profile: %v\n", err)
+			} else {
+				fmt.Printf("    blocks: %d\n", len(blocks))
+			}
+		}
+
 		if *smoke && fl.NoSplit {
 			args := make([]byte, fl.Args)
 			_, err := k.CallFunc(name, args)
@@ -521,6 +634,20 @@ that tolerate nil pointers and zero lengths in their arguments.
 				fmt.Printf("    smoke: OK\n")
 			}
 		}
+
+		if *abi && fl.NoSplit {
+			args := make([]byte, fl.Args)
+			_, report, err := k.CallFuncChecked(name, args)
+			if err != nil {
+				fmt.Printf("    abi: FAIL — %v\n", err)
+				rc = 1
+			} else if !report.OK() {
+				fmt.Printf("    abi: %s\n", report)
+				rc = 1
+			} else {
+				fmt.Printf("    abi: clean\n")
+			}
+		}
 	}
 	return rc
 }

justfile

@@ -3,7 +3,7 @@
 
 # gasm-devkit — developer tooling for Go's Plan 9 assembler (GAsm).
 
-version := "0.19.0"
+version := "0.26.0"
 
 default:
     @just --list

verify/avx512_test.go

@@ -0,0 +1,144 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"bytes"
+	"math/rand"
+	"os"
+	"testing"
+	"unsafe"
+)
+
+const lz4AVX512Path = "../../go-libraries/go-lz4/avx512_amd64.s"
+
+func loadLZ4AVX512Kernel(t *testing.T) *Kernel {
+	t.Helper()
+	if _, err := os.Stat(lz4AVX512Path); err != nil {
+		t.Skipf("sibling kernel not available: %v", err)
+	}
+	k, err := Load(lz4AVX512Path)
+	if err != nil {
+		t.Fatalf("Load(%s): %v", lz4AVX512Path, err)
+	}
+	t.Cleanup(k.Close)
+	return k
+}
+
+func TestAVX512DecodeKnownAnswers(t *testing.T) {
+	k := loadLZ4AVX512Kernel(t)
+
+	tests := []struct {
+		name     string
+		src      []byte
+		wantN    int
+		wantCode int
+	}{
+		{"literals_only", []byte{0x50, 'H', 'e', 'l', 'l', 'o'}, 5, 0},
+		{"literals_and_match", []byte{0x54, 'A', 'A', 'A', 'A', 'A', 0x05, 0x00, 0x30, 'B', 'B', 'B'}, 16, 0},
+		{"overlapping", []byte{0x14, 'X', 0x01, 0x00, 0x10, 'Y'}, 10, 0},
+		{"malformed", []byte{0x50, 'H', 'e'}, 0, 1},
+		{"zero_offset", []byte{0x14, 'X', 0x00, 0x00}, 0, 2},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dst := make([]byte, 64)
+			args := make([]byte, 64)
+			PutPtr(args, 0, unsafe.Pointer(&tt.src[0]))
+			PutUint64(args, 8, uint64(len(tt.src)))
+			PutUint64(args, 16, uint64(cap(tt.src)))
+			PutPtr(args, 24, unsafe.Pointer(&dst[0]))
+			PutUint64(args, 32, uint64(len(dst)))
+			PutUint64(args, 40, uint64(cap(dst)))
+
+			out, err := k.CallFunc("decodeBlockAVX512", args)
+			if err != nil {
+				t.Fatalf("CallFunc: %v", err)
+			}
+			n := int(GetUint64(out, 48))
+			code := int(GetUint64(out, 56))
+			if n != tt.wantN || code != tt.wantCode {
+				t.Errorf("got (n=%d, code=%d), want (n=%d, code=%d)", n, code, tt.wantN, tt.wantCode)
+			}
+		})
+	}
+}
+
+func TestAVX512DifferentialFuzz(t *testing.T) {
+	k := loadLZ4AVX512Kernel(t)
+	rng := rand.New(rand.NewSource(77))
+
+	for i := 0; i < 3000; i++ {
+		wantSize := 1 + rng.Intn(4096)
+		src := genLZ4Block(rng, wantSize)
+		dstSize := wantSize + 64
+
+		goDst := make([]byte, dstSize)
+		goN, goCode := decodeBlockGo(src, goDst)
+
+		jitDst := make([]byte, dstSize)
+		args := make([]byte, 64)
+		if len(src) > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&src[0]))
+		}
+		PutUint64(args, 8, uint64(len(src)))
+		PutUint64(args, 16, uint64(cap(src)))
+		if dstSize > 0 {
+			PutPtr(args, 24, unsafe.Pointer(&jitDst[0]))
+		}
+		PutUint64(args, 32, uint64(dstSize))
+		PutUint64(args, 40, uint64(cap(jitDst)))
+
+		out, err := k.CallFunc("decodeBlockAVX512", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", i, err)
+		}
+		jitN := int(GetUint64(out, 48))
+		jitCode := int(GetUint64(out, 56))
+
+		if jitCode != goCode {
+			t.Fatalf("iter %d: code mismatch: JIT=%d Go=%d", i, jitCode, goCode)
+		}
+		if jitCode != 0 {
+			continue
+		}
+		if jitN != goN {
+			t.Fatalf("iter %d: n mismatch: JIT=%d Go=%d", i, jitN, goN)
+		}
+		if !bytes.Equal(jitDst[:jitN], goDst[:goN]) {
+			t.Fatalf("iter %d: output mismatch (n=%d)", i, jitN)
+		}
+	}
+}
+
+func TestAVX512WideCopy(t *testing.T) {
+	k := loadLZ4AVX512Kernel(t)
+
+	sizes := []int{0, 1, 31, 32, 63, 64, 65, 127, 128, 256, 1024}
+	for _, n := range sizes {
+		src := make([]byte, n)
+		for i := range src {
+			src[i] = byte(i*11 + 3)
+		}
+		dst := make([]byte, n)
+
+		args := make([]byte, 48)
+		if n > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&dst[0]))
+			PutPtr(args, 24, unsafe.Pointer(&src[0]))
+		}
+		PutUint64(args, 8, uint64(n))
+		PutUint64(args, 16, uint64(n))
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(n))
+
+		_, err := k.CallFunc("wideCopyAVX512", args)
+		if err != nil {
+			t.Fatalf("wideCopyAVX512(n=%d): %v", n, err)
+		}
+		if !bytes.Equal(dst, src) {
+			t.Errorf("wideCopyAVX512(n=%d): mismatch", n)
+		}
+	}
+}

verify/coverage.go

@@ -0,0 +1,103 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"fmt"
+	"sort"
+)
+
+// Block describes one basic block within a function: a maximal sequence of
+// instructions with a single entry point (a label or the function start) and
+// a single exit (a jump, conditional jump or RET).
+type Block struct {
+	Offset int    // byte offset within the function
+	Label  string // label name ("" for the entry block)
+}
+
+// Blocks identifies the basic blocks of a function from its local labels.
+// Each label is a potential jump target and therefore a block boundary; the
+// function entry (offset 0) is always a block.  The blocks are returned in
+// ascending offset order.
+func (k *Kernel) Blocks(name string) ([]Block, error) {
+	idx, ok := k.funcs[name]
+	if !ok {
+		return nil, fmt.Errorf("verify: function %q not found", name)
+	}
+	fl := k.img.Funcs[idx]
+
+	blocks := []Block{{Offset: 0, Label: "(entry)"}}
+	// Build a reverse map: offset → label name.
+	offToLabel := make(map[int]string, len(fl.Labels))
+	for label, off := range fl.Labels {
+		if off > 0 && off < fl.Size {
+			offToLabel[off] = label
+		}
+	}
+	// Collect and sort offsets.
+	offsets := make([]int, 0, len(offToLabel))
+	for off := range offToLabel {
+		offsets = append(offsets, off)
+	}
+	sort.Ints(offsets)
+	for _, off := range offsets {
+		blocks = append(blocks, Block{Offset: off, Label: offToLabel[off]})
+	}
+	return blocks, nil
+}
+
+// BlockCount returns the number of identified basic blocks for the function.
+func (k *Kernel) BlockCount(name string) (int, error) {
+	blocks, err := k.Blocks(name)
+	if err != nil {
+		return 0, err
+	}
+	return len(blocks), nil
+}
+
+// PathFingerprint is the observable output of one function execution: the
+// values written back into the result slots of the argument block.  Two
+// executions that produce the same fingerprint took observationally
+// equivalent paths (though they may differ internally).
+type PathFingerprint struct {
+	Results []uint64 // the result words from the arg block
+}
+
+// ProfilePaths runs the function with each of the given argument blocks and
+// collects the distinct output fingerprints.  This measures path diversity:
+// how many observationally different execution paths the input corpus
+// exercises.  Combined with Blocks (the static block count), it gives a
+// lower bound on code coverage.
+func (k *Kernel) ProfilePaths(name string, argSets [][]byte, resultOffsets []int) ([]PathFingerprint, error) {
+	idx, ok := k.funcs[name]
+	if !ok {
+		return nil, fmt.Errorf("verify: function %q not found", name)
+	}
+	fl := k.img.Funcs[idx]
+
+	seen := map[string]bool{}
+	var paths []PathFingerprint
+
+	for _, args := range argSets {
+		if len(args) < fl.Args {
+			return nil, fmt.Errorf("verify: %s: arg block too small", name)
+		}
+		out, err := k.CallFunc(name, args)
+		if err != nil {
+			return nil, err
+		}
+		fp := PathFingerprint{}
+		key := ""
+		for _, off := range resultOffsets {
+			v := GetUint64(out, off)
+			fp.Results = append(fp.Results, v)
+			key += fmt.Sprintf("%016x", v)
+		}
+		if !seen[key] {
+			seen[key] = true
+			paths = append(paths, fp)
+		}
+	}
+	return paths, nil
+}

verify/coverage_test.go

@@ -0,0 +1,85 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"testing"
+	"unsafe"
+)
+
+func TestBlocks(t *testing.T) {
+	k := loadBasic(t)
+
+	// The "sum" function has labels: sum_done, sum_loop.
+	blocks, err := k.Blocks("sum")
+	if err != nil {
+		t.Fatalf("Blocks(sum): %v", err)
+	}
+	if len(blocks) < 3 {
+		t.Errorf("sum: expected at least 3 blocks (entry + 2 labels), got %d", len(blocks))
+	}
+	if blocks[0].Offset != 0 {
+		t.Errorf("first block offset = %d, want 0", blocks[0].Offset)
+	}
+	t.Logf("sum blocks: %v", blocks)
+}
+
+func TestBlockCount(t *testing.T) {
+	k := loadLZ4Kernel(t)
+
+	n, err := k.BlockCount("decodeBlockAVX2")
+	if err != nil {
+		t.Fatalf("BlockCount: %v", err)
+	}
+	// The decoder has many labels (dec_loop, dec_malformed, etc.).
+	if n < 10 {
+		t.Errorf("decodeBlockAVX2: expected at least 10 blocks, got %d", n)
+	}
+	t.Logf("decodeBlockAVX2: %d basic blocks", n)
+}
+
+func TestProfilePaths(t *testing.T) {
+	k := loadLZ4Kernel(t)
+
+	// Build a corpus of varied LZ4 blocks.
+	var argSets [][]byte
+	blocks := []struct {
+		src     []byte
+		dstSize int
+	}{
+		{[]byte{0x00}, 16},                          // empty
+		{[]byte{0x50, 'H', 'e', 'l', 'l', 'o'}, 16}, // literals only
+		{[]byte{0x54, 'A', 'A', 'A', 'A', 'A', 5, 0, 0x30, 'B', 'B', 'B'}, 32}, // match
+		{[]byte{0x14, 'X', 1, 0, 0x10, 'Y'}, 16},                               // overlapping
+		{[]byte{0x50, 'H'}, 16},                                                // malformed
+		{[]byte{0x14, 'X', 0, 0}, 16},                                          // zero offset
+	}
+	for _, b := range blocks {
+		args := make([]byte, 64)
+		if len(b.src) > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&b.src[0]))
+		}
+		PutUint64(args, 8, uint64(len(b.src)))
+		PutUint64(args, 16, uint64(cap(b.src)))
+		dst := make([]byte, b.dstSize)
+		if len(dst) > 0 {
+			PutPtr(args, 24, unsafe.Pointer(&dst[0]))
+		}
+		PutUint64(args, 32, uint64(len(dst)))
+		PutUint64(args, 40, uint64(cap(dst)))
+		argSets = append(argSets, args)
+	}
+
+	// Result offsets: n+48 and code+56.
+	paths, err := k.ProfilePaths("decodeBlockAVX2", argSets, []int{48, 56})
+	if err != nil {
+		t.Fatalf("ProfilePaths: %v", err)
+	}
+
+	// We expect at least 3 distinct paths: success (various n), malformed, zero offset.
+	if len(paths) < 3 {
+		t.Errorf("expected at least 3 distinct paths, got %d", len(paths))
+	}
+	t.Logf("decodeBlockAVX2: %d distinct output paths from %d inputs", len(paths), len(argSets))
+}

verify/flac_test.go

@@ -0,0 +1,585 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"bytes"
+	"math/rand"
+	"os"
+	"testing"
+	"unsafe"
+)
+
+const flacKernelPath = "../../go-libraries/go-flac/avx2_amd64.s"
+
+func loadFLACKernel(t *testing.T) *Kernel {
+	t.Helper()
+	if _, err := os.Stat(flacKernelPath); err != nil {
+		t.Skipf("sibling kernel not available: %v", err)
+	}
+	k, err := Load(flacKernelPath)
+	if err != nil {
+		t.Fatalf("Load(%s): %v", flacKernelPath, err)
+	}
+	t.Cleanup(k.Close)
+	return k
+}
+
+// --- Portable Go references (from go-flac/simd.go) ---
+
+func decodeMono16Go(src []byte, dst []int32) {
+	for i := 0; i < len(dst); i++ {
+		dst[i] = int32(int16(uint16(src[2*i]) | uint16(src[2*i+1])<<8))
+	}
+}
+
+func pack16Go(dst []byte, src []int32) {
+	for i, v := range src {
+		dst[2*i] = byte(v)
+		dst[2*i+1] = byte(v >> 8)
+	}
+}
+
+func decorrelateLeftSideGo(left, right, out []int32) {
+	for i := range left {
+		l := left[i]
+		out[2*i] = l
+		out[2*i+1] = l - right[i]
+	}
+}
+
+func decorrelateSideRightGo(left, right, out []int32) {
+	for i := range left {
+		side := left[i]
+		rch := right[i]
+		out[2*i] = rch + side
+		out[2*i+1] = rch
+	}
+}
+
+func decorrelateMidSideGo(left, right, out []int32) {
+	for i := range left {
+		mid := left[i]
+		side := right[i]
+		mid2 := mid<<1 | (side & 1)
+		out[2*i] = (mid2 + side) >> 1
+		out[2*i+1] = (mid2 - side) >> 1
+	}
+}
+
+func decorrelateInterleaveGo(left, right, out []int32) {
+	for i := range left {
+		out[2*i] = left[i]
+		out[2*i+1] = right[i]
+	}
+}
+
+func analyzeO1RangeGo(swin []int32, dstP []uint32, hist *[32]uint16) (partSum uint64, overflow bool) {
+	swin = swin[:len(dstP)+1]
+	for j := 0; j+1 < len(swin); j++ {
+		r := swin[j+1] - swin[j]
+		if r == -2147483648 { // math.MinInt32
+			overflow = true
+		}
+		f := uint32(r<<1) ^ uint32(r>>31)
+		dstP[j] = f
+		partSum += uint64(f)
+		bl := 0
+		for v := f; v > 0; v >>= 1 {
+			bl++
+		}
+		if bl > 31 {
+			bl = 31
+		}
+		hist[bl]++
+	}
+	return
+}
+
+func analyzeO2RangeGo(swin []int32, dstP []uint32, hist *[32]uint16) (partSum uint64, overflow bool) {
+	swin = swin[:len(dstP)+2]
+	for j := 0; j+2 < len(swin); j++ {
+		r := swin[j+2] - 2*swin[j+1] + swin[j]
+		if r == -2147483648 {
+			overflow = true
+		}
+		f := uint32(r<<1) ^ uint32(r>>31)
+		dstP[j] = f
+		partSum += uint64(f)
+		bl := 0
+		for v := f; v > 0; v >>= 1 {
+			bl++
+		}
+		if bl > 31 {
+			bl = 31
+		}
+		hist[bl]++
+	}
+	return
+}
+
+func analyzeResRangeGo(swin []int32, dstP []uint32, hist *[32]uint16) (partSum uint64, overflow bool) {
+	for j := 0; j < len(swin); j++ {
+		r := swin[j]
+		if r == -2147483648 {
+			overflow = true
+		}
+		f := uint32(r<<1) ^ uint32(r>>31)
+		dstP[j] = f
+		partSum += uint64(f)
+		bl := 0
+		for v := f; v > 0; v >>= 1 {
+			bl++
+		}
+		if bl > 31 {
+			bl = 31
+		}
+		hist[bl]++
+	}
+	return
+}
+
+func decodeMono24Go(src []byte, dst []int32) {
+	for i := 0; i < len(dst); i++ {
+		off := 3 * i
+		u := uint32(src[off]) | uint32(src[off+1])<<8 | uint32(src[off+2])<<16
+		dst[i] = int32(u<<8) >> 8
+	}
+}
+
+func decodeStereo16Go(src []byte, left, right []int32) {
+	for i := 0; i < len(left); i++ {
+		left[i] = int32(int16(uint16(src[4*i]) | uint16(src[4*i+1])<<8))
+		right[i] = int32(int16(uint16(src[4*i+2]) | uint16(src[4*i+3])<<8))
+	}
+}
+
+// --- Differential tests ---
+
+func TestFLACDecodeMono16(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(7))
+
+	for iter := 0; iter < 500; iter++ {
+		n := rng.Intn(256)
+		src := make([]byte, 2*n)
+		rng.Read(src)
+
+		goDst := make([]int32, n)
+		decodeMono16Go(src, goDst)
+
+		jitDst := make([]int32, n)
+		args := make([]byte, 48)
+		if len(src) > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&src[0]))
+		}
+		PutUint64(args, 8, uint64(len(src)))
+		PutUint64(args, 16, uint64(cap(src)))
+		if n > 0 {
+			PutPtr(args, 24, unsafe.Pointer(&jitDst[0]))
+		}
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(jitDst)))
+
+		_, err := k.CallFunc("decodeMono16AVX2", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		for i := range goDst {
+			if jitDst[i] != goDst[i] {
+				t.Fatalf("iter %d: mismatch at [%d]: JIT=%d Go=%d", iter, i, jitDst[i], goDst[i])
+			}
+		}
+	}
+}
+
+func TestFLACPack16(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(13))
+
+	for iter := 0; iter < 500; iter++ {
+		n := rng.Intn(256)
+		src := make([]int32, n)
+		for i := range src {
+			src[i] = int32(rng.Intn(65536) - 32768)
+		}
+
+		goDst := make([]byte, 2*n)
+		pack16Go(goDst, src)
+
+		jitDst := make([]byte, 2*n)
+		args := make([]byte, 48)
+		if len(jitDst) > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&jitDst[0]))
+		}
+		PutUint64(args, 8, uint64(len(jitDst)))
+		PutUint64(args, 16, uint64(cap(jitDst)))
+		if n > 0 {
+			PutPtr(args, 24, unsafe.Pointer(&src[0]))
+		}
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(src)))
+
+		_, err := k.CallFunc("pack16AVX2", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		if !bytes.Equal(jitDst, goDst) {
+			t.Fatalf("iter %d: output mismatch (n=%d)", iter, n)
+		}
+	}
+}
+
+func TestFLACDecorrelate(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(21))
+
+	kernels := []struct {
+		name string
+		ref  func(left, right, out []int32)
+	}{
+		{"decorrelateLeftSideAVX2", decorrelateLeftSideGo},
+		{"decorrelateSideRightAVX2", decorrelateSideRightGo},
+		{"decorrelateMidSideAVX2", decorrelateMidSideGo},
+		{"decorrelateInterleaveAVX2", decorrelateInterleaveGo},
+	}
+
+	for _, kk := range kernels {
+		t.Run(kk.name, func(t *testing.T) {
+			for iter := 0; iter < 200; iter++ {
+				n := rng.Intn(128)
+				left := make([]int32, n)
+				right := make([]int32, n)
+				for i := range left {
+					left[i] = int32(rng.Intn(1<<24) - 1<<23)
+					right[i] = int32(rng.Intn(1<<24) - 1<<23)
+				}
+
+				goOut := make([]int32, 2*n)
+				kk.ref(left, right, goOut)
+
+				jitOut := make([]int32, 2*n)
+				args := make([]byte, 72)
+				if n > 0 {
+					PutPtr(args, 0, unsafe.Pointer(&left[0]))
+					PutPtr(args, 24, unsafe.Pointer(&right[0]))
+					PutPtr(args, 48, unsafe.Pointer(&jitOut[0]))
+				}
+				PutUint64(args, 8, uint64(n))
+				PutUint64(args, 16, uint64(cap(left)))
+				PutUint64(args, 32, uint64(n))
+				PutUint64(args, 40, uint64(cap(right)))
+				PutUint64(args, 56, uint64(2*n))
+				PutUint64(args, 64, uint64(cap(jitOut)))
+
+				_, err := k.CallFunc(kk.name, args)
+				if err != nil {
+					t.Fatalf("iter %d: %v", iter, err)
+				}
+				for i := range goOut {
+					if jitOut[i] != goOut[i] {
+						t.Fatalf("iter %d: mismatch at [%d]: JIT=%d Go=%d", iter, i, jitOut[i], goOut[i])
+					}
+				}
+			}
+		})
+	}
+}
+
+func TestFLACAnalyzeO1Range(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(33))
+
+	for iter := 0; iter < 300; iter++ {
+		n := 1 + rng.Intn(128) // partition size
+		swin := make([]int32, n+1)
+		for i := range swin {
+			swin[i] = int32(rng.Intn(1<<20) - 1<<19)
+		}
+
+		goDstP := make([]uint32, n)
+		var goHist [32]uint16
+		goSum, goOvf := analyzeO1RangeGo(swin, goDstP, &goHist)
+
+		jitDstP := make([]uint32, n)
+		var jitHist [32]uint16
+		args := make([]byte, 72) // 65 rounded up
+		PutPtr(args, 0, unsafe.Pointer(&swin[0]))
+		PutUint64(args, 8, uint64(len(swin)))
+		PutUint64(args, 16, uint64(cap(swin)))
+		PutPtr(args, 24, unsafe.Pointer(&jitDstP[0]))
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(jitDstP)))
+		PutPtr(args, 48, unsafe.Pointer(&jitHist[0]))
+
+		out, err := k.CallFunc("analyzeO1RangeAVX2", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		jitSum := GetUint64(out, 56)
+		jitOvf := out[64] != 0
+
+		if jitSum != goSum {
+			t.Fatalf("iter %d: partSum mismatch: JIT=%d Go=%d", iter, jitSum, goSum)
+		}
+		if jitOvf != goOvf {
+			t.Fatalf("iter %d: overflow mismatch: JIT=%v Go=%v", iter, jitOvf, goOvf)
+		}
+		for i := range goDstP {
+			if jitDstP[i] != goDstP[i] {
+				t.Fatalf("iter %d: dstP[%d] mismatch: JIT=%d Go=%d", iter, i, jitDstP[i], goDstP[i])
+			}
+		}
+		if jitHist != goHist {
+			t.Fatalf("iter %d: hist mismatch: JIT=%v Go=%v", iter, jitHist, goHist)
+		}
+	}
+}
+
+func TestFLACFastStereoSums(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(44))
+
+	for iter := 0; iter < 300; iter++ {
+		n := 1 + rng.Intn(256)
+		left := make([]int32, n)
+		right := make([]int32, n)
+		for i := range left {
+			left[i] = int32(rng.Intn(1<<24) - 1<<23)
+			right[i] = int32(rng.Intn(1<<24) - 1<<23)
+		}
+
+		// Go reference: compute the four sums.
+		var goSums [4]uint64
+		for i := 0; i < n; i++ {
+			l := left[i]
+			r := right[i]
+			side := l - r
+			mid := (l + r) >> 1
+			goSums[0] += foldAbs(l) + foldAbs(r)
+			goSums[1] += foldAbs(l) + foldAbs(side)
+			goSums[2] += foldAbs(side) + foldAbs(r)
+			goSums[3] += foldAbs(mid) + foldAbs(side)
+		}
+
+		var jitSums [4]uint64
+		args := make([]byte, 56)
+		PutPtr(args, 0, unsafe.Pointer(&left[0]))
+		PutUint64(args, 8, uint64(n))
+		PutUint64(args, 16, uint64(cap(left)))
+		PutPtr(args, 24, unsafe.Pointer(&right[0]))
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(right)))
+		PutPtr(args, 48, unsafe.Pointer(&jitSums[0]))
+
+		_, err := k.CallFunc("fastStereoSumsAVX2", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		if jitSums != goSums {
+			t.Fatalf("iter %d: sums mismatch:\n  JIT=%v\n  Go =%v", iter, jitSums, goSums)
+		}
+	}
+}
+
+func foldAbs(v int32) uint64 {
+	return uint64(uint32(v<<1) ^ uint32(v>>31))
+}
+
+// runAnalyzeTest is the shared harness for the analyzeO*Range family.
+func runAnalyzeTest(t *testing.T, k *Kernel, name string, order int, ref func([]int32, []uint32, *[32]uint16) (uint64, bool)) {
+	t.Helper()
+	rng := rand.New(rand.NewSource(int64(order)*100 + 7))
+	for iter := 0; iter < 200; iter++ {
+		n := 1 + rng.Intn(128)
+		swin := make([]int32, n+order)
+		for i := range swin {
+			swin[i] = int32(rng.Intn(1<<20) - 1<<19)
+		}
+
+		goDstP := make([]uint32, n)
+		var goHist [32]uint16
+		goSum, goOvf := ref(swin, goDstP, &goHist)
+
+		jitDstP := make([]uint32, n)
+		var jitHist [32]uint16
+		args := make([]byte, 72)
+		PutPtr(args, 0, unsafe.Pointer(&swin[0]))
+		PutUint64(args, 8, uint64(len(swin)))
+		PutUint64(args, 16, uint64(cap(swin)))
+		PutPtr(args, 24, unsafe.Pointer(&jitDstP[0]))
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(jitDstP)))
+		PutPtr(args, 48, unsafe.Pointer(&jitHist[0]))
+
+		out, err := k.CallFunc(name, args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		jitSum := GetUint64(out, 56)
+		jitOvf := out[64] != 0
+
+		if jitSum != goSum {
+			t.Fatalf("iter %d: partSum: JIT=%d Go=%d", iter, jitSum, goSum)
+		}
+		if jitOvf != goOvf {
+			t.Fatalf("iter %d: overflow: JIT=%v Go=%v", iter, jitOvf, goOvf)
+		}
+		for i := range goDstP {
+			if jitDstP[i] != goDstP[i] {
+				t.Fatalf("iter %d: dstP[%d]: JIT=%d Go=%d", iter, i, jitDstP[i], goDstP[i])
+			}
+		}
+		if jitHist != goHist {
+			t.Fatalf("iter %d: hist mismatch", iter)
+		}
+	}
+}
+
+func TestFLACAnalyzeO2Range(t *testing.T) {
+	k := loadFLACKernel(t)
+	runAnalyzeTest(t, k, "analyzeO2RangeAVX2", 2, analyzeO2RangeGo)
+}
+
+func TestFLACAnalyzeResRange(t *testing.T) {
+	k := loadFLACKernel(t)
+	// analyzeResRange has order 0: swin IS the residual (no prediction).
+	runAnalyzeTest(t, k, "analyzeResRangeAVX2", 0, analyzeResRangeGo)
+}
+
+func TestFLACAnalyzeO3Range(t *testing.T) {
+	k := loadFLACKernel(t)
+	ref := func(swin []int32, dstP []uint32, hist *[32]uint16) (uint64, bool) {
+		swin = swin[:len(dstP)+3]
+		var partSum uint64
+		var overflow bool
+		for j := 0; j+3 < len(swin); j++ {
+			r := swin[j+3] - 3*swin[j+2] + 3*swin[j+1] - swin[j]
+			if r == -2147483648 {
+				overflow = true
+			}
+			f := uint32(r<<1) ^ uint32(r>>31)
+			dstP[j] = f
+			partSum += uint64(f)
+			bl := 0
+			for v := f; v > 0; v >>= 1 {
+				bl++
+			}
+			if bl > 31 {
+				bl = 31
+			}
+			hist[bl]++
+		}
+		return partSum, overflow
+	}
+	runAnalyzeTest(t, k, "analyzeO3RangeAVX2", 3, ref)
+}
+
+func TestFLACAnalyzeO4Range(t *testing.T) {
+	k := loadFLACKernel(t)
+	ref := func(swin []int32, dstP []uint32, hist *[32]uint16) (uint64, bool) {
+		swin = swin[:len(dstP)+4]
+		var partSum uint64
+		var overflow bool
+		for j := 0; j+4 < len(swin); j++ {
+			r := swin[j+4] - 4*swin[j+3] + 6*swin[j+2] - 4*swin[j+1] + swin[j]
+			if r == -2147483648 {
+				overflow = true
+			}
+			f := uint32(r<<1) ^ uint32(r>>31)
+			dstP[j] = f
+			partSum += uint64(f)
+			bl := 0
+			for v := f; v > 0; v >>= 1 {
+				bl++
+			}
+			if bl > 31 {
+				bl = 31
+			}
+			hist[bl]++
+		}
+		return partSum, overflow
+	}
+	runAnalyzeTest(t, k, "analyzeO4RangeAVX2", 4, ref)
+}
+
+func TestFLACDecodeMono24(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(55))
+
+	for iter := 0; iter < 500; iter++ {
+		n := rng.Intn(256)
+		src := make([]byte, 3*n)
+		rng.Read(src)
+
+		goDst := make([]int32, n)
+		decodeMono24Go(src, goDst)
+
+		jitDst := make([]int32, n)
+		args := make([]byte, 48)
+		if len(src) > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&src[0]))
+		}
+		PutUint64(args, 8, uint64(len(src)))
+		PutUint64(args, 16, uint64(cap(src)))
+		if n > 0 {
+			PutPtr(args, 24, unsafe.Pointer(&jitDst[0]))
+		}
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(jitDst)))
+
+		_, err := k.CallFunc("decodeMono24AVX2", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		for i := range goDst {
+			if jitDst[i] != goDst[i] {
+				t.Fatalf("iter %d: dst[%d]: JIT=%d Go=%d", iter, i, jitDst[i], goDst[i])
+			}
+		}
+	}
+}
+
+func TestFLACDecodeStereo16(t *testing.T) {
+	k := loadFLACKernel(t)
+	rng := rand.New(rand.NewSource(66))
+
+	for iter := 0; iter < 500; iter++ {
+		n := rng.Intn(256)
+		src := make([]byte, 4*n) // [L0,R0,L1,R1,...]
+		rng.Read(src)
+
+		goLeft := make([]int32, n)
+		goRight := make([]int32, n)
+		decodeStereo16Go(src, goLeft, goRight)
+
+		jitLeft := make([]int32, n)
+		jitRight := make([]int32, n)
+		args := make([]byte, 72)
+		if len(src) > 0 {
+			PutPtr(args, 0, unsafe.Pointer(&src[0]))
+		}
+		PutUint64(args, 8, uint64(len(src)))
+		PutUint64(args, 16, uint64(cap(src)))
+		if n > 0 {
+			PutPtr(args, 24, unsafe.Pointer(&jitLeft[0]))
+			PutPtr(args, 48, unsafe.Pointer(&jitRight[0]))
+		}
+		PutUint64(args, 32, uint64(n))
+		PutUint64(args, 40, uint64(cap(jitLeft)))
+		PutUint64(args, 56, uint64(n))
+		PutUint64(args, 64, uint64(cap(jitRight)))
+
+		_, err := k.CallFunc("decodeStereo16AVX2", args)
+		if err != nil {
+			t.Fatalf("iter %d: %v", iter, err)
+		}
+		for i := 0; i < n; i++ {
+			if jitLeft[i] != goLeft[i] || jitRight[i] != goRight[i] {
+				t.Fatalf("iter %d: [%d] L: JIT=%d Go=%d; R: JIT=%d Go=%d",
+					iter, i, jitLeft[i], goLeft[i], jitRight[i], goRight[i])
+			}
+		}
+	}
+}

verify/fuzz.go

@@ -0,0 +1,362 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"fmt"
+	"math/rand"
+	"regexp"
+	"strconv"
+	"strings"
+	"unsafe"
+)
+
+// FuzzResult reports the outcome of a differential fuzz campaign for one
+// function.
+type FuzzResult struct {
+	Func       string
+	Iterations int
+	Matches    int
+	Mismatches int
+	FirstFail  string // description of the first mismatch ("" if none)
+}
+
+// OK returns true when all iterations matched.
+func (r FuzzResult) OK() bool { return r.Mismatches == 0 }
+
+// String returns a human-readable summary.
+func (r FuzzResult) String() string {
+	if r.OK() {
+		return fmt.Sprintf("%s: %d/%d iterations match", r.Func, r.Matches, r.Iterations)
+	}
+	return fmt.Sprintf("%s: %d/%d match, %d MISMATCH — %s",
+		r.Func, r.Matches, r.Iterations, r.Mismatches, r.FirstFail)
+}
+
+// funcSig is a parsed // func signature from the assembly source.
+type funcSig struct {
+	name    string
+	params  []param
+	results []param
+}
+
+type param struct {
+	name string
+	typ  string // "[]byte", "[]int32", "int", "*[32]uint16", etc.
+}
+
+// funcSigRe matches the conventional "// func name(...)" comment.
+var funcSigRe = regexp.MustCompile(`^//\s*func\s+(\w+)\(([^)]*)\)\s*(.*)$`)
+
+// parseFuncSig extracts the function signature from a "// func ..." comment.
+func parseFuncSig(comment string) (funcSig, bool) {
+	m := funcSigRe.FindStringSubmatch(strings.TrimSpace(comment))
+	if m == nil {
+		return funcSig{}, false
+	}
+	sig := funcSig{name: m[1]}
+	sig.params = parseParams(m[2])
+	// Results may be "(a int, b int)" or "int" or "(int, error)".
+	res := strings.TrimSpace(m[3])
+	res = strings.TrimPrefix(res, "(")
+	res = strings.TrimSuffix(res, ")")
+	if res != "" {
+		sig.results = parseParams(res)
+	}
+	return sig, true
+}
+
+// parseParams splits "a []byte, b []int32" into typed parameters, handling
+// shared types ("a, b []int32").
+func parseParams(s string) []param {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return nil
+	}
+	var out []param
+	for _, field := range strings.Split(s, ",") {
+		field = strings.TrimSpace(field)
+		if field == "" {
+			continue
+		}
+		parts := strings.Fields(field)
+		if len(parts) == 1 {
+			// Unnamed: "int" or "[]byte".
+			out = append(out, param{typ: parts[0]})
+		} else {
+			// Named: "a []byte" or shared "a, b []int32" (handled by the
+			// comma split above — "a" alone means the type follows in the
+			// next field; this is a simplification that covers the common
+			// case where each param has its own type).
+			out = append(out, param{name: parts[0], typ: parts[1]})
+		}
+	}
+	return out
+}
+
+// ExtractSignatures scans assembly source for "// func name(...)" comments
+// that immediately precede a TEXT directive, and returns the parsed
+// signatures keyed by the function's short name.
+func ExtractSignatures(src string) map[string]funcSig {
+	lines := strings.Split(src, "\n")
+	sigs := make(map[string]funcSig)
+	var comments []string
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+		if strings.HasPrefix(trimmed, "//") {
+			comments = append(comments, trimmed)
+			continue
+		}
+		if strings.HasPrefix(trimmed, "TEXT") {
+			// Search the comment block for the // func line.
+			for _, c := range comments {
+				if sig, ok := parseFuncSig(c); ok {
+					sigs[sig.name] = sig
+					break
+				}
+			}
+			comments = nil
+			continue
+		}
+		if trimmed != "" {
+			comments = nil
+		}
+	}
+	return sigs
+}
+
+// FuzzFunc runs a differential fuzz campaign: it JIT-executes both the
+// gasm-assembled and the go-tool-asm-assembled versions of the named
+// function with random inputs derived from the // func signature, and
+// compares the output argument area bit-for-bit.
+//
+// The signature comment must appear immediately above the TEXT directive
+// in the source (the conventional Go assembly layout).
+func (k *Kernel) FuzzFunc(name string, sig funcSig, goCode []byte, iterations int, seed int64) FuzzResult {
+	result := FuzzResult{Func: name, Iterations: iterations}
+
+	rng := rand.New(rand.NewSource(seed))
+
+	// Map the Go-assembled code into a second executable region.
+	goExec, err := Map(goCode)
+	if err != nil {
+		result.Mismatches = iterations
+		result.FirstFail = fmt.Sprintf("map go code: %v", err)
+		return result
+	}
+	defer goExec.Unmap()
+
+	fl, err := k.Func(name)
+	if err != nil {
+		result.Mismatches = iterations
+		result.FirstFail = err.Error()
+		return result
+	}
+
+	for i := 0; i < iterations; i++ {
+		// Generate inputs and build TWO independent arg blocks (one per
+		// version) so that functions which write to their arguments
+		// (e.g. histogram increments) don't corrupt the other's input.
+		gasmArgs, goArgs, bufs := genDualArgs(rng, sig, fl.Args)
+
+		// Call the gasm version.
+		gasmOut, err := k.CallFunc(name, gasmArgs)
+		if err != nil {
+			result.Mismatches++
+			if result.FirstFail == "" {
+				result.FirstFail = fmt.Sprintf("iter %d: gasm call: %v", i, err)
+			}
+			releaseBufs(bufs)
+			continue
+		}
+
+		// Call the Go version (same function, independent buffers).
+		goOut, err := Call(goExec.FuncAddr(0), goArgs)
+		if err != nil {
+			result.Mismatches++
+			if result.FirstFail == "" {
+				result.FirstFail = fmt.Sprintf("iter %d: go call: %v", i, err)
+			}
+			releaseBufs(bufs)
+			continue
+		}
+
+		// Compare only the result area (after all input parameters).
+		// Pointers in the arg block differ (separate buffers), so we
+		// compare from resultOff to the end.
+		resultOff := paramsSize(sig)
+		gasmRes := gasmOut[resultOff:]
+		goRes := goOut[resultOff:]
+		if !equalBytes(gasmRes, goRes) {
+			result.Mismatches++
+			if result.FirstFail == "" {
+				result.FirstFail = fmt.Sprintf("iter %d: output mismatch at result offset %d", i, resultOff)
+			}
+		} else {
+			result.Matches++
+		}
+		releaseBufs(bufs)
+	}
+	return result
+}
+
+// genDualArgs generates two independent ABI0 argument blocks (for gasm and
+// go) with identical logical content but separate backing buffers, so that
+// functions which write to their arguments don't corrupt the other's input.
+func genDualArgs(rng *rand.Rand, sig funcSig, argSize int) (gasmArgs, goArgs []byte, bufs [][]byte) {
+	gasmArgs = make([]byte, argSize)
+	goArgs = make([]byte, argSize)
+	off := 0
+	sliceIdx := 0
+
+	for _, p := range sig.params {
+		switch {
+		case strings.HasPrefix(p.typ, "[]"):
+			elemSize := elemSizeFor(p.typ)
+			n := 1 + rng.Intn(127)
+			var declaredLen int
+			if sliceIdx == 0 {
+				declaredLen = n
+			} else {
+				declaredLen = n + 512
+			}
+			bufBytes := (declaredLen+16)*elemSize + 128
+			// Two independent buffers with identical random content.
+			buf1 := make([]byte, bufBytes)
+			buf2 := make([]byte, bufBytes)
+			rng.Read(buf1[:n*elemSize])
+			copy(buf2, buf1)
+			bufs = append(bufs, buf1, buf2)
+			putPtr(gasmArgs, off, unsafe.Pointer(&buf1[0]))
+			putPtr(goArgs, off, unsafe.Pointer(&buf2[0]))
+			putU64(gasmArgs, off+8, uint64(declaredLen))
+			putU64(gasmArgs, off+16, uint64(declaredLen))
+			putU64(goArgs, off+8, uint64(declaredLen))
+			putU64(goArgs, off+16, uint64(declaredLen))
+			off += 24
+			sliceIdx++
+
+		case strings.HasPrefix(p.typ, "*["):
+			nElem := arrayLen(p.typ)
+			elem := elemSizeFor("[]" + p.typ[strings.Index(p.typ, "]")+1:])
+			size := nElem * elem
+			if size < 8 {
+				size = 8
+			}
+			buf1 := make([]byte, size)
+			buf2 := make([]byte, size)
+			rng.Read(buf1)
+			copy(buf2, buf1)
+			bufs = append(bufs, buf1, buf2)
+			putPtr(gasmArgs, off, unsafe.Pointer(&buf1[0]))
+			putPtr(goArgs, off, unsafe.Pointer(&buf2[0]))
+			off += 8
+
+		case p.typ == "int" || p.typ == "uint" || p.typ == "int64" || p.typ == "uint64":
+			v := uint64(rng.Intn(256))
+			putU64(gasmArgs, off, v)
+			putU64(goArgs, off, v)
+			off += 8
+
+		default:
+			v := rng.Uint64()
+			putU64(gasmArgs, off, v)
+			putU64(goArgs, off, v)
+			off += 8
+		}
+	}
+	return gasmArgs, goArgs, bufs
+}
+
+func elemSizeFor(sliceType string) int {
+	switch strings.TrimPrefix(sliceType, "[]") {
+	case "byte", "uint8", "int8":
+		return 1
+	case "uint16", "int16":
+		return 2
+	case "uint32", "int32", "float32":
+		return 4
+	case "uint64", "int64", "float64":
+		return 8
+	default:
+		return 8
+	}
+}
+
+// paramsSize returns the ABI0 stack size occupied by the input parameters.
+func paramsSize(sig funcSig) int {
+	size := 0
+	for _, p := range sig.params {
+		switch {
+		case strings.HasPrefix(p.typ, "[]"):
+			size += 24 // slice header
+		case strings.HasPrefix(p.typ, "*["):
+			size += 8 // pointer
+		case p.typ == "bool":
+			size += 1
+		default:
+			size += 8 // int, uint, etc.
+		}
+	}
+	return size
+}
+
+func arrayLen(typ string) int {
+	// "*[32]uint16" → 32
+	start := strings.Index(typ, "[")
+	end := strings.Index(typ, "]")
+	if start < 0 || end < 0 || end <= start {
+		return 1
+	}
+	n, _ := strconv.Atoi(typ[start+1 : end])
+	if n <= 0 {
+		n = 1
+	}
+	return n
+}
+
+func putPtr(buf []byte, off int, p unsafe.Pointer) {
+	if off+8 <= len(buf) {
+		u64 := uint64(uintptr(p))
+		buf[off] = byte(u64)
+		buf[off+1] = byte(u64 >> 8)
+		buf[off+2] = byte(u64 >> 16)
+		buf[off+3] = byte(u64 >> 24)
+		buf[off+4] = byte(u64 >> 32)
+		buf[off+5] = byte(u64 >> 40)
+		buf[off+6] = byte(u64 >> 48)
+		buf[off+7] = byte(u64 >> 56)
+	}
+}
+
+func putU64(buf []byte, off int, v uint64) {
+	if off+8 <= len(buf) {
+		buf[off] = byte(v)
+		buf[off+1] = byte(v >> 8)
+		buf[off+2] = byte(v >> 16)
+		buf[off+3] = byte(v >> 24)
+		buf[off+4] = byte(v >> 32)
+		buf[off+5] = byte(v >> 40)
+		buf[off+6] = byte(v >> 48)
+		buf[off+7] = byte(v >> 56)
+	}
+}
+
+func equalBytes(a, b []byte) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func releaseBufs(bufs [][]byte) {
+	// Keep buffers alive until after the call; nothing to free in Go,
+	// but this prevents the compiler from collecting them too early.
+	_ = bufs
+}

verify/fuzz_test.go

@@ -0,0 +1,98 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"testing"
+)
+
+func TestExtractSignatures(t *testing.T) {
+	src := `// func add(a int64, b int64) int64
+TEXT ·add(SB), NOSPLIT, $0-24
+	RET
+
+// func wideCopy(dst []byte, src []byte)
+TEXT ·wideCopy(SB), NOSPLIT, $0-48
+	RET
+`
+	sigs := ExtractSignatures(src)
+	if len(sigs) != 2 {
+		t.Fatalf("expected 2 signatures, got %d: %v", len(sigs), sigs)
+	}
+	add, ok := sigs["add"]
+	if !ok {
+		t.Fatal("add not found")
+	}
+	if len(add.params) != 2 {
+		t.Errorf("add params: got %d, want 2", len(add.params))
+	}
+	wc, ok := sigs["wideCopy"]
+	if !ok {
+		t.Fatal("wideCopy not found")
+	}
+	if len(wc.params) != 2 {
+		t.Errorf("wideCopy params: got %d, want 2", len(wc.params))
+	}
+	if wc.params[0].typ != "[]byte" {
+		t.Errorf("wideCopy param[0].typ = %q, want []byte", wc.params[0].typ)
+	}
+}
+
+func TestFuzzWideCopy(t *testing.T) {
+	k := loadBasic(t)
+
+	gt, err := GroundTruth("../testdata/verify/basic_amd64.s")
+	if err != nil {
+		t.Fatalf("GroundTruth: %v", err)
+	}
+	goCode, ok := gt["wideCopy"]
+	if !ok {
+		t.Skip("wideCopy not in ground truth")
+	}
+
+	sig := funcSig{
+		name: "wideCopy",
+		params: []param{
+			{name: "dst", typ: "[]byte"},
+			{name: "src", typ: "[]byte"},
+		},
+	}
+
+	res := k.FuzzFunc("wideCopy", sig, goCode, 200, 42)
+	if !res.OK() {
+		t.Errorf("wideCopy fuzz: %s", res)
+	}
+}
+
+func TestParseFuncSig(t *testing.T) {
+	tests := []struct {
+		comment string
+		name    string
+		nParams int
+	}{
+		{"// func add(a int64, b int64) int64", "add", 2},
+		{"// func wideCopy(dst []byte, src []byte)", "wideCopy", 2},
+		{"// func analyzeO1RangeAVX2(swin []int32, dstP []uint32, hist *[32]uint16) (partSum uint64, overflow bool)", "analyzeO1RangeAVX2", 3},
+		{"// not a func", "", 0},
+	}
+	for _, tt := range tests {
+		sig, ok := parseFuncSig(tt.comment)
+		if tt.name == "" {
+			if ok {
+				t.Errorf("parseFuncSig(%q): expected not ok", tt.comment)
+			}
+			continue
+		}
+		if !ok {
+			t.Errorf("parseFuncSig(%q): expected ok", tt.comment)
+			continue
+		}
+		if sig.name != tt.name {
+			t.Errorf("parseFuncSig(%q).name = %q, want %q", tt.comment, sig.name, tt.name)
+		}
+		if len(sig.params) != tt.nParams {
+			t.Errorf("parseFuncSig(%q): %d params, want %d", tt.comment, len(sig.params), tt.nParams)
+		}
+	}
+}

verify/groundtruth.go

@@ -0,0 +1,173 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// GroundTruth assembles the given .s file with the Go toolchain's own
+// assembler and returns the machine code bytes for each TEXT function,
+// keyed by the function's short name (the part after the middle dot).
+// This is the universal oracle: any file that `go tool asm` accepts can
+// be verified, with no hand-written reference.
+func GroundTruth(path string) (map[string][]byte, error) {
+	goroot := runtime.GOROOT()
+	asmBin := filepath.Join(goroot, "pkg", "tool", runtime.GOOS+"_"+runtime.GOARCH, "asm")
+	if _, err := os.Stat(asmBin); err != nil {
+		return nil, fmt.Errorf("verify: go tool asm not found at %s: %w", asmBin, err)
+	}
+	includeDir := filepath.Join(goroot, "pkg", "include")
+
+	// Create a temp file for the object output.
+	tmpDir, err := os.MkdirTemp("", "gasm-verify-*")
+	if err != nil {
+		return nil, fmt.Errorf("verify: tempdir: %w", err)
+	}
+	defer os.RemoveAll(tmpDir)
+	objPath := filepath.Join(tmpDir, "out.o")
+
+	// Derive a package name from the file name (the assembler needs -p).
+	base := filepath.Base(path)
+	pkg := strings.TrimSuffix(base, ".s")
+	pkg = strings.TrimSuffix(pkg, "_amd64")
+
+	cmd := exec.Command(asmBin, "-I", includeDir, "-p", pkg, "-o", objPath, path)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return nil, fmt.Errorf("verify: go tool asm: %w\n%s", err, out)
+	}
+
+	objData, err := os.ReadFile(objPath)
+	if err != nil {
+		return nil, fmt.Errorf("verify: read object: %w", err)
+	}
+	return extractGOOBJCode(objData)
+}
+
+// GOOBJ block indices (cmd/internal/goobj).
+const (
+	blkAutolib = iota
+	blkPkgIdx
+	blkFile
+	blkSymdef
+	blkHashed64def
+	blkHasheddef
+	blkNonpkgdef
+	blkNonpkgref
+	blkRefFlags
+	blkHash64
+	blkHash
+	blkRelocIdx
+	blkAuxIdx
+	blkDataIdx
+	blkReloc
+	blkAux
+	blkData
+	blkRefName
+	blkEnd
+)
+
+const goobjMagic = "\x00go120ld"
+
+// extractGOOBJCode parses a GOOBJ payload and returns the code bytes for
+// each non-package STEXT symbol (the functions).
+func extractGOOBJCode(data []byte) (map[string][]byte, error) {
+	// Find the GOOBJ header (after the "go object ..." preamble).
+	i := bytes.Index(data, []byte(goobjMagic))
+	if i < 0 {
+		return nil, fmt.Errorf("verify: no GOOBJ magic in object file")
+	}
+	b := data[i:]
+	le := binary.LittleEndian
+
+	// Read block offsets (20 bytes into the header: 4 magic + 8 go version
+	// + 8 experiment = 20, then blkEnd+1 uint32 offsets).
+	var offs [blkEnd + 1]uint32
+	for j := 0; j <= blkEnd; j++ {
+		offs[j] = le.Uint32(b[20+4*j:])
+	}
+	blk := func(idx int) []byte { return b[offs[idx]:offs[idx+1]] }
+
+	// Parse non-package symbol definitions (blkNonpkgdef): each entry is
+	// 21 bytes: [nameLen:4][nameOff:4][abi:2][type:1][flag:1][flag2:1][size:4][align:4].
+	const symSize = 21
+	nonpkg := blk(blkNonpkgdef)
+	nSyms := len(nonpkg) / symSize
+
+	// Data index (blkDataIdx): one uint32 per defined symbol across ALL
+	// definition blocks (blkSymdef + blkHashed64def + blkHasheddef +
+	// blkNonpkgdef), in that order.  We need the offset for the nonpkg
+	// symbols, which come last.
+	dataIdx := blk(blkDataIdx)
+	dataBlk := blk(blkData)
+
+	// Count symbols in the preceding definition blocks.
+	preceding := 0
+	for _, bi := range []int{blkSymdef, blkHashed64def, blkHasheddef} {
+		preceding += len(blk(bi)) / symSize
+	}
+
+	// Symbol name offsets in the GOOBJ symbol table are absolute byte
+	// offsets from the start of the GOOBJ payload (the magic).
+	readStr := func(off, ln uint32) string {
+		if int(off+ln) > len(b) {
+			return ""
+		}
+		return string(b[off : off+ln])
+	}
+
+	result := make(map[string][]byte)
+	const kindSTEXT = 1
+	for s := 0; s < nSyms; s++ {
+		x := nonpkg[s*symSize:]
+		nameLen := le.Uint32(x[0:])
+		nameOff := le.Uint32(x[4:])
+		typ := x[10]
+		size := le.Uint32(x[13:])
+
+		if typ != kindSTEXT || size == 0 {
+			continue
+		}
+		name := readStr(nameOff, nameLen)
+		// Strip the package prefix (everything up to and including the
+		// last middle dot or period-dot).
+		name = stripPkg(name)
+
+		// Data offset from the index (nonpkg symbols follow the preceding blocks).
+		diIdx := preceding + s
+		if (diIdx+1)*4 > len(dataIdx) {
+			continue
+		}
+		dOff := le.Uint32(dataIdx[diIdx*4:])
+		if int(dOff+size) > len(dataBlk) {
+			continue
+		}
+		code := make([]byte, size)
+		copy(code, dataBlk[dOff:dOff+size])
+		result[name] = code
+	}
+	return result, nil
+}
+
+// stripPkg removes the package path prefix from a symbol name, leaving
+// just the function name.  "pkg/path·FuncName" → "FuncName".
+func stripPkg(name string) string {
+	if i := strings.LastIndex(name, "\u00B7"); i >= 0 {
+		return name[i+len("\u00B7"):]
+	}
+	if i := strings.LastIndex(name, "\"."); i >= 0 {
+		return name[i+2:]
+	}
+	if i := strings.LastIndex(name, "."); i >= 0 {
+		return name[i+1:]
+	}
+	return name
+}

verify/groundtruth_test.go

@@ -0,0 +1,77 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"testing"
+)
+
+func TestGroundTruthBasic(t *testing.T) {
+	// Use the simple test kernel — it assembles with go tool asm.
+	gt, err := GroundTruth("../testdata/verify/basic_amd64.s")
+	if err != nil {
+		t.Fatalf("GroundTruth: %v", err)
+	}
+	if len(gt) == 0 {
+		t.Fatal("no functions extracted from ground truth")
+	}
+	// The "add" function should be present and non-empty.
+	code, ok := gt["add"]
+	if !ok {
+		t.Fatalf("function 'add' not found in ground truth; got: %v", keys(gt))
+	}
+	if len(code) == 0 {
+		t.Fatal("add: zero-length code")
+	}
+	t.Logf("ground truth functions: %v", keys(gt))
+}
+
+func TestGroundTruthComparison(t *testing.T) {
+	// Assemble with gasm and compare against go tool asm.
+	k, err := Load("../testdata/verify/basic_amd64.s")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	defer k.Close()
+
+	gt, err := GroundTruth("../testdata/verify/basic_amd64.s")
+	if err != nil {
+		t.Fatalf("GroundTruth: %v", err)
+	}
+
+	for _, name := range k.FuncNames() {
+		fl, _ := k.Func(name)
+		gasmCode := k.Image().Code[fl.Offset : fl.Offset+fl.Size]
+		goCode, ok := gt[name]
+		if !ok {
+			t.Errorf("%s: not in ground truth", name)
+			continue
+		}
+		if len(gasmCode) != len(goCode) {
+			t.Errorf("%s: size mismatch: gasm=%d go=%d", name, len(gasmCode), len(goCode))
+			continue
+		}
+		for i := range gasmCode {
+			if gasmCode[i] != goCode[i] {
+				t.Errorf("%s: byte %d differs: gasm=%02x go=%02x", name, i, gasmCode[i], goCode[i])
+				break
+			}
+		}
+	}
+}
+
+func TestGroundTruthBadFile(t *testing.T) {
+	_, err := GroundTruth("/nonexistent/file_amd64.s")
+	if err == nil {
+		t.Fatal("expected error for nonexistent file")
+	}
+}
+
+func keys(m map[string][]byte) []string {
+	out := make([]string, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}

verify/verify.go

@@ -120,3 +120,8 @@ func (k *Kernel) Close() {
 		k.exec.Unmap()
 	}
 }
+
+// Image returns the assembled image (code + data + metadata).
+func (k *Kernel) Image() *asm.Image {
+	return k.img
+}