Imported Upstream version 1.4upstream/1.4

1 files changed, 811 insertions, 0 deletions

diff --git a/src/cmd/internal/rsc.io/x86/x86asm/ext_test.go b/src/cmd/internal/rsc.io/x86/x86asm/ext_test.go
new file mode 100644
index 000000000..f65d6b2d5
--- /dev/null
+++ b/src/cmd/internal/rsc.io/x86/x86asm/ext_test.go
@@ -0,0 +1,811 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Support for testing against external disassembler program.
+
+package x86asm
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/hex"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"math/rand"
+	"os"
+	"os/exec"
+	"regexp"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+var (
+	printTests = flag.Bool("printtests", false, "print test cases that exercise new code paths")
+	dumpTest   = flag.Bool("dump", false, "dump all encodings")
+	mismatch   = flag.Bool("mismatch", false, "log allowed mismatches")
+	longTest   = flag.Bool("long", false, "long test")
+	keep       = flag.Bool("keep", false, "keep object files around")
+	debug      = false
+)
+
+// A ExtInst represents a single decoded instruction parsed
+// from an external disassembler's output.
+type ExtInst struct {
+	addr uint32
+	enc  [32]byte
+	nenc int
+	text string
+}
+
+func (r ExtInst) String() string {
+	return fmt.Sprintf("%#x: % x: %s", r.addr, r.enc, r.text)
+}
+
+// An ExtDis is a connection between an external disassembler and a test.
+type ExtDis struct {
+	Arch     int
+	Dec      chan ExtInst
+	File     *os.File
+	Size     int
+	KeepFile bool
+	Cmd      *exec.Cmd
+}
+
+// Run runs the given command - the external disassembler - and returns
+// a buffered reader of its standard output.
+func (ext *ExtDis) Run(cmd ...string) (*bufio.Reader, error) {
+	if *keep {
+		log.Printf("%s\n", strings.Join(cmd, " "))
+	}
+	ext.Cmd = exec.Command(cmd[0], cmd[1:]...)
+	out, err := ext.Cmd.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stdoutpipe: %v", err)
+	}
+	if err := ext.Cmd.Start(); err != nil {
+		return nil, fmt.Errorf("exec: %v", err)
+	}
+
+	b := bufio.NewReaderSize(out, 1<<20)
+	return b, nil
+}
+
+// Wait waits for the command started with Run to exit.
+func (ext *ExtDis) Wait() error {
+	return ext.Cmd.Wait()
+}
+
+// testExtDis tests a set of byte sequences against an external disassembler.
+// The disassembler is expected to produce the given syntax and be run
+// in the given architecture mode (16, 32, or 64-bit).
+// The extdis function must start the external disassembler
+// and then parse its output, sending the parsed instructions on ext.Dec.
+// The generate function calls its argument f once for each byte sequence
+// to be tested. The generate function itself will be called twice, and it must
+// make the same sequence of calls to f each time.
+// When a disassembly does not match the internal decoding,
+// allowedMismatch determines whether this mismatch should be
+// allowed, or else considered an error.
+func testExtDis(
+	t *testing.T,
+	syntax string,
+	arch int,
+	extdis func(ext *ExtDis) error,
+	generate func(f func([]byte)),
+	allowedMismatch func(text string, size int, inst *Inst, dec ExtInst) bool,
+) {
+	start := time.Now()
+	ext := &ExtDis{
+		Dec:  make(chan ExtInst),
+		Arch: arch,
+	}
+	errc := make(chan error)
+
+	// First pass: write instructions to input file for external disassembler.
+	file, f, size, err := writeInst(generate)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ext.Size = size
+	ext.File = f
+	defer func() {
+		f.Close()
+		if !*keep {
+			os.Remove(file)
+		}
+	}()
+
+	// Second pass: compare disassembly against our decodings.
+	var (
+		totalTests  = 0
+		totalSkips  = 0
+		totalErrors = 0
+
+		errors = make([]string, 0, 100) // sampled errors, at most cap
+	)
+	go func() {
+		errc <- extdis(ext)
+	}()
+	generate(func(enc []byte) {
+		dec, ok := <-ext.Dec
+		if !ok {
+			t.Errorf("decoding stream ended early")
+			return
+		}
+		inst, text := disasm(syntax, arch, pad(enc))
+		totalTests++
+		if *dumpTest {
+			fmt.Printf("%x -> %s [%d]\n", enc[:len(enc)], dec.text, dec.nenc)
+		}
+		if text != dec.text || inst.Len != dec.nenc {
+			suffix := ""
+			if allowedMismatch(text, size, &inst, dec) {
+				totalSkips++
+				if !*mismatch {
+					return
+				}
+				suffix += " (allowed mismatch)"
+			}
+			totalErrors++
+			if len(errors) >= cap(errors) {
+				j := rand.Intn(totalErrors)
+				if j >= cap(errors) {
+					return
+				}
+				errors = append(errors[:j], errors[j+1:]...)
+			}
+			errors = append(errors, fmt.Sprintf("decode(%x) = %q, %d, want %q, %d%s", enc, text, inst.Len, dec.text, dec.nenc, suffix))
+		}
+	})
+
+	if *mismatch {
+		totalErrors -= totalSkips
+	}
+
+	for _, b := range errors {
+		t.Log(b)
+	}
+
+	if totalErrors > 0 {
+		t.Fail()
+	}
+	t.Logf("%d test cases, %d expected mismatches, %d failures; %.0f cases/second", totalTests, totalSkips, totalErrors, float64(totalTests)/time.Since(start).Seconds())
+
+	if err := <-errc; err != nil {
+		t.Fatal("external disassembler: %v", err)
+	}
+
+}
+
+const start = 0x8000 // start address of text
+
+// writeInst writes the generated byte sequences to a new file
+// starting at offset start. That file is intended to be the input to
+// the external disassembler.
+func writeInst(generate func(func([]byte))) (file string, f *os.File, size int, err error) {
+	f, err = ioutil.TempFile("", "x86map")
+	if err != nil {
+		return
+	}
+
+	file = f.Name()
+
+	f.Seek(start, 0)
+	w := bufio.NewWriter(f)
+	defer w.Flush()
+	size = 0
+	generate(func(x []byte) {
+		if len(x) > 16 {
+			x = x[:16]
+		}
+		if debug {
+			fmt.Printf("%#x: %x%x\n", start+size, x, pops[len(x):])
+		}
+		w.Write(x)
+		w.Write(pops[len(x):])
+		size += len(pops)
+	})
+	return file, f, size, nil
+}
+
+// 0x5F is a single-byte pop instruction.
+// We pad the bytes we want decoded with enough 0x5Fs
+// that no matter what state the instruction stream is in
+// after reading our bytes, the pops will get us back to
+// a forced instruction boundary.
+var pops = []byte{
+	0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f,
+	0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f,
+	0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f,
+	0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f, 0x5f,
+}
+
+// pad pads the code sequenc with pops.
+func pad(enc []byte) []byte {
+	return append(enc[:len(enc):len(enc)], pops...)
+}
+
+// disasm returns the decoded instruction and text
+// for the given source bytes, using the given syntax and mode.
+func disasm(syntax string, mode int, src []byte) (inst Inst, text string) {
+	// If printTests is set, we record the coverage value
+	// before and after, and we write out the inputs for which
+	// coverage went up, in the format expected in testdata/decode.text.
+	// This produces a fairly small set of test cases that exercise nearly
+	// all the code.
+	var cover float64
+	if *printTests {
+		cover -= coverage()
+	}
+
+	inst, err := decode1(src, mode, syntax == "gnu")
+	if err != nil {
+		text = "error: " + err.Error()
+	} else {
+		switch syntax {
+		case "gnu":
+			text = GNUSyntax(inst)
+		case "intel":
+			text = IntelSyntax(inst)
+		case "plan9":
+			text = Plan9Syntax(inst, 0, nil)
+		default:
+			text = "error: unknown syntax " + syntax
+		}
+	}
+
+	if *printTests {
+		cover += coverage()
+		if cover > 0 {
+			max := len(src)
+			if max > 16 && inst.Len <= 16 {
+				max = 16
+			}
+			fmt.Printf("%x|%x\t%d\t%s\t%s\n", src[:inst.Len], src[inst.Len:max], mode, syntax, text)
+		}
+	}
+
+	return
+}
+
+// coverage returns a floating point number denoting the
+// test coverage until now. The number increases when new code paths are exercised,
+// both in the Go program and in the decoder byte code.
+func coverage() float64 {
+	/*
+		testing.Coverage is not in the main distribution.
+		The implementation, which must go in package testing, is:
+
+		// Coverage reports the current code coverage as a fraction in the range [0, 1].
+		func Coverage() float64 {
+			var n, d int64
+			for _, counters := range cover.Counters {
+				for _, c := range counters {
+					if c > 0 {
+						n++
+					}
+					d++
+				}
+			}
+			if d == 0 {
+				return 0
+			}
+			return float64(n) / float64(d)
+		}
+	*/
+
+	var f float64
+	// f += testing.Coverage()
+	f += decodeCoverage()
+	return f
+}
+
+func decodeCoverage() float64 {
+	n := 0
+	for _, t := range decoderCover {
+		if t {
+			n++
+		}
+	}
+	return float64(1+n) / float64(1+len(decoderCover))
+}
+
+// Helpers for writing disassembler output parsers.
+
+// isPrefix reports whether text is the name of an instruction prefix.
+func isPrefix(text string) bool {
+	return prefixByte[text] > 0
+}
+
+// prefixByte maps instruction prefix text to actual prefix byte values.
+var prefixByte = map[string]byte{
+	"es":       0x26,
+	"cs":       0x2e,
+	"ss":       0x36,
+	"ds":       0x3e,
+	"fs":       0x64,
+	"gs":       0x65,
+	"data16":   0x66,
+	"addr16":   0x67,
+	"lock":     0xf0,
+	"repn":     0xf2,
+	"repne":    0xf2,
+	"rep":      0xf3,
+	"repe":     0xf3,
+	"xacquire": 0xf2,
+	"xrelease": 0xf3,
+	"bnd":      0xf2,
+	"addr32":   0x66,
+	"data32":   0x67,
+}
+
+// hasPrefix reports whether any of the space-separated words in the text s
+// begins with any of the given prefixes.
+func hasPrefix(s string, prefixes ...string) bool {
+	for _, prefix := range prefixes {
+		for s := s; s != ""; {
+			if strings.HasPrefix(s, prefix) {
+				return true
+			}
+			i := strings.Index(s, " ")
+			if i < 0 {
+				break
+			}
+			s = s[i+1:]
+		}
+	}
+	return false
+}
+
+// contains reports whether the text s contains any of the given substrings.
+func contains(s string, substrings ...string) bool {
+	for _, sub := range substrings {
+		if strings.Contains(s, sub) {
+			return true
+		}
+	}
+	return false
+}
+
+// isHex reports whether b is a hexadecimal character (0-9A-Fa-f).
+func isHex(b byte) bool { return b == '0' || unhex[b] > 0 }
+
+// parseHex parses the hexadecimal byte dump in hex,
+// appending the parsed bytes to raw and returning the updated slice.
+// The returned bool signals whether any invalid hex was found.
+// Spaces and tabs between bytes are okay but any other non-hex is not.
+func parseHex(hex []byte, raw []byte) ([]byte, bool) {
+	hex = trimSpace(hex)
+	for j := 0; j < len(hex); {
+		for hex[j] == ' ' || hex[j] == '\t' {
+			j++
+		}
+		if j >= len(hex) {
+			break
+		}
+		if j+2 > len(hex) || !isHex(hex[j]) || !isHex(hex[j+1]) {
+			return nil, false
+		}
+		raw = append(raw, unhex[hex[j]]<<4|unhex[hex[j+1]])
+		j += 2
+	}
+	return raw, true
+}
+
+var unhex = [256]byte{
+	'0': 0,
+	'1': 1,
+	'2': 2,
+	'3': 3,
+	'4': 4,
+	'5': 5,
+	'6': 6,
+	'7': 7,
+	'8': 8,
+	'9': 9,
+	'A': 10,
+	'B': 11,
+	'C': 12,
+	'D': 13,
+	'E': 14,
+	'F': 15,
+	'a': 10,
+	'b': 11,
+	'c': 12,
+	'd': 13,
+	'e': 14,
+	'f': 15,
+}
+
+// index is like bytes.Index(s, []byte(t)) but avoids the allocation.
+func index(s []byte, t string) int {
+	i := 0
+	for {
+		j := bytes.IndexByte(s[i:], t[0])
+		if j < 0 {
+			return -1
+		}
+		i = i + j
+		if i+len(t) > len(s) {
+			return -1
+		}
+		for k := 1; k < len(t); k++ {
+			if s[i+k] != t[k] {
+				goto nomatch
+			}
+		}
+		return i
+	nomatch:
+		i++
+	}
+}
+
+// fixSpace rewrites runs of spaces, tabs, and newline characters into single spaces in s.
+// If s must be rewritten, it is rewritten in place.
+func fixSpace(s []byte) []byte {
+	s = trimSpace(s)
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\t' || s[i] == '\n' || i > 0 && s[i] == ' ' && s[i-1] == ' ' {
+			goto Fix
+		}
+	}
+	return s
+
+Fix:
+	b := s
+	w := 0
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c == '\t' || c == '\n' {
+			c = ' '
+		}
+		if c == ' ' && w > 0 && b[w-1] == ' ' {
+			continue
+		}
+		b[w] = c
+		w++
+	}
+	if w > 0 && b[w-1] == ' ' {
+		w--
+	}
+	return b[:w]
+}
+
+// trimSpace trims leading and trailing space from s, returning a subslice of s.
+func trimSpace(s []byte) []byte {
+	j := len(s)
+	for j > 0 && (s[j-1] == ' ' || s[j-1] == '\t' || s[j-1] == '\n') {
+		j--
+	}
+	i := 0
+	for i < j && (s[i] == ' ' || s[i] == '\t') {
+		i++
+	}
+	return s[i:j]
+}
+
+// pcrel and pcrelw match instructions using relative addressing mode.
+var (
+	pcrel  = regexp.MustCompile(`^((?:.* )?(?:j[a-z]+|call|ljmp|loopn?e?w?|xbegin)q?(?:,p[nt])?) 0x([0-9a-f]+)$`)
+	pcrelw = regexp.MustCompile(`^((?:.* )?(?:callw|jmpw|xbeginw|ljmpw)(?:,p[nt])?) 0x([0-9a-f]+)$`)
+)
+
+// Generators.
+//
+// The test cases are described as functions that invoke a callback repeatedly,
+// with a new input sequence each time. These helpers make writing those
+// a little easier.
+
+// hexCases generates the cases written in hexadecimal in the encoded string.
+// Spaces in 'encoded' separate entire test cases, not individual bytes.
+func hexCases(t *testing.T, encoded string) func(func([]byte)) {
+	return func(try func([]byte)) {
+		for _, x := range strings.Fields(encoded) {
+			src, err := hex.DecodeString(x)
+			if err != nil {
+				t.Errorf("parsing %q: %v", x, err)
+			}
+			try(src)
+		}
+	}
+}
+
+// testdataCases generates the test cases recorded in testdata/decode.txt.
+// It only uses the inputs; it ignores the answers recorded in that file.
+func testdataCases(t *testing.T) func(func([]byte)) {
+	var codes [][]byte
+	data, err := ioutil.ReadFile("testdata/decode.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		f := strings.Fields(line)[0]
+		i := strings.Index(f, "|")
+		if i < 0 {
+			t.Errorf("parsing %q: missing | separator", f)
+			continue
+		}
+		if i%2 != 0 {
+			t.Errorf("parsing %q: misaligned | separator", f)
+		}
+		code, err := hex.DecodeString(f[:i] + f[i+1:])
+		if err != nil {
+			t.Errorf("parsing %q: %v", f, err)
+			continue
+		}
+		codes = append(codes, code)
+	}
+
+	return func(try func([]byte)) {
+		for _, code := range codes {
+			try(code)
+		}
+	}
+}
+
+// manyPrefixes generates all possible 2⁹ combinations of nine chosen prefixes.
+// The relative ordering of the prefixes within the combinations varies deterministically.
+func manyPrefixes(try func([]byte)) {
+	var prefixBytes = []byte{0x66, 0x67, 0xF0, 0xF2, 0xF3, 0x3E, 0x36, 0x66, 0x67}
+	var enc []byte
+	for i := 0; i < 1<<uint(len(prefixBytes)); i++ {
+		enc = enc[:0]
+		for j, p := range prefixBytes {
+			if i&(1<<uint(j)) != 0 {
+				enc = append(enc, p)
+			}
+		}
+		if len(enc) > 0 {
+			k := i % len(enc)
+			enc[0], enc[k] = enc[k], enc[0]
+		}
+		try(enc)
+	}
+}
+
+// basicPrefixes geneartes 8 different possible prefix cases: no prefix
+// and then one each of seven different prefix bytes.
+func basicPrefixes(try func([]byte)) {
+	try(nil)
+	for _, b := range []byte{0x66, 0x67, 0xF0, 0xF2, 0xF3, 0x3E, 0x36} {
+		try([]byte{b})
+	}
+}
+
+func rexPrefixes(try func([]byte)) {
+	try(nil)
+	for _, b := range []byte{0x40, 0x48, 0x43, 0x4C} {
+		try([]byte{b})
+	}
+}
+
+// concat takes two generators and returns a generator for the
+// cross product of the two, concatenating the results from each.
+func concat(gen1, gen2 func(func([]byte))) func(func([]byte)) {
+	return func(try func([]byte)) {
+		gen1(func(enc1 []byte) {
+			gen2(func(enc2 []byte) {
+				try(append(enc1[:len(enc1):len(enc1)], enc2...))
+			})
+		})
+	}
+}
+
+// concat3 takes three generators and returns a generator for the
+// cross product of the three, concatenating the results from each.
+func concat3(gen1, gen2, gen3 func(func([]byte))) func(func([]byte)) {
+	return func(try func([]byte)) {
+		gen1(func(enc1 []byte) {
+			gen2(func(enc2 []byte) {
+				gen3(func(enc3 []byte) {
+					try(append(append(enc1[:len(enc1):len(enc1)], enc2...), enc3...))
+				})
+			})
+		})
+	}
+}
+
+// concat4 takes four generators and returns a generator for the
+// cross product of the four, concatenating the results from each.
+func concat4(gen1, gen2, gen3, gen4 func(func([]byte))) func(func([]byte)) {
+	return func(try func([]byte)) {
+		gen1(func(enc1 []byte) {
+			gen2(func(enc2 []byte) {
+				gen3(func(enc3 []byte) {
+					gen4(func(enc4 []byte) {
+						try(append(append(append(enc1[:len(enc1):len(enc1)], enc2...), enc3...), enc4...))
+					})
+				})
+			})
+		})
+	}
+}
+
+// filter generates the sequences from gen that satisfy ok.
+func filter(gen func(func([]byte)), ok func([]byte) bool) func(func([]byte)) {
+	return func(try func([]byte)) {
+		gen(func(enc []byte) {
+			if ok(enc) {
+				try(enc)
+			}
+		})
+	}
+}
+
+// enum8bit generates all possible 1-byte sequences, followed by distinctive padding.
+func enum8bit(try func([]byte)) {
+	for i := 0; i < 1<<8; i++ {
+		try([]byte{byte(i), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88})
+	}
+}
+
+// enum8bit generates all possible 2-byte sequences, followed by distinctive padding.
+func enum16bit(try func([]byte)) {
+	for i := 0; i < 1<<16; i++ {
+		try([]byte{byte(i), byte(i >> 8), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88})
+	}
+}
+
+// enum24bit generates all possible 3-byte sequences, followed by distinctive padding.
+func enum24bit(try func([]byte)) {
+	for i := 0; i < 1<<24; i++ {
+		try([]byte{byte(i), byte(i >> 8), byte(i >> 16), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88})
+	}
+}
+
+// enumModRM generates all possible modrm bytes and, for modrm values that indicate
+// a following sib byte, all possible modrm, sib combinations.
+func enumModRM(try func([]byte)) {
+	for i := 0; i < 256; i++ {
+		if (i>>3)&07 == 04 && i>>6 != 3 { // has sib
+			for j := 0; j < 256; j++ {
+				try([]byte{0, byte(i), byte(j), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88}) // byte encodings
+				try([]byte{1, byte(i), byte(j), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88}) // word encodings
+			}
+		} else {
+			try([]byte{0, byte(i), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88}) // byte encodings
+			try([]byte{1, byte(i), 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88}) // word encodings
+		}
+	}
+}
+
+// fixed generates the single case b.
+// It's mainly useful to prepare an argument for concat or concat3.
+func fixed(b ...byte) func(func([]byte)) {
+	return func(try func([]byte)) {
+		try(b)
+	}
+}
+
+// testBasic runs the given test function with cases all using opcode as the initial opcode bytes.
+// It runs three phases:
+//
+// First, zero-or-one prefixes followed by opcode followed by all possible 1-byte values.
+// If in -short mode, that's all.
+//
+// Second, zero-or-one prefixes followed by opcode followed by all possible 2-byte values.
+// If not in -long mode, that's all. This phase and the next run in parallel with other tests
+// (using t.Parallel).
+//
+// Finally, opcode followed by all possible 3-byte values. The test can take a very long time
+// and prints progress messages to package log.
+func testBasic(t *testing.T, testfn func(*testing.T, func(func([]byte))), opcode ...byte) {
+	testfn(t, concat3(basicPrefixes, fixed(opcode...), enum8bit))
+	if testing.Short() {
+		return
+	}
+
+	t.Parallel()
+	testfn(t, concat3(basicPrefixes, fixed(opcode...), enum16bit))
+	if !*longTest {
+		return
+	}
+
+	name := caller(2)
+	op1 := make([]byte, len(opcode)+1)
+	copy(op1, opcode)
+	for i := 0; i < 256; i++ {
+		log.Printf("%s 24-bit: %d/256\n", name, i)
+		op1[len(opcode)] = byte(i)
+		testfn(t, concat(fixed(op1...), enum16bit))
+	}
+}
+
+func testBasicREX(t *testing.T, testfn func(*testing.T, func(func([]byte))), opcode ...byte) {
+	testfn(t, filter(concat4(basicPrefixes, rexPrefixes, fixed(opcode...), enum8bit), isValidREX))
+	if testing.Short() {
+		return
+	}
+
+	t.Parallel()
+	testfn(t, filter(concat4(basicPrefixes, rexPrefixes, fixed(opcode...), enum16bit), isValidREX))
+	if !*longTest {
+		return
+	}
+
+	name := caller(2)
+	op1 := make([]byte, len(opcode)+1)
+	copy(op1, opcode)
+	for i := 0; i < 256; i++ {
+		log.Printf("%s 24-bit: %d/256\n", name, i)
+		op1[len(opcode)] = byte(i)
+		testfn(t, filter(concat3(rexPrefixes, fixed(op1...), enum16bit), isValidREX))
+	}
+}
+
+// testPrefix runs the given test function for all many prefix possibilities
+// followed by all possible 1-byte sequences.
+//
+// If in -long mode, it then runs a test of all the prefix possibilities followed
+// by all possible 2-byte sequences.
+func testPrefix(t *testing.T, testfn func(*testing.T, func(func([]byte)))) {
+	t.Parallel()
+	testfn(t, concat(manyPrefixes, enum8bit))
+	if testing.Short() || !*longTest {
+		return
+	}
+
+	name := caller(2)
+	for i := 0; i < 256; i++ {
+		log.Printf("%s 16-bit: %d/256\n", name, i)
+		testfn(t, concat3(manyPrefixes, fixed(byte(i)), enum8bit))
+	}
+}
+
+func testPrefixREX(t *testing.T, testfn func(*testing.T, func(func([]byte)))) {
+	t.Parallel()
+	testfn(t, filter(concat3(manyPrefixes, rexPrefixes, enum8bit), isValidREX))
+	if testing.Short() || !*longTest {
+		return
+	}
+
+	name := caller(2)
+	for i := 0; i < 256; i++ {
+		log.Printf("%s 16-bit: %d/256\n", name, i)
+		testfn(t, filter(concat4(manyPrefixes, rexPrefixes, fixed(byte(i)), enum8bit), isValidREX))
+	}
+}
+
+func caller(skip int) string {
+	pc, _, _, _ := runtime.Caller(skip)
+	f := runtime.FuncForPC(pc)
+	name := "?"
+	if f != nil {
+		name = f.Name()
+		if i := strings.LastIndex(name, "."); i >= 0 {
+			name = name[i+1:]
+		}
+	}
+	return name
+}
+
+func isValidREX(x []byte) bool {
+	i := 0
+	for i < len(x) && isPrefixByte(x[i]) {
+		i++
+	}
+	if i < len(x) && Prefix(x[i]).IsREX() {
+		i++
+		if i < len(x) {
+			return !isPrefixByte(x[i]) && !Prefix(x[i]).IsREX()
+		}
+	}
+	return true
+}
+
+func isPrefixByte(b byte) bool {
+	switch b {
+	case 0x26, 0x2E, 0x36, 0x3E, 0x64, 0x65, 0x66, 0x67, 0xF0, 0xF2, 0xF3:
+		return true
+	}
+	return false
+}