golang benchmark 代码
文件路径：/src/testing/benchmark.go
/ Copyright 2009 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.
package testing
import (
"flag"
"fmt"
"internal/race"
"internal/sysinfo"
"io"
"math"
"os"
"runtime"
"sort"
"strconv"
"strings"
"sync"
"sync/atomic"
"time"
"unicode"
)
func initBenchmarkFlags() {
matchBenchmarks = flag.String("test.bench", "", "run only benchmarks matching `regexp`")
benchmarkMemory = flag.Bool("test.benchmem", false, "print memory allocations for benchmarks")
flag.Var(&benchTime, "test.benchtime", "run each benchmark for duration `d`")
}
var (
matchBenchmarks *string
benchmarkMemory *bool
benchTime = durationOrCountFlag{d: 1 * time.Second} / changed during test of testing package
)
type durationOrCountFlag struct {
d time.Duration
n int
allowZero bool
}
func (f *durationOrCountFlag) String() string {
if f.n > 0 {
return fmt.Sprintf("%dx", f.n)
}
return f.d.String()
}
func (f *durationOrCountFlag) Set(s string) error {
if strings.HasSuffix(s, "x") {
n, err := strconv.ParseInt(s[:len(s)-1], 10, 0)
if err != nil || n < 0 || (!f.allowZero && n == 0) {
return fmt.Errorf("invalid count")
}
*f = durationOrCountFlag{n: int(n)}
return nil
}
d, err := time.ParseDuration(s)
if err != nil || d < 0 || (!f.allowZero && d == 0) {
return fmt.Errorf("invalid duration")
}
*f = durationOrCountFlag{d: d}
return nil
}
/ Global lock to ensure only one benchmark runs at a time.
var benchmarkLock sync.Mutex
/ Used for every benchmark for measuring memory.
var memStats runtime.MemStats
/ InternalBenchmark is an internal type but exported because it is cross-package;
/ it is part of the implementation of the "go test" command.
type InternalBenchmark struct {
Name string
F func(b *B)
}
/ B is a type passed to Benchmark functions to manage benchmark
/ timing and to specify the number of iterations to run.
/
/ A benchmark ends when its Benchmark function returns or calls any of the methods
/ FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods must be called
/ only from the goroutine running the Benchmark function.
/ The other reporting methods, such as the variations of Log and Error,
/ may be called simultaneously from multiple goroutines.
/
/ Like in tests, benchmark logs are accumulated during execution
/ and dumped to standard output when done. Unlike in tests, benchmark logs
/ are always printed, so as not to hide output whose existence may be
/ affecting benchmark results.
type B struct {
common
importPath string / import path of the package containing the benchmark
context *benchContext
N int
previousN int / number of iterations in the previous run
previousDuration time.Duration / total duration of the previous run
benchFunc func(b *B)
benchTime durationOrCountFlag
bytes int64
missingBytes bool / one of the subbenchmarks does not have bytes set.
timerOn bool
showAllocResult bool
result BenchmarkResult
parallelism int / RunParallel creates parallelism*GOMAXPROCS goroutines
/ The initial states of memStats.Mallocs and memStats.TotalAlloc.
startAllocs uint64
startBytes uint64
/ The net total of this test after being run.
netAllocs uint64
netBytes uint64
/ Extra metrics collected by ReportMetric.
extra map[string]float64
}
/ StartTimer starts timing a test. This function is called automatically
/ before a benchmark starts, but it can also be used to resume timing after
/ a call to StopTimer.
func (b *B) StartTimer() {
if !b.timerOn {
runtime.ReadMemStats(&memStats)
b.startAllocs = memStats.Mallocs
b.startBytes = memStats.TotalAlloc
b.start = time.Now()
b.timerOn = true
}
}
/ StopTimer stops timing a test. This can be used to pause the timer
/ while performing complex initialization that you don't
/ want to measure.
func (b *B) StopTimer() {
if b.timerOn {
b.duration += time.Since(b.start)
runtime.ReadMemStats(&memStats)
b.netAllocs += memStats.Mallocs - b.startAllocs
b.netBytes += memStats.TotalAlloc - b.startBytes
b.timerOn = false
}
}
/ ResetTimer zeroes the elapsed benchmark time and memory allocation counters
/ and deletes user-reported metrics.
/ It does not affect whether the timer is running.
func (b *B) ResetTimer() {
if b.extra == nil {
/ Allocate the extra map before reading memory stats.
/ Pre-size it to make more allocation unlikely.
b.extra = make(map[string]float64, 16)
} else {
for k := range b.extra {
delete(b.extra, k)
}
}
if b.timerOn {
runtime.ReadMemStats(&memStats)
b.startAllocs = memStats.Mallocs
b.startBytes = memStats.TotalAlloc
b.start = time.Now()
}
b.duration = 0
b.netAllocs = 0
b.netBytes = 0
}
/ SetBytes records the number of bytes processed in a single operation.
/ If this is called, the benchmark will report ns/op and MB/s.
func (b *B) SetBytes(n int64) { b.bytes = n }
/ ReportAllocs enables malloc statistics for this benchmark.
/ It is equivalent to setting -test.benchmem, but it only affects the
/ benchmark function that calls ReportAllocs.
func (b *B) ReportAllocs() {
b.showAllocResult = true
}
/ runN runs a single benchmark for the specified number of iterations.
func (b *B) runN(n int) {
benchmarkLock.Lock()
defer benchmarkLock.Unlock()
defer b.runCleanup(normalPanic)
/ Try to get a comparable environment for each run
/ by clearing garbage from previous runs.
runtime.GC()
b.raceErrors = -race.Errors()
b.N = n
b.parallelism = 1
b.ResetTimer()
b.StartTimer()
b.benchFunc(b)
b.StopTimer()
b.previousN = n
b.previousDuration = b.duration
b.raceErrors += race.Errors()
if b.raceErrors > 0 {
b.Errorf("race detected during execution of benchmark")
}
}
func min(x, y int64) int64 {
if x > y {
return y
}
return x
}
func max(x, y int64) int64 {
if x < y {
return y
}
return x
}
/ run1 runs the first iteration of benchFunc. It reports whether more
/ iterations of this benchmarks should be run.
func (b *B) run1() bool {
if ctx := b.context; ctx != nil {
/ Extend maxLen, if needed.
if n := len(b.name) + ctx.extLen + 1; n > ctx.maxLen {
ctx.maxLen = n + 8 / Add additional slack to avoid too many jumps in size.
}
}
go func() {
/ Signal that we're done whether we return normally
/ or by FailNow's runtime.Goexit.
defer func() {
b.signal <- true
}()
b.runN(1)
}()
<-b.signal
if b.failed {
fmt.Fprintf(b.w, "--- FAIL: %s\n%s", b.name, b.output)
return false
}
/ Only print the output if we know we are not going to proceed.
/ Otherwise it is printed in processBench.
b.mu.RLock()
finished := b.finished
b.mu.RUnlock()
if atomic.LoadInt32(&b.hasSub) != 0 || finished {
tag := "BENCH"
if b.skipped {
tag = "SKIP"
}
if b.chatty != nil && (len(b.output) > 0 || finished) {
b.trimOutput()
fmt.Fprintf(b.w, "--- %s: %s\n%s", tag, b.name, b.output)
}
return false
}
return true
}
var labelsOnce sync.Once
/ run executes the benchmark in a separate goroutine, including all of its
/ subbenchmarks. b must not have subbenchmarks.
func (b *B) run() {
labelsOnce.Do(func() {
fmt.Fprintf(b.w, "goos: %s\n", runtime.GOOS)
fmt.Fprintf(b.w, "goarch: %s\n", runtime.GOARCH)
if b.importPath != "" {
fmt.Fprintf(b.w, "pkg: %s\n", b.importPath)
}
if cpu := sysinfo.CPU.Name(); cpu != "" {
fmt.Fprintf(b.w, "cpu: %s\n", cpu)
}
})
if b.context != nil {
/ Running go test --test.bench
b.context.processBench(b) / Must call doBench.
} else {
/ Running func Benchmark.
b.doBench()
}
}
func (b *B) doBench() BenchmarkResult {
go b.launch()
<-b.signal
return b.result
}
/ launch launches the benchmark function. It gradually increases the number
/ of benchmark iterations until the benchmark runs for the requested benchtime.
/ launch is run by the doBench function as a separate goroutine.
/ run1 must have been called on b.
func (b *B) launch() {
/ Signal that we're done whether we return normally
/ or by FailNow's runtime.Goexit.
defer func() {
b.signal <- true
}()
/ Run the benchmark for at least the specified amount of time.
if b.benchTime.n > 0 {
/ We already ran a single iteration in run1.
/ If -benchtime=1x was requested, use that result.
/ See https://golang.org/issue/32051.
if b.benchTime.n > 1 {
b.runN(b.benchTime.n)
}
} else {
d := b.benchTime.d
for n := int64(1); !b.failed && b.duration < d && n < 1e9; {
last := n
/ Predict required iterations.
goalns := d.Nanoseconds()
prevIters := int64(b.N)
prevns := b.duration.Nanoseconds()
if prevns <= 0 {
/ Round up, to avoid div by zero.
prevns = 1
}
/ Order of operations matters.
/ For very fast benchmarks, prevIters ~= prevns.
/ If you divide first, you get 0 or 1,
/ which can hide an order of magnitude in execution time.
/ So multiply first, then divide.
n = goalns * prevIters / prevns
/ Run more iterations than we think we'll need (1.2x).
n += n / 5
/ Don't grow too fast in case we had timing errors previously.
n = min(n, 100*last)
/ Be sure to run at least one more than last time.
n = max(n, last+1)
/ Don't run more than 1e9 times. (This also keeps n in int range on 32 bit platforms.)
n = min(n, 1e9)
b.runN(int(n))
}
}
b.result = BenchmarkResult{b.N, b.duration, b.bytes, b.netAllocs, b.netBytes, b.extra}
}
/ ReportMetric adds "n unit" to the reported benchmark results.
/ If the metric is per-iteration, the caller should divide by b.N,
/ and by convention units should end in "/op".
/ ReportMetric overrides any previously reported value for the same unit.
/ ReportMetric panics if unit is the empty string or if unit contains
/ any whitespace.
/ If unit is a unit normally reported by the benchmark framework itself
/ (such as "allocs/op"), ReportMetric will override that metric.
/ Setting "ns/op" to 0 will suppress that built-in metric.
func (b *B) ReportMetric(n float64, unit string) {
if unit == "" {
panic("metric unit must not be empty")
}
if strings.IndexFunc(unit, unicode.IsSpace) >= 0 {
panic("metric unit must not contain whitespace")
}
b.extra[unit] = n
}
/ BenchmarkResult contains the results of a benchmark run.
type BenchmarkResult struct {
N int / The number of iterations.
T time.Duration / The total time taken.
Bytes int64 / Bytes processed in one iteration.
MemAllocs uint64 / The total number of memory allocations.
MemBytes uint64 / The total number of bytes allocated.
/ Extra records additional metrics reported by ReportMetric.
Extra map[string]float64
}
/ NsPerOp returns the "ns/op" metric.
func (r BenchmarkResult) NsPerOp() int64 {
if v, ok := r.Extra["ns/op"]; ok {
return int64(v)
}
if r.N <= 0 {
return 0
}
return r.T.Nanoseconds() / int64(r.N)
}
/ mbPerSec returns the "MB/s" metric.
func (r BenchmarkResult) mbPerSec() float64 {
if v, ok := r.Extra["MB/s"]; ok {
return v
}
if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
return 0
}
return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
}
/ AllocsPerOp returns the "allocs/op" metric,
/ which is calculated as r.MemAllocs / r.N.
func (r BenchmarkResult) AllocsPerOp() int64 {
if v, ok := r.Extra["allocs/op"]; ok {
return int64(v)
}
if r.N <= 0 {
return 0
}
return int64(r.MemAllocs) / int64(r.N)
}
/ AllocedBytesPerOp returns the "B/op" metric,
/ which is calculated as r.MemBytes / r.N.
func (r BenchmarkResult) AllocedBytesPerOp() int64 {
if v, ok := r.Extra["B/op"]; ok {
return int64(v)
}
if r.N <= 0 {
return 0
}
return int64(r.MemBytes) / int64(r.N)
}
/ String returns a summary of the benchmark results.
/ It follows the benchmark result line format from
/ https://golang.org/design/14313-benchmark-format, not including the
/ benchmark name.
/ Extra metrics override built-in metrics of the same name.
/ String does not include allocs/op or B/op, since those are reported
/ by MemString.
func (r BenchmarkResult) String() string {
buf := new(strings.Builder)
fmt.Fprintf(buf, "%8d", r.N)
/ Get ns/op as a float.
ns, ok := r.Extra["ns/op"]
if !ok {
ns = float64(r.T.Nanoseconds()) / float64(r.N)
}
if ns != 0 {
buf.WriteByte('\t')
prettyPrint(buf, ns, "ns/op")
}
if mbs := r.mbPerSec(); mbs != 0 {
fmt.Fprintf(buf, "\t%7.2f MB/s", mbs)
}
/ Print extra metrics that aren't represented in the standard
/ metrics.
var extraKeys []string
for k := range r.Extra {
switch k {
case "ns/op", "MB/s", "B/op", "allocs/op":
/ Built-in metrics reported elsewhere.
continue
}
extraKeys = append(extraKeys, k)
}
sort.Strings(extraKeys)
for _, k := range extraKeys {
buf.WriteByte('\t')
prettyPrint(buf, r.Extra[k], k)
}
return buf.String()
}
func prettyPrint(w io.Writer, x float64, unit string) {
/ Print all numbers with 10 places before the decimal point
/ and small numbers with four sig figs. Field widths are
/ chosen to fit the whole part in 10 places while aligning
/ the decimal point of all fractional formats.
var format string
switch y := math.Abs(x); {
case y == 0 || y >= 999.95:
format = "%10.0f %s"
case y >= 99.995:
format = "%12.1f %s"
case y >= 9.9995:
format = "%13.2f %s"
case y >= 0.99995:
format = "%14.3f %s"
case y >= 0.099995:
format = "%15.4f %s"
case y >= 0.0099995:
format = "%16.5f %s"
case y >= 0.00099995:
format = "%17.6f %s"
default:
format = "%18.7f %s"
}
fmt.Fprintf(w, format, x, unit)
}
/ MemString returns r.AllocedBytesPerOp and r.AllocsPerOp in the same format as 'go test'.
func (r BenchmarkResult) MemString() string {
return fmt.Sprintf("%8d B/op\t%8d allocs/op",
r.AllocedBytesPerOp(), r.AllocsPerOp())
}
/ benchmarkName returns full name of benchmark including procs suffix.
func benchmarkName(name string, n int) string {
if n != 1 {
return fmt.Sprintf("%s-%d", name, n)
}
return name
}
type benchContext struct {
match *matcher
maxLen int / The largest recorded benchmark name.
extLen int / Maximum extension length.
}
/ RunBenchmarks is an internal function but exported because it is cross-package;
/ it is part of the implementation of the "go test" command.
func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
runBenchmarks("", matchString, benchmarks)
}
func runBenchmarks(importPath string, matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) bool {
/ If no flag was specified, don't run benchmarks.
if len(*matchBenchmarks) == 0 {
return true
}
/ Collect matching benchmarks and determine longest name.
maxprocs := 1
for _, procs := range cpuList {
if procs > maxprocs {
maxprocs = procs
}
}
ctx := &benchContext{
match: newMatcher(matchString, *matchBenchmarks, "-test.bench"),
extLen: len(benchmarkName("", maxprocs)),
}
var bs []InternalBenchmark
for _, Benchmark := range benchmarks {
if _, matched, _ := ctx.match.fullName(nil, Benchmark.Name); matched {
bs = append(bs, Benchmark)
benchName := benchmarkName(Benchmark.Name, maxprocs)
if l := len(benchName) + ctx.extLen + 1; l > ctx.maxLen {
ctx.maxLen = l
}
}
}
main := &B{
common: common{
name: "Main",
w: os.Stdout,
bench: true,
},
importPath: importPath,
benchFunc: func(b *B) {
for _, Benchmark := range bs {
b.Run(Benchmark.Name, Benchmark.F)
}
},
benchTime: benchTime,
context: ctx,
}
if Verbose() {
main.chatty = newChattyPrinter(main.w)
}
main.runN(1)
return !main.failed
}
/ processBench runs bench b for the configured CPU counts and prints the results.
func (ctx *benchContext) processBench(b *B) {
for i, procs := range cpuList {
for j := uint(0); j < *count; j++ {
runtime.GOMAXPROCS(procs)
benchName := benchmarkName(b.name, procs)
/ If it's chatty, we've already printed this information.
if b.chatty == nil {
fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
}
/ Recompute the running time for all but the first iteration.
if i > 0 || j > 0 {
b = &B{
common: common{
signal: make(chan bool),
name: b.name,
w: b.w,
chatty: b.chatty,
bench: true,
},
benchFunc: b.benchFunc,
benchTime: b.benchTime,
}
b.run1()
}
r := b.doBench()
if b.failed {
/ The output could be very long here, but probably isn't.
/ We print it all, regardless, because we don't want to trim the reason
/ the benchmark failed.
fmt.Fprintf(b.w, "--- FAIL: %s\n%s", benchName, b.output)
continue
}
results := r.String()
if b.chatty != nil {
fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
}
if *benchmarkMemory || b.showAllocResult {
results += "\t" + r.MemString()
}
fmt.Fprintln(b.w, results)
/ Unlike with tests, we ignore the -chatty flag and always print output for
/ benchmarks since the output generation time will skew the results.
if len(b.output) > 0 {
b.trimOutput()
fmt.Fprintf(b.w, "--- BENCH: %s\n%s", benchName, b.output)
}
if p := runtime.GOMAXPROCS(-1); p != procs {
fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
}
}
}
}
/ Run benchmarks f as a subbenchmark with the given name. It reports
/ whether there were any failures.
/
/ A subbenchmark is like any other benchmark. A benchmark that calls Run at
/ least once will not be measured itself and will be called once with N=1.
func (b *B) Run(name string, f func(b *B)) bool {
/ Since b has subbenchmarks, we will no longer run it as a benchmark itself.
/ Release the lock and acquire it on exit to ensure locks stay paired.
atomic.StoreInt32(&b.hasSub, 1)
benchmarkLock.Unlock()
defer benchmarkLock.Lock()
benchName, ok, partial := b.name, true, false
if b.context != nil {
benchName, ok, partial = b.context.match.fullName(&b.common, name)
}
if !ok {
return true
}
var pc [maxStackLen]uintptr
n := runtime.Callers(2, pc[:])
sub := &B{
common: common{
signal: make(chan bool),
name: benchName,
parent: &b.common,
level: b.level + 1,
creator: pc[:n],
w: b.w,
chatty: b.chatty,
bench: true,
},
importPath: b.importPath,
benchFunc: f,
benchTime: b.benchTime,
context: b.context,
}
if partial {
/ Partial name match, like -bench=X/Y matching BenchmarkX.
/ Only process sub-benchmarks, if any.
atomic.StoreInt32(&sub.hasSub, 1)
}
if b.chatty != nil {
labelsOnce.Do(func() {
fmt.Printf("goos: %s\n", runtime.GOOS)
fmt.Printf("goarch: %s\n", runtime.GOARCH)
if b.importPath != "" {
fmt.Printf("pkg: %s\n", b.importPath)
}
if cpu := sysinfo.CPU.Name(); cpu != "" {
fmt.Printf("cpu: %s\n", cpu)
}
})
fmt.Println(benchName)
}
if sub.run1() {
sub.run()
}
b.add(sub.result)
return !sub.failed
}
/ add simulates running benchmarks in sequence in a single iteration. It is
/ used to give some meaningful results in case func Benchmark is used in
/ combination with Run.
func (b *B) add(other BenchmarkResult) {
r := &b.result
/ The aggregated BenchmarkResults resemble running all subbenchmarks as
/ in sequence in a single benchmark.
r.N = 1
r.T += time.Duration(other.NsPerOp())
if other.Bytes == 0 {
/ Summing Bytes is meaningless in aggregate if not all subbenchmarks
/ set it.
b.missingBytes = true
r.Bytes = 0
}
if !b.missingBytes {
r.Bytes += other.Bytes
}
r.MemAllocs += uint64(other.AllocsPerOp())
r.MemBytes += uint64(other.AllocedBytesPerOp())
}
/ trimOutput shortens the output from a benchmark, which can be very long.
func (b *B) trimOutput() {
/ The output is likely to appear multiple times because the benchmark
/ is run multiple times, but at least it will be seen. This is not a big deal
/ because benchmarks rarely print, but just in case, we trim it if it's too long.
const maxNewlines = 10
for nlCount, j := 0, 0; j < len(b.output); j++ {
if b.output[j] == '\n' {
nlCount++
if nlCount >= maxNewlines {
b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
break
}
}
}
}
/ A PB is used by RunParallel for running parallel benchmarks.
type PB struct {
globalN *uint64 / shared between all worker goroutines iteration counter
grain uint64 / acquire that many iterations from globalN at once
cache uint64 / local cache of acquired iterations
bN uint64 / total number of iterations to execute (b.N)
}
/ Next reports whether there are more iterations to execute.
func (pb *PB) Next() bool {
if pb.cache == 0 {
n := atomic.AddUint64(pb.globalN, pb.grain)
if n <= pb.bN {
pb.cache = pb.grain
} else if n < pb.bN+pb.grain {
pb.cache = pb.bN + pb.grain - n
} else {
return false
}
}
pb.cache--
return true
}
/ RunParallel runs a benchmark in parallel.
/ It creates multiple goroutines and distributes b.N iterations among them.
/ The number of goroutines defaults to GOMAXPROCS. To increase parallelism for
/ non-CPU-bound benchmarks, call SetParallelism before RunParallel.
/ RunParallel is usually used with the go test -cpu flag.
/
/ The body function will be run in each goroutine. It should set up any
/ goroutine-local state and then iterate until pb.Next returns false.
/ It should not use the StartTimer, StopTimer, or ResetTimer functions,
/ because they have global effect. It should also not call Run.
func (b *B) RunParallel(body func(*PB)) {
if b.N == 0 {
return / Nothing to do when probing.
}
/ Calculate grain size as number of iterations that take ~100µs.
/ 100µs is enough to amortize the overhead and provide sufficient
/ dynamic load balancing.
grain := uint64(0)
if b.previousN > 0 && b.previousDuration > 0 {
grain = 1e5 * uint64(b.previousN) / uint64(b.previousDuration)
}
if grain < 1 {
grain = 1
}
/ We expect the inner loop and function call to take at least 10ns,
/ so do not do more than 100µs/10ns=1e4 iterations.
if grain > 1e4 {
grain = 1e4
}
n := uint64(0)
numProcs := b.parallelism * runtime.GOMAXPROCS(0)
var wg sync.WaitGroup
wg.Add(numProcs)
for p := 0; p < numProcs; p++ {
go func() {
defer wg.Done()
pb := &PB{
globalN: &n,
grain: grain,
bN: uint64(b.N),
}
body(pb)
}()
}
wg.Wait()
if n <= uint64(b.N) && !b.Failed() {
b.Fatal("RunParallel: body exited without pb.Next() == false")
}
}
/ SetParallelism sets the number of goroutines used by RunParallel to p*GOMAXPROCS.
/ There is usually no need to call SetParallelism for CPU-bound benchmarks.
/ If p is less than 1, this call will have no effect.
func (b *B) SetParallelism(p int) {
if p >= 1 {
b.parallelism = p
}
}
/ Benchmark benchmarks a single function. It is useful for creating
/ custom benchmarks that do not use the "go test" command.
/
/ If f depends on testing flags, then Init must be used to register
/ those flags before calling Benchmark and before calling flag.Parse.
/
/ If f calls Run, the result will be an estimate of running all its
/ subbenchmarks that don't call Run in sequence in a single benchmark.
func Benchmark(f func(b *B)) BenchmarkResult {
b := &B{
common: common{
signal: make(chan bool),
w: discard{},
},
benchFunc: f,
benchTime: benchTime,
}
if b.run1() {
b.run()
}
return b.result
}
type discard struct{}
func (discard) Write(b []byte) (n int, err error) { return len(b), nil }