cmd/go: parallelize fmt

Currently go fmt formats all files sequentially.
That's a shame. Parallelize it over files.

Reduces time of go fmt ./... in std lib
from ~6.1s to ~0.9s.

Reduces time of go fmt github.com/google/syzkaller/...
from ~5.2s to ~1.8s.

Change-Id: I3d27fc25326106b2a4781e13506a25c12d5bcdc5
Reviewed-on: https://go-review.googlesource.com/45491
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/fmtcmd/fmt.go

@@ -8,6 +8,8 @@ package fmtcmd
 import (
 	"os"
 	"path/filepath"
+	"runtime"
+	"sync"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
@@ -41,13 +43,29 @@ See also: go fix, go vet.
 
 func runFmt(cmd *base.Command, args []string) {
 	gofmt := gofmtPath()
+	procs := runtime.GOMAXPROCS(0)
+	var wg sync.WaitGroup
+	wg.Add(procs)
+	fileC := make(chan string, 2*procs)
+	for i := 0; i < procs; i++ {
+		go func() {
+			defer wg.Done()
+			for file := range fileC {
+				base.Run(str.StringList(gofmt, "-l", "-w", file))
+			}
+		}()
+	}
 	for _, pkg := range load.Packages(args) {
 		// Use pkg.gofiles instead of pkg.Dir so that
 		// the command only applies to this package,
 		// not to packages in subdirectories.
 		files := base.FilterDotUnderscoreFiles(base.RelPaths(pkg.Internal.AllGoFiles))
-		base.Run(str.StringList(gofmt, "-l", "-w", files))
+		for _, file := range files {
+			fileC <- file
+		}
 	}
+	close(fileC)
+	wg.Wait()
 }
 
 func gofmtPath() string {