cmd/cgo: parallelize loadDefines calls

```
export CC="zig cc -target x86_64-linux"
hyperfine '../pkg/tool/darwin_arm64/cgo -objdir /tmp net/cgo_linux.go net/cgo_resnew.go net/cgo_socknew.go net/cgo_unix_cgo.go net/cgo_unix_cgo_res.go'
```

**Before**
```
  Time (mean ± sig):      1.293 s ±  0.017 s    [User: 0.472 s, System: 0.451 s]
  Range (min ... max):    1.263 s ...  1.316 s    10 runs

```

**After**
```
  Time (mean ±sig):     986.5 ms ±  22.6 ms    [User: 487.0 ms, System: 519.5 ms]
  Range (min ... max):   950.7 ms ... 1022.2 ms    10 runs

```

The version after changes is 25% faster for 5 input files (std "net" package).
I also tried to make CC artifictially slower (wrapper with sleep 0.2) and it showes same 25% performance increase.

Change-Id: I7a26fdc8d8a23b0df9bc71d30b96e82e2ddb943b
Reviewed-on: https://go-review.googlesource.com/c/go/+/581336
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/gcc.go

@@ -193,7 +193,6 @@ func (p *Package) Translate(f *File) {
 	var conv typeConv
 	conv.Init(p.PtrSize, p.IntSize)
 
-	p.loadDefines(f)
 	p.typedefs = map[string]bool{}
 	p.typedefList = nil
 	numTypedefs := -1
@@ -233,12 +232,14 @@ func (p *Package) Translate(f *File) {
 
 // loadDefines coerces gcc into spitting out the #defines in use
 // in the file f and saves relevant renamings in f.Name[name].Define.
-func (p *Package) loadDefines(f *File) {
+// Returns true if env:CC is Clang
+func (f *File) loadDefines(gccOptions []string) bool {
 	var b bytes.Buffer
 	b.WriteString(builtinProlog)
 	b.WriteString(f.Preamble)
-	stdout := p.gccDefines(b.Bytes())
+	stdout := gccDefines(b.Bytes(), gccOptions)
 
+	var gccIsClang bool
 	for _, line := range strings.Split(stdout, "\n") {
 		if len(line) < 9 || line[0:7] != "#define" {
 			continue
@@ -261,7 +262,7 @@ func (p *Package) loadDefines(f *File) {
 		}
 
 		if key == "__clang__" {
-			p.GccIsClang = true
+			gccIsClang = true
 		}
 
 		if n := f.Name[key]; n != nil {
@@ -271,6 +272,7 @@ func (p *Package) loadDefines(f *File) {
 			n.Define = val
 		}
 	}
+	return gccIsClang
 }
 
 // guessKinds tricks gcc into revealing the kind of each
@@ -1722,7 +1724,7 @@ func checkGCCBaseCmd() ([]string, error) {
 }
 
 // gccMachine returns the gcc -m flag to use, either "-m32", "-m64" or "-marm".
-func (p *Package) gccMachine() []string {
+func gccMachine() []string {
 	switch goarch {
 	case "amd64":
 		if goos == "darwin" {
@@ -1795,7 +1797,7 @@ func (p *Package) gccCmd() []string {
 	}
 
 	c = append(c, p.GccOptions...)
-	c = append(c, p.gccMachine()...)
+	c = append(c, gccMachine()...)
 	if goos == "aix" {
 		c = append(c, "-maix64")
 		c = append(c, "-mcmodel=large")
@@ -2187,10 +2189,10 @@ func (p *Package) gccDebug(stdin []byte, nnames int) (d *dwarf.Data, ints []int6
 // and returns the corresponding standard output, which is the
 // #defines that gcc encountered while processing the input
 // and its included files.
-func (p *Package) gccDefines(stdin []byte) string {
+func gccDefines(stdin []byte, gccOptions []string) string {
 	base := append(gccBaseCmd, "-E", "-dM", "-xc")
-	base = append(base, p.gccMachine()...)
+	base = append(base, gccMachine()...)
-	stdout, _ := runGcc(stdin, append(append(base, p.GccOptions...), "-"))
+	stdout, _ := runGcc(stdin, append(append(base, gccOptions...), "-"))
 	return stdout
 }
 

src/cmd/cgo/main.go

@@ -24,6 +24,7 @@ import (
 	"runtime"
 	"sort"
 	"strings"
+	"sync"
 
 	"cmd/internal/edit"
 	"cmd/internal/notsha256"
@@ -390,6 +391,8 @@ func main() {
 	// Use the beginning of the notsha256 of the input to disambiguate.
 	h := notsha256.New()
 	io.WriteString(h, *importPath)
+	var once sync.Once
+	var wg sync.WaitGroup
 	fs := make([]*File, len(goFiles))
 	for i, input := range goFiles {
 		if *srcDir != "" {
@@ -411,6 +414,9 @@ func main() {
 			fatalf("%s", err)
 		}
 
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
 			// Apply trimpath to the file path. The path won't be read from after this point.
 			input, _ = objabi.ApplyRewrites(input, *trimpath)
 			if strings.ContainsAny(input, "\r\n") {
@@ -425,9 +431,17 @@ func main() {
 			f.Edit = edit.NewBuffer(b)
 			f.ParseGo(input, b)
 			f.ProcessCgoDirectives()
+			gccIsClang := f.loadDefines(p.GccOptions)
+			once.Do(func() {
+				p.GccIsClang = gccIsClang
+			})
+
 			fs[i] = f
+		}()
 	}
 
+	wg.Wait()
+
 	cPrefix = fmt.Sprintf("_%x", h.Sum(nil)[0:6])
 
 	if *objDir == "" {