cmd/cgo: read source files once

Now cgo reads source files twice: for c prefix generation and parsing
go code to an ast node. It can be narrowed down to single loop.

Change-Id: Ie05452a3a12106aaab863244727390037e69e8e6
Reviewed-on: https://go-review.googlesource.com/40939
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/ast.go

@@ -17,8 +17,8 @@ import (
 	"strings"
 )
 
-func parse(name string, flags parser.Mode) *ast.File {
-	ast1, err := parser.ParseFile(fset, name, nil, flags)
+func parse(name string, src []byte, flags parser.Mode) *ast.File {
+	ast1, err := parser.ParseFile(fset, name, src, flags)
 	if err != nil {
 		if list, ok := err.(scanner.ErrorList); ok {
 			// If err is a scanner.ErrorList, its String will print just
@@ -39,12 +39,12 @@ func sourceLine(n ast.Node) int {
 	return fset.Position(n.Pos()).Line
 }
 
-// ReadGo populates f with information learned from reading the
-// Go source file with the given file name. It gathers the C preamble
+// ParseGo populates f with information learned from the Go source code
+// which was read from the named file. It gathers the C preamble
 // attached to the import "C" comment, a list of references to C.xxx,
 // a list of exported functions, and the actual AST, to be rewritten and
 // printed.
-func (f *File) ReadGo(name string) {
+func (f *File) ParseGo(name string, src []byte) {
 	// Create absolute path for file, so that it will be used in error
 	// messages and recorded in debug line number information.
 	// This matches the rest of the toolchain. See golang.org/issue/5122.
@@ -58,8 +58,8 @@ func (f *File) ReadGo(name string) {
 	// so we use ast1 to look for the doc comments on import "C"
 	// and on exported functions, and we use ast2 for translating
 	// and reprinting.
-	ast1 := parse(name, parser.ParseComments)
-	ast2 := parse(name, 0)
+	ast1 := parse(name, src, parser.ParseComments)
+	ast2 := parse(name, src, 0)
 
 	f.Package = ast1.Name.Name
 	f.Name = make(map[string]*Name)

src/cmd/cgo/main.go

@@ -17,7 +17,7 @@ import (
 	"go/ast"
 	"go/printer"
 	"go/token"
-	"io"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -265,30 +265,28 @@ func main() {
 	// concern is other cgo wrappers for the same functions.
 	// Use the beginning of the md5 of the input to disambiguate.
 	h := md5.New()
-	for _, input := range goFiles {
-		if *srcDir != "" {
-			input = filepath.Join(*srcDir, input)
-		}
-		f, err := os.Open(input)
-		if err != nil {
-			fatalf("%s", err)
-		}
-		io.Copy(h, f)
-		f.Close()
-	}
-	cPrefix = fmt.Sprintf("_%x", h.Sum(nil)[0:6])
-
 	fs := make([]*File, len(goFiles))
 	for i, input := range goFiles {
 		if *srcDir != "" {
 			input = filepath.Join(*srcDir, input)
 		}
+
+		b, err := ioutil.ReadFile(input)
+		if err != nil {
+			fatalf("%s", err)
+		}
+		if _, err = h.Write(b); err != nil {
+			fatalf("%s", err)
+		}
+
 		f := new(File)
-		f.ReadGo(input)
+		f.ParseGo(input, b)
 		f.DiscardCgoDirectives()
 		fs[i] = f
 	}
 
+	cPrefix = fmt.Sprintf("_%x", h.Sum(nil)[0:6])
+
 	if *objDir == "" {
 		// make sure that _obj directory exists, so that we can write
 		// all the output files there.