cmd/link: optimize int parsing

Speeds up linking cmd/go by ~1.5%:

name       old s/op   new s/op   delta
LinkCmdGo  0.58 ± 6%  0.57 ± 5%  -1.21%  (p=0.000 n=98+99)

Less noisy benchmark, with garbage collection off:

name       old s/op   new s/op   delta
LinkCmdGo  0.49 ± 2%  0.49 ± 2%  -1.79%  (p=0.000 n=98+99)

Change-Id: I0123bcb66a87cbc4d703356e4c5a4035032012ec
Reviewed-on: https://go-review.googlesource.com/20916Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/util.go

@@ -33,6 +33,8 @@ type Biobuf struct {
 	linelen int
 }
 
+func (b *Biobuf) Reader() *bufio.Reader { return b.r }
+
 func Bopenw(name string) (*Biobuf, error) {
 	f, err := os.Create(name)
 	if err != nil {

src/cmd/link/internal/ld/objfile.go

@@ -395,15 +395,17 @@ func readref(ctxt *Link, f *obj.Biobuf, pkg string, pn string) {
 }
 
 func rdint64(f *obj.Biobuf) int64 {
-	var c int
-
+	r := f.Reader()
 	uv := uint64(0)
-	for shift := 0; ; shift += 7 {
+	for shift := uint(0); ; shift += 7 {
 		if shift >= 64 {
 			log.Fatalf("corrupt input")
 		}
-		c = obj.Bgetc(f)
-		uv |= uint64(c&0x7F) << uint(shift)
+		c, err := r.ReadByte()
+		if err != nil {
+			log.Fatalln("error reading input: ", err)
+		}
+		uv |= uint64(c&0x7F) << shift
 		if c&0x80 == 0 {
 			break
 		}