debug/dwarf: optimize buf.string

(*buf).string previously manually searched
through its underlying byte slice until we
encountered a '0'. This change instead uses
bytes.IndexByte that results in a speed up:

$ benchstat before.txt after.txt

name         old time/op    new time/op    delta
BufString-8     257ns ± 1%     174ns ± 1%  -32.37%  (p=0.000 n=9+8)

name         old speed      new speed      delta
BufString-8   495MB/s ± 1%   732MB/s ± 1%  +47.76%  (p=0.000 n=10+8)

name         old alloc/op   new alloc/op   delta
BufString-8      162B ± 0%      162B ± 0%     ~     (all equal)

name         old allocs/op  new allocs/op  delta
BufString-8      3.00 ± 0%      3.00 ± 0%     ~     (all equal)

Change-Id: I7cf241742cc091d5d30d987a168b02d83955b1cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/196657
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/debug/dwarf/buf.go

@@ -7,6 +7,7 @@
 package dwarf
 
 import (
+	"bytes"
 	"encoding/binary"
 	"strconv"
 )
@@ -79,16 +80,16 @@ func (b *buf) bytes(n int) []byte {
 func (b *buf) skip(n int) { b.bytes(n) }
 
 func (b *buf) string() string {
-	for i := 0; i < len(b.data); i++ {
-		if b.data[i] == 0 {
-			s := string(b.data[0:i])
-			b.data = b.data[i+1:]
-			b.off += Offset(i + 1)
-			return s
-		}
+	i := bytes.IndexByte(b.data, 0)
+	if i < 0 {
+		b.error("underflow")
+		return ""
 	}
-	b.error("underflow")
-	return ""
+
+	s := string(b.data[0:i])
+	b.data = b.data[i+1:]
+	b.off += Offset(i + 1)
+	return s
 }
 
 func (b *buf) uint16() uint16 {