bytes: improve WriteRune performance

Remove the runeBytes buffer and write the utf8 encoding directly
to the internal buf byte slice.

name         old time/op   new time/op   delta
WriteRune-4   80.5µs ± 2%   57.1µs ± 2%  -29.06%  (p=0.000 n=20+20)

name         old speed     new speed     delta
WriteRune-4  153MB/s ± 2%  215MB/s ± 2%  +40.96%  (p=0.000 n=20+20)

Change-Id: Ic15f6e2d6e56a3d15c74f56159e2eae020ba73ba
Reviewed-on: https://go-review.googlesource.com/28816
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/buffer.go

@@ -15,11 +15,10 @@ import (
 // A Buffer is a variable-sized buffer of bytes with Read and Write methods.
 // The zero value for Buffer is an empty buffer ready to use.
 type Buffer struct {
-	buf       []byte            // contents are the bytes buf[off : len(buf)]
-	off       int               // read at &buf[off], write at &buf[len(buf)]
-	runeBytes [utf8.UTFMax]byte // avoid allocation of slice on each call to WriteRune
-	bootstrap [64]byte          // memory to hold first slice; helps small buffers avoid allocation.
-	lastRead  readOp            // last read operation, so that Unread* can work correctly.
+	buf       []byte   // contents are the bytes buf[off : len(buf)]
+	off       int      // read at &buf[off], write at &buf[len(buf)]
+	bootstrap [64]byte // memory to hold first slice; helps small buffers avoid allocation.
+	lastRead  readOp   // last read operation, so that Unread* can work correctly.
 }
 
 // The readOp constants describe the last action performed on
@@ -246,8 +245,10 @@ func (b *Buffer) WriteRune(r rune) (n int, err error) {
 		b.WriteByte(byte(r))
 		return 1, nil
 	}
-	n = utf8.EncodeRune(b.runeBytes[0:], r)
-	b.Write(b.runeBytes[0:n])
+	b.lastRead = opInvalid
+	m := b.grow(utf8.UTFMax)
+	n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
+	b.buf = b.buf[:m+n]
 	return n, nil
 }