bufio: optimize bufio.Reader.ReadString to avoid an allocation and copy

name                old time/op    new time/op    delta
ReaderReadString-4     226ns ±12%     161ns ±11%  -28.76%  (p=0.008 n=5+5)

name                old alloc/op   new alloc/op   delta
ReaderReadString-4      288B ± 0%      144B ± 0%  -50.00%  (p=0.008 n=5+5)

name                old allocs/op  new allocs/op  delta
ReaderReadString-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)

Change-Id: I77f330b8340c2bfbfff1f6f1000170b65953a200
GitHub-Last-Rev: 65d65302a7b80504b4d37b81a3843fe1439e638a
GitHub-Pull-Request: golang/go#34706
Reviewed-on: https://go-review.googlesource.com/c/go/+/199257
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index 4c4522c660..cb68f3ba23 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -535,6 +535,23 @@ func TestReadWriteRune(t *testing.T) {
 	}
 }
 
+func TestReadStringAllocs(t *testing.T) {
+	r := strings.NewReader("       foo       foo        42        42        42        42        42        42        42        42       4.2       4.2       4.2       4.2\n")
+	buf := NewReader(r)
+	allocs := testing.AllocsPerRun(100, func() {
+		r.Seek(0, io.SeekStart)
+		buf.Reset(r)
+
+		_, err := buf.ReadString('\n')
+		if err != nil {
+			t.Fatal(err)
+		}
+	})
+	if allocs != 1 {
+		t.Errorf("Unexpected number of allocations, got %f, want 1", allocs)
+	}
+}
+
 func TestWriter(t *testing.T) {
 	var data [8192]byte
 
@@ -1644,6 +1661,21 @@ func BenchmarkReaderWriteToOptimal(b *testing.B) {
 	}
 }
 
+func BenchmarkReaderReadString(b *testing.B) {
+	r := strings.NewReader("       foo       foo        42        42        42        42        42        42        42        42       4.2       4.2       4.2       4.2\n")
+	buf := NewReader(r)
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		r.Seek(0, io.SeekStart)
+		buf.Reset(r)
+
+		_, err := buf.ReadString('\n')
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
 func BenchmarkWriterCopyOptimal(b *testing.B) {
 	// Optimal case is where the underlying writer implements io.ReaderFrom
 	srcBuf := bytes.NewBuffer(make([]byte, 8192))