encoding/csv: update and add CSV reading benchmarks

Benchmarks broken off from https://golang.org/cl/24723 and modified to
allocate less in the places we're not trying to measure.

Updates #16791

Change-Id: I508e4cfeac60322d56f1d71ff1912f6a6f183a63
Reviewed-on: https://go-review.googlesource.com/30357
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/csv/reader_test.go

@@ -5,6 +5,7 @@
 package csv
 
 import (
+	"io"
 	"reflect"
 	"strings"
 	"testing"
@@ -292,8 +293,52 @@ func TestRead(t *testing.T) {
 	}
 }
 
-func BenchmarkRead(b *testing.B) {
-	data := `x,y,z,w
+// nTimes is an io.Reader which yields the string s n times.
+type nTimes struct {
+	s   string
+	n   int
+	off int
+}
+
+func (r *nTimes) Read(p []byte) (n int, err error) {
+	for {
+		if r.n <= 0 || r.s == "" {
+			return n, io.EOF
+		}
+		n0 := copy(p, r.s[r.off:])
+		p = p[n0:]
+		n += n0
+		r.off += n0
+		if r.off == len(r.s) {
+			r.off = 0
+			r.n--
+		}
+		if len(p) == 0 {
+			return
+		}
+	}
+}
+
+// benchmarkRead measures reading the provided CSV rows data.
+// initReader, if non-nil, modifies the Reader before it's used.
+func benchmarkRead(b *testing.B, initReader func(*Reader), rows string) {
+	b.ReportAllocs()
+	r := NewReader(&nTimes{s: rows, n: b.N})
+	if initReader != nil {
+		initReader(r)
+	}
+	for {
+		_, err := r.Read()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+const benchmarkCSVData = `x,y,z,w
 x,y,z,
 x,y,,
 x,,,
@@ -305,11 +350,22 @@ x,,,
 "","","",""
 `
 
-	for i := 0; i < b.N; i++ {
-		_, err := NewReader(strings.NewReader(data)).ReadAll()
+func BenchmarkRead(b *testing.B) {
+	benchmarkRead(b, nil, benchmarkCSVData)
+}
 
-		if err != nil {
-			b.Fatalf("could not read data: %s", err)
-		}
-	}
+func BenchmarkReadWithFieldsPerRecord(b *testing.B) {
+	benchmarkRead(b, func(r *Reader) { r.FieldsPerRecord = 4 }, benchmarkCSVData)
+}
+
+func BenchmarkReadWithoutFieldsPerRecord(b *testing.B) {
+	benchmarkRead(b, func(r *Reader) { r.FieldsPerRecord = -1 }, benchmarkCSVData)
+}
+
+func BenchmarkReadLargeFields(b *testing.B) {
+	benchmarkRead(b, nil, strings.Repeat(`xxxxxxxxxxxxxxxx,yyyyyyyyyyyyyyyy,zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww,vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
+xxxxxxxxxxxxxxxxxxxxxxxx,yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy,zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww,vvvv
+,,zzzz,wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww,vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy,zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww,vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
+`, 3))
 }