encoding/csv: Preallocate records slice

Currently parseRecord will always start with a nil
slice and then resize the slice on append. For input
with a fixed number of fields per record we can preallocate
the slice to avoid having to resize the slice.

This change implements this optimization by using
FieldsPerRecord as capacity if it's > 0 and also adds a
benchmark to better show the differences.

benchmark         old ns/op     new ns/op     delta
BenchmarkRead     19741         17909         -9.28%

benchmark         old allocs     new allocs     delta
BenchmarkRead     59             41             -30.51%

benchmark         old bytes     new bytes     delta
BenchmarkRead     6276          5844          -6.88%

Change-Id: I7c2abc9c80a23571369bcfcc99a8ffc474eae7ab
Reviewed-on: https://go-review.googlesource.com/8880Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/csv/reader.go

@@ -228,6 +228,12 @@ func (r *Reader) parseRecord() (fields []string, err error) {
 	}
 	r.r.UnreadRune()
 
+	// If FieldsPerRecord is greater then 0 we can assume the final
+	// length of fields to be equal to FieldsPerRecord.
+	if r.FieldsPerRecord > 0 {
+		fields = make([]string, 0, r.FieldsPerRecord)
+	}
+
 	// At this point we have at least one field.
 	for {
 		haveField, delim, err := r.parseField()

src/encoding/csv/reader_test.go

@@ -282,3 +282,25 @@ func TestRead(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkRead(b *testing.B) {
+	data := `x,y,z,w
+x,y,z,
+x,y,,
+x,,,
+,,,
+"x","y","z","w"
+"x","y","z",""
+"x","y","",""
+"x","","",""
+"","","",""
+`
+
+	for i := 0; i < b.N; i++ {
+		_, err := NewReader(strings.NewReader(data)).ReadAll()
+
+		if err != nil {
+			b.Fatalf("could not read data: %s", err)
+		}
+	}
+}