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/8880
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/encoding/csv/reader.go b/src/encoding/csv/reader.go
index d9432954ac..d0a09044fb 100644
--- a/src/encoding/csv/reader.go
+++ b/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()