Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go 1.8.1

What operating system and processor architecture are you using (go env)?

amd64, darwin

What did you do?

go test encoding/base64 --bench=BenchmarkEncodeToString

What did you expect to see?

It should be possible to increase the speed of the encoder by 3 or 4 times by using SIMD instructions. So ideally, I'd like to see something like

go test --bench=BenchmarkEncodeToString
BenchmarkEncodeToString-8 500000 3777 ns/op 2168.51 MB/s
PASS

What did you see instead?

BenchmarkEncodeToString-8 200000 11393 ns/op 719.01 MB/s
PASS
ok encoding/base64 2.406s