encoding/base64: lift nil check out of encode loop
Most of the encoding time is spent in the first Encode loop, since the
rest of the function only deals with the few remaining bytes. Any
unnecessary work done in that loop body matters tremendously.
One such unnecessary bottleneck was the use of the enc.encode table.
Since enc is a pointer receiver, and the field is first used within the
loop, the encoder must perform a nil check at every iteration.
Add a dummy use of the field before the start of the loop, to move the
nil check there. After that line, the compiler now knows that enc can't
be nil, and thus the hot loop is free of nil checks.
name old time/op new time/op delta
EncodeToString-4 14.7µs ± 0% 13.7µs ± 1% -6.53% (p=0.000 n=10+10)
name old speed new speed delta
EncodeToString-4 559MB/s ± 0% 598MB/s ± 1% +6.99% (p=0.000 n=10+10)