I made Go bindings for that library https://github.com/myfreeweb/go-base64-simd …

…to use in a mail indexer project…

…only to discover that half of my mail was not being decoded, because that decoder does not skip whitespace.

Daniel Lemire has done brilliant work. His https://github.com/lemire/despacer looks great.

I wonder though if their Base64 SIMD algorithms could be tweaked to do the decode in a single pass?

Like you, also email related, I have written a native Base64 addon for Node in C optimized for decoding MIME wrapped Base64 in a single pass (https://github.com/ronomon/base64). It's not yet using SIMD instructions but it's 1.5-2x as fast as Node's routines with top speeds of 1398.18 MB/s decoding and 1048.58 MB/s encoding on a single core (Intel Xeon E5-1620 v3 @ 3.50GHz). Here is an explanation of the difference: https://github.com/nodejs/node/issues/12114

The Ronomon Base64 addon also ships with a fuzz test (https://github.com/ronomon/base64/blob/master/test.js) which you are welcome to try out on your Go binding. It picked up an interesting bug in Node's decode routine where some white space was actually decoded as data (https://github.com/nodejs/node/issues/11987).