tree b887b06eddfd75a343031a86ffc8fff83026b86b
parent 4937f05ccd0b97ff5aeaa1d038cf7f6bc8f7dd36
author David Benjamin <davidben@google.com> 1632256395 -0400
committer Adam Langley <agl@google.com> 1632344374 +0000

Speed up constant-time base64 decoding.

I was inspired to look at this again recently and noticed we could do a
bit better. Instead of a tower of selects, rely on all the cases being
mutually exclusive and use the ret |= mask & value formulation without
loss in clarity. We do need to fixup the invalid case slightly, but
since that computation is mostly independent, I'm guessing the CPU and
compiler are able to schedule it effectively.

Before:
Did 251000 base64 decode operations in 2002569us (159.4 MB/sec)
After:
Did 346000 base64 decode operations in 2005426us (219.5 MB/sec) [+37.7%]

Change-Id: I542167202fd4e94c93dd5a2519a97bc388072c89
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/49525
Reviewed-by: Adam Langley <agl@google.com>