A collection of declarative macros, which makes working with bitfields and similar things trivial.
Every macro which makes your live simpler should have zero performance penalty in a sense, that even though they might look complex and intimidating. They actually just evaluate, to exactly the same code you'd write by hand and LLVM can optimize them to dust.
Frame formats, like the one used by IEEE802.11, use a metric crap ton(definetly an SI unit) of bitfields and, since I appear to be addicted to writing binary parsers, have developed it to takeover some repetitive tasks.