c++ - When is it worthwhile to use bit fields?

Question

Is it worthwhile using C's bit-field implementation? If so, when is it ever used?

I was looking through some emulator code and it looks like the registers for the chips are not being implemented using bit fields.

Is this something that is avoided for performance reasons (or some other reason)?

Are there still times when bit-fields are used? (ie firmware to put on actual chips, etc)

Answer 1

Bit-fields are typically only used when there's a need to map structure fields to specific bit slices, where some hardware will be interpreting the raw bits. An example might be assembling an IP packet header. I can't see a compelling reason for an emulator to model a register using bit-fields, as it's never going to touch real hardware!

Whilst bit-fields can lead to neat syntax, they're pretty platform-dependent, and therefore non-portable. A more portable, but yet more verbose, approach is to use direct bitwise manipulation, using shifts and bit-masks.

If you use bit-fields for anything other than assembling (or disassembling) structures at some physical interface, performance may suffer. This is because every time you read or write from a bit-field, the compiler will have to generate code to do the masking and shifting, which will burn cycles.