Yep. Another thing to watch out for is using bitfields to access device registers. One time, I was working with an older embedded PPC architecture on a driver

Yep. Another thing to watch out for is using bitfields to access device registers. One time, I was working with an older embedded PPC architecture on a driver to talk to an FPGA that was attached to a 32 bit local bus. The problem is that it didn’t support unaligned accesses. The lower two address bits simply weren’t hooked up, so any access to byte addresses that weren’t a multiple of 4 would just behave as if you masked the two lower address bits off. There was a structure that used bitfields to access bits in a register on that FPGA. It worked fine til I updated GCC, then it stopped working. It turns out that the newer version of GCC would do a single byte unaligned read if you were accessing, say, bits 8-15 in a 32 bit bitfield, whereas the older GCC would read the full 32 bit word and shift/mask as needed. It turns out you can force the older behavior with -fstrict-volatile-bitfields. Took a minute to figure out, but I learned my lesson. That said, I don’t think you should really be doing IO that way anyway. I typically use IO accessor methods, sometimes with raw addresses, sometimes with struct overlays and taking the address of the member.