Missed optimizations in C compilers
Among other things, I seen lots of inefficiencies with bitfields. I tend to use them a lot in hardware register access and packing of data in embedded development. Imagine a bitfield that fits into one word and setting each of the bitfields to a constant. A good compiler should be able to set all the values with one load operations. Some compilers would break this into many separate loads. I think the ARM compilers were worse at this while Clang would optimize it much better. Many times had to forgo bitfields and use macro definitions and masking to get the best code generation at the cost of readability.