Whats an ABI and why is it so messy?
“ABI” stands for “Application Binary Interface.” Conventionally, it refers to a great mass of details about how arguments are arranged on the call stack and/or in registers, and how various types are arranged and padded in structs. A single CPU design may suffer multiple ABIs designed by different development tool vendors who made different choices, or even by the same vendor for different target applications or compiler versions. In ideal circumstances the CPU designer presents one ABI and all the OSes and compilers use it. In practice every ABI omits details that compiler implementers (consciously or accidentally) must choose for themselves. That ABI definition suffices for compilers to generate code so a program can interact safely with an OS and its lowest-level libraries. Users usually want an ABI to encompass more detail, allowing libraries built with different compilers (or different releases of the same compiler!) to be linked together. For C++, this includes many more details
