It's not just that. If you can't trust the ABI then everything else is wrong too. Everything. Not just zeroing registers. It only just so happens that in this c

It's not just that. If you can't trust the ABI then everything else is wrong too. Everything. Not just zeroing registers. It only just so happens that in this case XMM7 is used for zeroing, but it could be used to save a variable across a function call. Then there is no trick to get it set back to the right value. The ABI is not optional, or best effort, or best practice, or any other BS that passes in the ordinary world. It is just as required as the correct operation of instructions (e.g., add should actually add things, mul should actually multiply them, and so on).