Assume I want to reserve 8 bytes on the stack and I also want to make sure current stack pointer is 8 byte aligned. I have seen some codes that assure current sp is 8 bye aligned using this logic:
sp = sp & -8;
They AND it with the amount they are going to reserve on the stack (which of course is negative).
How does this logic work?
It works because negative numbers are represented in two's complement, so -8 is equivalent to ~7, of which the 3 least significant bits are 0s and the rest are 1s. ANDing this with a value clears the 3 least significant bits, which obviously results in it being 8-byte aligned. By the way, this trick only works to align things to powers of 2. If you had some strange reason to align things to a 12-byte boundary, for example, sp = sp & -12 would not work as intended.
