Function rounds 

pub fn rounds(func: &mut Func, to: u32)

Rounds up the bytes every variable length array asks for, so that the stack pointer stays where a call can be made from.

The size of one is whatever the program wrote in the brackets times the size of an element, so it is any number at all, and the bytes come off the stack pointer where the declaration stands. A stack pointer moved by an odd number is one no call can be made through afterwards: the convention says a call arrives with the stack pointer on a multiple of to, and every argument passed on the stack, every spill of a vector register and the alignment of every local below the array is counted from there. So the number that comes off is the size rounded up, which is (size + to - 1) & -to and is three instructions the machine already has.

Here rather than in the front end because to is the convention’s and the front end writes one IR for every target. Here rather than in crate::lower because it is arithmetic, which is the one thing a lowering rule deliberately cannot do, and that is what this module is for.

An array asking for more alignment than to is not rounded any further and is refused later by name. Giving it what it asked for means masking the stack pointer as well as subtracting from it, and after that nothing in the frame has a constant distance from anywhere. See Growing in crate::frame.