Alignment and padding

Learn alignment and padding.

Alignment and padding

Every type has an alignment: a value can only sit at addresses that are multiples of that number. For primitives, alignment is usually equal to size (u8: 1, u16: 2, u32: 4, u64: 8). A compound type's alignment is the largest alignment among its fields; the empty bytes in between are called padding:

align_of::<u64>()       // 8
size_of::<(u8, u32)>()  // 8: 1 + 3 padding + 4
size_of::<[u16; 3]>()   // 6: no padding between array elements

Unaligned access is slow on some processors and a hardware fault on others; that's why Rust guarantees alignment through the type system.

Cache and size

Small data fits into more cache lines and is processed faster. In a vector with millions of elements, 8 bytes of padding per struct means megabytes of wasted memory. Common improvements: reordering fields, packing bool flags into bit fields, using a u32 index instead of usize where enough, switching to a "struct of arrays" layout.

// array of structs           // struct of arrays
struct P { x: f32, y: f32 }   struct Ps { x: Vec<f32>, y: Vec<f32> }
Vec<P>                        // a loop touching only x is faster

Exercise Alignment and size

Print align_of::<u64>(), size_of::<(u8, u32)>() and size_of::<[u16; 3]>() on one line separated by spaces.

Show example solution

use std::mem::{align_of, size_of};

fn main() {
    println!("{} {} {}", align_of::<u64>(), size_of::<(u8, u32)>(), size_of::<[u16; 3]>());
}

Test yourself

What can be done in repr(C) to shrink a struct's size?
- Use a Box
- Order the fields by alignment from largest to smallest by hand
- Nothing
- Make the fields private
Show answer

Order the fields by alignment from largest to smallest by hand — repr(C) keeps the order; the programmer must reorder. repr(Rust) does it automatically.

Ask for the alignment

println!("{}", std::mem::____::<u64>());   // 8

Show answer

println!("{}", std::mem::align_of::<u64>());   // 8

Type and size

(u8, u32)

8
[u16; 3]

6
(u8, u8)

2
(u64, u8)

16

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