A bug story: data alignment on x86

Post by wyldfire » Mon, Nov 07, 2016, 2:13 PM UTC

alkonaut wrote: post #19

d33 wrote: post #17

Earlier quoted context omitted.

Anybody knows how Rust handles this problem?

Not sure if this is what you are asking: Last time I tried alignment in Rust I worked around the lack of explicit alignment support by adding a zero length array of the correct size to the end of the struct. Not sure if alignment support from proper attributes has landed yet. [repr(C)] struct Something { pub foo: f32, pub _alignment: [EightBytes, 0] } where "EightBytes" is a data type of size 8, to align the whole st…

Post by steveklabnik » Mon, Nov 07, 2016, 2:34 PM UTC

alkonaut wrote: post #19

d33 wrote: post #17

Earlier quoted context omitted.

Anybody knows how Rust handles this problem?

Not sure if this is what you are asking: Last time I tried alignment in Rust I worked around the lack of explicit alignment support by adding a zero length array of the correct size to the end of the struct. Not sure if alignment support from proper attributes has landed yet. [repr(C)] struct Something { pub foo: f32, pub _alignment: [EightBytes, 0] } where "EightBytes" is a data type of size 8, to align the whole st…

    struct One {
      foo: u8,
    }
    
    struct Two {
      bar: u16,
    }
    
    struct Three {
      foo: u8,
      bar: u16,
    }
    
    struct Four {
      foo: u16,
      bar: u16,
    }
    
    fn main() {
        assert_eq!(1, std::mem::size_of::());
        assert_eq!(2, std::mem::size_of::());
        assert_eq!(4, std::mem::size_of::());
        assert_eq!(4, std::mem::size_of::());
    }

Post by alkonaut » Mon, Nov 07, 2016, 2:43 PM UTC

steveklabnik wrote: post #23

alkonaut wrote: post #19

Earlier quoted context omitted.

Not sure if this is what you are asking: Last time I tried alignment in Rust I worked around the lack of explicit alignment support by adding a zero length array of the correct size to the end of the struct. Not sure if alignment support from proper attributes has landed yet. [repr(C)] struct Something { pub foo: f32, pub _alignment: [EightBytes, 0] } where "EightBytes" is a data type of size 8, to align the whole st…

Structs already insert padding to give them alignment: struct One { foo: u8, } struct Two { bar: u16, } struct Three { foo: u8, bar: u16, } struct Four { foo: u16, bar: u16, } fn main() { assert_eq!(1, std::mem::size_of:: ()); assert_eq!(2, std::mem::size_of:: ()); assert_eq!(4, std::mem::size_of:: ()); assert_eq!(4, std::mem::size_of:: ()); }

Post by steveklabnik » Mon, Nov 07, 2016, 3:10 PM UTC

alkonaut wrote: post #25

steveklabnik wrote: post #23

Earlier quoted context omitted.

Structs already insert padding to give them alignment: struct One { foo: u8, } struct Two { bar: u16, } struct Three { foo: u8, bar: u16, } struct Four { foo: u16, bar: u16, } fn main() { assert_eq!(1, std::mem::size_of:: ()); assert_eq!(2, std::mem::size_of:: ()); assert_eq!(4, std::mem::size_of:: ()); assert_eq!(4, std::mem::size_of:: ()); }

What I needed to do was to place e.g. a N byte struct exactly on an M byte alignment (e.g. 11 byte struct on 32 byte alignment etc).

Post by novia » Mon, Nov 07, 2016, 3:22 PM UTC