Packed structs

[FireFox317]

You should use packed structs for mmio instead of normal structs, since normal structs don't have a in-memory defined layout and the compiler is allowed to change the order of fields for performance or other reasons. Packed structs will always have the correct layout as specified.

[markfirmware]

Yes but I’ve found that a regular struct with just u32 fields retain the coded order. I will review this - if packed structs are not defective in those cases then I might as well use packed struct now.

ziglang/zig/issues/2713
ziglang/zig/issues/2627

[markfirmware]

@FireFox317

Discovered another issue with using packed struct for mmio: ziglang/zig#4056

[fengb]

Zig currently retains coded order as an implementation detail, but it's almost guaranteed to change in the future. As a possible workaround, maybe use extern struct as that also guarantees consistent (C ABI implementation defined) memory layout while not having as many buggy edge cases.

[FireFox317]

And regarding ziglang/zig#2627, as long as your fields in the packed struct are a multiple of u8's, it should work fine.

[markfirmware]

ziglang/zig#3133 (comment)

[markfirmware]

@FireFox317 @fengb now using extern struct