Returning non-standard sized packed struct from a function can lead to data corruption

[frmdstryr]

In the following code, the "State" struct (with three u12 fields) becomes invalid if it's defined using a function:

const std = @import("std");

pub const CtrlRegister = packed struct {
    ENBL: bool,
    RDIR: bool,
    RSTEP: bool, // Cleared after write
    MODE: u4,
    EXSTALL: bool,
    ISGAIN: u2,
    DTIME: u2,
};

pub const TorqueRegister = packed struct {
    TORQUE: u8,
    SMPLTH: u3,
    reserved: u1,
};

pub const OffRegister = packed struct {
    TOFF: u8,
    PWMMODE: u1,
    reserved: u3,
};

// Note: Works if the struct is not packed!
pub const State = packed struct {
    ctrl: CtrlRegister,
    torque: TorqueRegister,
    off: OffRegister,
};

pub fn loadState(a: u16, b: u16, c: u16) State {
    return State{
        .ctrl = @bitCast(CtrlRegister, @truncate(u12, a)),
        .torque = @bitCast(TorqueRegister, @truncate(u12, b)),
        .off = @bitCast(TorqueRegister, @truncate(u12, c)),
    };
}

pub export fn main() void {
    const a = @truncate(u12, 0x8C10);
    const b = @truncate(u12, 0x01FF);
    const c = @truncate(u12, 0x0030);
    
    // This works fine
    const x = State{
        .ctrl = @bitCast(CtrlRegister, @truncate(u12, a)),
        .torque = @bitCast(TorqueRegister, @truncate(u12, b)),
        .off = @bitCast(TorqueRegister, @truncate(u12, c)),
    };
    std.debug.warn("{}\n", .{x});    

    // When I load in a fn.. 
    const s  = loadState(a, b, c);
    
    // The ctrl resgister is now off!!!!
    std.debug.warn("{}\n", .{s});    

}

https://godbolt.org/z/5hvjvz

After some messing around I found that if I change State to not be a packed struct it works (which is fine for my use case). Just wanted to report it.

// This is correct
State{ .ctrl = CtrlRegister{ .ENBL = false, .RDIR = false, .RSTEP = false, .MODE = 2, .EXSTALL = false, .ISGAIN = 0, .DTIME = 3 }, .torque = TorqueRegister{ .TORQUE = 255, .SMPLTH = 1, .reserved = 0 }, .off = OffRegister{ .TOFF = 48, .PWMMODE = 0, .reserved = 0 } }

// This is not!!!
State{ .ctrl = CtrlRegister{ .ENBL = false, .RDIR = false, .RSTEP = false, .MODE = 6, .EXSTALL = false, .ISGAIN = 0, .DTIME = 0 }, .torque = TorqueRegister{ .TORQUE = 0, .SMPLTH = 2, .reserved = 0 }, .off = OffRegister{ .TOFF = 0, .PWMMODE = 0, .reserved = 0 } }

[fengb]

https://godbolt.org/z/EoTabT

Packed struct error: the compiler has decided each of the 12 bit fields should be padded to 2 bytes.

• Nested misaligned packed struct causes "Assertion failed. This is a bug in the Zig compiler." #2784
• Incorrect byte offset and struct size for packed structs #2627

[Vexu]

Fixed in master.