@sizeOf() packed struct changes depending on relative sizes of members

[mirrexagon]

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• I searched for this bug and did not find it in the issue tracker, and furthermore, the title I used above will make this new bug report turn up in the search results for my query.

Zig Version

0.9.0 (built with Nix from master commit e97feb9)

Steps to Reproduce

const std = @import("std");

const txdata = packed struct {
    /// Transmit data (RW)
    data: u8,
    _reserved_8: u23,
    /// Transmit FIFO full (RO)
    full: bool = false,
};

const txdata_passes = packed struct {
    /// Transmit data (RW)
    data: u9, // Changed to u9
    _reserved_8: u22, // Changed to u22
    /// Transmit FIFO full (RO)
    full: bool = false,
};

const register_size_bits = 32;
const register_size_bytes = register_size_bits / 8;

test "txdata size correct but fails" {
    try std.testing.expectEqual(register_size_bits, @bitSizeOf(txdata));
    try std.testing.expectEqual(register_size_bytes, @sizeOf(txdata));
}

test "txdata size adjusted to pass" {
    try std.testing.expectEqual(register_size_bits, @bitSizeOf(txdata_passes));
    try std.testing.expectEqual(register_size_bytes, @sizeOf(txdata_passes));
}

Run zig test on the above.

Expected Behavior

@bitSizeOf(txdata) and @bitSizeOf(txdata_passes) should be 32, and @sizeOf(txdata) and @sizeOf(txdata_passes) should be 4.

Actual Behavior

@sizeOf(txdata) is 5.

---

This came up when implementing MMIO registers here: https://github.com/mirrexagon/luxos/blob/a0d46e992ac256685ee36a85f9f255299f0fbe7d/src/target/soc/fe310-g002/uart.zig#L50

Trying to @bitCast() between txdata and usize (32 bits on riscv32) failed due to this. I can work around this by increasing the size of the data member from u8 to u9, and decreasing the size of the next member by one bit (u23 to u22) - doing that makes the sizes match up.

Also affects this register: https://github.com/mirrexagon/luxos/blob/a0d46e992ac256685ee36a85f9f255299f0fbe7d/src/target/arch/riscv/mcsr.zig#L17

Is this something that should work? My understanding is that packed structs that fit exactly into a whole number of bytes should not have any padding.

I'm unsure if this is related to or would be fixed by #8102.

[igor84]

I just noticed that this code:

export fn run() u8 {
    const TestStruct = packed struct {
        a: u32,
        b: [3]u8,
    };
    return @sizeOf(TestStruct);
}

returns 8 instead of 7, but if I change b array to have 2 elements it returns correct 6.

[mirrexagon]

#11279 appears to fix my use case! Thanks @igor84 for your work!