make packed structs versatile

[andrewrk]

Currently, packed structs are always align(1) and each field of a packed struct is either align(1) or a special kind of alignment that indicates bit offsets. I propose the following changes:

• implement ability to specify explicit field alignment of packed structs (related to ability to set alignment on struct fields #1512), including even bit offsets. This can cause padding to be inserted (possibly even just bits to the next byte).
• packed structs, like normal structs, have the alignment of the most-aligned field. Minimum 1 byte.
• without explicit alignment specified, fields get bit packed. However fields can be specified to be byte aligned and pointers to these fields can even be naturally aligned. A packed struct with every field like this: field: T align(T), would be equivalent to an extern struct.
• ability for packed structs to have fields that are normal structs. In this case whether a packed struct has well-defined memory layout is determined by whether or not all the fields it has have well-defined memory layout. Normal structs and types can still be bit-packed though!
• when a packed struct has no well-defined memory layout, it can participate in debug safety features (see add safety checks for pointer casting #2414). Perhaps there could even be a special type that could be used as a field, that allows Zig to put the debug safety in that field's location, and packed structs would have well-defined memory layout until the first field that does not have well defined memory layout. This would allow putting this debug safety field at the end, and still doing @bitCast, @ptrCast, or other useful things with packed structs.
• fix the issues regarding packed structs not ending with byte-alignment. This should be handled semantically equivalent to, for example, u7.

Related bug: #2627

[andrewrk]

Here are some nice tests provided by @MasterQ32:

const std = @import("std");

test "sizeOf, variant 1" { // breaks in master
    const T = packed struct {
        one: u8,
        three: [3]u8,
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(T));
}

test "sizeOf, variant 2" { // doesn't break in master
    const T = packed struct {
        three: [3]u8,
        one: u8,
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(T));
}

test "daurnimator (original)" { // breaks in master
    const T = packed struct {
        _1: u1,
        x: u7,
        _: u24,
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(T));
}

test "daurnimator (variant 1)" { // doesn't break in master
    const T = packed struct {
        _1: u1,
        x: u7,
        _2: u8,
        _3: u16,
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(T));
}

test "daurnimator (variant 2)" { // doesn't break in master
    const T = packed struct {
        _1: u1,
        x: u7,
        _2: u16,
        _3: u8,
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(T));
}

test "MasterQ32'1" {
    const Flags1 = packed struct {
        // byte 0
        b0_0: u1,
        b0_1: u1,
        b0_2: u1,
        b0_3: u1,
        b0_4: u1,
        b0_5: u1,
        b0_6: u1,
        b0_7: u1,

        // partial byte 1 (but not 8 bits)
        b1_0: u1,
        b1_1: u1,
        b1_2: u1,
        b1_3: u1,
        b1_4: u1,
        b1_5: u1,
        b1_6: u1,

        // some padding to fill to size 3
        _: u9,
    };
    // TODO: This still breaks
    // std.testing.expectEqual(@as(usize, 4), @sizeOf(Flags1));
}


test "MasterQ32'2" {
    const Flags2 = packed struct {
        // byte 0
        b0_0: u1,
        b0_1: u1,
        b0_2: u1,
        b0_3: u1,
        b0_4: u1,
        b0_5: u1,
        b0_6: u1,
        b0_7: u1,

        // partial byte 1 (but not 8 bits)
        b1_0: u1,
        b1_1: u1,
        b1_2: u1,
        b1_3: u1,
        b1_4: u1,
        b1_5: u1,
        b1_6: u1,

        // some padding that should yield @sizeOf(Flags2) == 4
        _: u10, // this *was* originally 17, but the error happens with 10 as well
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(Flags2));
}


test "MasterQ32'3" {
    const Flags3 = packed struct {
        // byte 0
        b0_0: u1,
        b0_1: u1,
        b0_2: u1,
        b0_3: u1,
        b0_4: u1,
        b0_5: u1,
        b0_6: u1,
        b0_7: u1,

        // byte 1
        b1_0: u1,
        b1_1: u1,
        b1_2: u1,
        b1_3: u1,
        b1_4: u1,
        b1_5: u1,
        b1_6: u1,
        b1_7: u1,

        // some padding that should yield @sizeOf(Flags2) == 4
        _: u16, // it works, if the padding is 8-based
    };
    std.testing.expectEqual(@as(usize, 4), @sizeOf(Flags3));
}

test "fix for #3651" {
    const T1 = packed struct {
        array: [3][3]u8, // also with align(1)
    };

    const T2 = packed struct {
        array: [9]u8,
    };
    std.testing.expectEqual(@as(usize, 9), @sizeOf(T1));
    std.testing.expectEqual(@as(usize, 9), @sizeOf(T2));
}

[markfirmware]

Another case:

const S = packed struct {
    a: u32,
    pad: [3]u32,
    b: u32,
};

test "packed struct @byteOffsetOf" {
    const offset = @byteOffsetOf(S, "b");
    warn("\n\noffset is {}\n\n", .{offset});
    assert(offset == 16);
}

const assert = @import("std").debug.assert;
const warn = @import("std").debug.warn;

@FireFox317 @fengb @andrewrk

[andrewrk]

Here's a helpful comment about how packed structs work

[mikdusan]

I have code producing these numbers; need some confirmation I'm on the correct path:

same struct with a single field align(N) changing { 0,1,2,4,8 }

// abi_size=7
// size_in_bits=54
// abi_align=1
const Foo = packed struct {
    a: u12,         // @bitSizeOf=12  @align= 0  @byteOffsetOf= 0  @bitOffsetOf=  0  → 1:0:2
    b: u1,          // @bitSizeOf= 1  @align= 0  @byteOffsetOf= 1  @bitOffsetOf= 12  → 1:4:1
    c: u32,         // @bitSizeOf=32  @align= 0  @byteOffsetOf= 1  @bitOffsetOf= 13  → 1:5:5
    d: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 5  @bitOffsetOf= 45  → 1:5:2
    e: bool,        // @bitSizeOf= 1  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 49  → 1:1:1
    f: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 50  → 1:2:1
};


// abi_size=8
// size_in_bits=58
// abi_align=1
const Foo = packed struct {
    a: u12,         // @bitSizeOf=12  @align= 0  @byteOffsetOf= 0  @bitOffsetOf=  0  → 1:0:2
    b: u1 align(1), // @bitSizeOf= 1  @align= 1  @byteOffsetOf= 2  @bitOffsetOf= 16  → 1:0:1
    c: u32,         // @bitSizeOf=32  @align= 0  @byteOffsetOf= 2  @bitOffsetOf= 17  → 1:1:5
    d: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 49  → 1:1:1
    e: bool,        // @bitSizeOf= 1  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 53  → 1:5:1
    f: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 54  → 1:6:2
};


// abi_size=8
// size_in_bits=58
// abi_align=2
const Foo = packed struct {
    a: u12,         // @bitSizeOf=12  @align= 0  @byteOffsetOf= 0  @bitOffsetOf=  0  → 2:0:2
    b: u1 align(2), // @bitSizeOf= 1  @align= 2  @byteOffsetOf= 2  @bitOffsetOf= 16  → 1:0:1
    c: u32,         // @bitSizeOf=32  @align= 0  @byteOffsetOf= 2  @bitOffsetOf= 17  → 2:1:6
    d: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 49  → 1:1:1
    e: bool,        // @bitSizeOf= 1  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 53  → 1:5:1
    f: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 6  @bitOffsetOf= 54  → 1:6:2
};


// abi_size=12
// size_in_bits=74
// abi_align=4
const Foo = packed struct {
    a: u12,         // @bitSizeOf=12  @align= 0  @byteOffsetOf= 0  @bitOffsetOf=  0  → 2:0:2
    b: u1 align(4), // @bitSizeOf= 1  @align= 4  @byteOffsetOf= 4  @bitOffsetOf= 32  → 1:0:1
    c: u32,         // @bitSizeOf=32  @align= 0  @byteOffsetOf= 4  @bitOffsetOf= 33  → 4:1:8
    d: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 8  @bitOffsetOf= 65  → 1:1:1
    e: bool,        // @bitSizeOf= 1  @align= 0  @byteOffsetOf= 8  @bitOffsetOf= 69  → 1:5:1
    f: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf= 8  @bitOffsetOf= 70  → 1:6:2
};  


// abi_size=16
// size_in_bits=106
// abi_align=8
const Foo = packed struct {
    a: u12,         // @bitSizeOf=12  @align= 0  @byteOffsetOf= 0  @bitOffsetOf=  0  → 2:0:2
    b: u1 align(8), // @bitSizeOf= 1  @align= 8  @byteOffsetOf= 8  @bitOffsetOf= 64  → 1:0:1
    c: u32,         // @bitSizeOf=32  @align= 0  @byteOffsetOf= 8  @bitOffsetOf= 65  → 4:1:8
    d: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf=12  @bitOffsetOf= 97  → 1:1:1
    e: bool,        // @bitSizeOf= 1  @align= 0  @byteOffsetOf=12  @bitOffsetOf=101  → 1:5:1
    f: u4,          // @bitSizeOf= 4  @align= 0  @byteOffsetOf=12  @bitOffsetOf=102  → 1:6:2
};  

[mikdusan]

Here's a packed struct with very inefficient field sizes. One thing that I need to point out is this shows how overlapped the host integers are. Is overlapped ok? If it is I think codegen will be using different <{ ... }> for each field access? If overlapping host integers aren't allowed, I'll need some more examples to understand this.

// abi_size=79
// size_in_bits=630
// abi_align=1
const Foo = packed struct {
    a: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf= 0  @bitOffsetOf=  0  → 1:0:8
    b: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf= 7  @bitOffsetOf= 63  → 1:7:9
    c: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=15  @bitOffsetOf=126  → 1:6:9
    d: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=23  @bitOffsetOf=189  → 1:5:9
    e: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=31  @bitOffsetOf=252  → 1:4:9
    f: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=39  @bitOffsetOf=315  → 1:3:9
    g: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=47  @bitOffsetOf=378  → 1:2:9
    h: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=55  @bitOffsetOf=441  → 1:1:8
    i: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=63  @bitOffsetOf=504  → 1:0:8
    j: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=70  @bitOffsetOf=567  → 1:7:9
};  

// abi_size=80
// size_in_bits=630
// abi_align=8
const Foo = packed struct {
    a: u63 align(8), // @bitSizeOf=63  @align= 8  @byteOffsetOf= 0  @bitOffsetOf=  0  → 8:0:8
    b: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf= 0  @bitOffsetOf= 63  → 8:63:16
    c: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf= 8  @bitOffsetOf=126  → 8:62:16
    d: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=16  @bitOffsetOf=189  → 8:61:16
    e: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=24  @bitOffsetOf=252  → 8:60:16
    f: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=32  @bitOffsetOf=315  → 8:59:16
    g: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=40  @bitOffsetOf=378  → 8:58:16
    h: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=48  @bitOffsetOf=441  → 8:57:16
    i: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=56  @bitOffsetOf=504  → 8:56:16
    j: u63,          // @bitSizeOf=63  @align= 0  @byteOffsetOf=64  @bitOffsetOf=567  → 8:55:16
};  

[s-ol]

Not sure if this is 100% related, but I was expecting this to work on 0.6.0:

comptime {
        const type = packed struct {
                asd: [8]u1,
        };
        @compileLog(@sizeOf(type));
}

...but error: array of 'u1' not allowed in packed struct due to padding bits (same for array of booleans).

This would be very nice to have though for memory-mapped IO; right now something like this:

// written from memory, dont use
const IOPort = packed struct {
  ddr0: bool,
  ddr1: bool,
  ...
  ddr7: bool,
  pin0: bool,
  pin1: bool,
  ...
  pin7: bool,
  port0: bool,
  port1: bool,
  ...
  port7: bool,
}
const PORTB = @intToPtr(*volatile IOPort, 0x28);

works great on AVR and compiles bit access to optimized single-bit instructions (set-bit, clear-bit), but it doesn't lend itself to meta-programming because you cannot (easily) lookup a specific pin given (port: *volatile IOPort, pin_index: u3).

[andrewrk]

Rejected in favor of #5049 and #10113.