Zig: Generalization of a struct/union that allows for explicitly defined memory layout

[mrakh]

When dealing with data that depends on fixed memory offsets, such as in MMIO or serialization routines, it's sometimes convenient to be able to explicitly specify field offsets, especially when they are non-contiguous or have unaligned overlap. Of course, any such composite type can be emulated with nested structs/unions and dummy padding fields. Or you could also use other workarounds, like indexing into arrays with descriptively named const variables, or hiding the logic behind getter and setter functions. However, such techniques are typically hard to read, repetitive to write, and/or error-prone, making them ill-suited for the aforementioned use case.

I believe it would be nice to have a generalized composite primitive that allows for a user-defined memory layout, where each field would be qualified with a type and an offset. This would make certain patterns easier to express: (making up possible syntax)

// Sparse structures
const MMIOControlRegisters = memblock {
    enable_interrupt : u8 : 0x00000100,
    dma_dest_physaddr : u48 : 0x00000320,
    desc_select : u16 : 0x00000005e,
};

Or perhaps the offset values could be made bit-addressable:

// Making some endianness assumptions here
const WebSocketHeader = memblock {
    fin : u1 : 0,
    rsv1 : u1 : 1,
    rsv2 : u1 : 2,
    rsv3 : u1 : 3,
    opcode : u4 : 4,
    mask_flag : u1 : 8,
    len_code : u7 : 9,

    payload_len_small : u7 : 9,
    mask_small : u32 : 16,

    payload_len_med : u16 : 16,
    mask_med : u32 : 32,

    payload_len_large : u64 : 16,
    mask_large : u32 : 80,
};

[daurnimator]

I think this should be done by re-introducing the offset field (removed in 0f677810ea9d5c8ba549f6c9948979a768a1eda6) to StructField so that @Type(.{.Struct = .{ .... }) can create custom layouts.

e.g.

const MMIOControlRegisterStruct = @Type(.{
    .Struct = .{
        // .layout =
        .fields = &[_]std.builtin.TypeInfo.StructField{
            .{
                .name = "enable_interrupt",
                .field_type = u8,
                .offset = 0x00000100,
            },
            .{
                .name = "dma_dest_physaddr",
                .field_type = u48,
                .offset = 0x00000320,
            },
            .{
                .name = "desc_select",
                .field_type = u16,
                .offset = 0x00000005e,
            },
        },
        // .decls = .{},
    },
});

[codehz]

(but use @Type directly for generate type is horrible, maybe we can add field tag or attribute in another language)

Technically we can parse string in comptime and generate struct, but it is still inconvenient...

proposal:
like

const MMIOControlRegisterStruct = transform(struct {
    enable_interrupt: u8 "offset: 0x00000100",
});
fn transform(comptime Original: type) type {
    var orig = @TypeInfo(Original);
    inline for (orig.fields) |*field| {
        field.offset = parseTag(field.tag, usize, "offset");
    }
    return @Type(orig);
}

Oh, there is already Tags proposal: https://github.com/ziglang/zig/issues/1099

[Rocknest]

@codehz name of the field can be a tag

const MMIOControlRegisterStruct = Transform(struct {
    enable_interrupt_OFFSET_0x00000100: u8,
        // or
        @"enable_interrupt$0x00000100": u8,
});
fn Transform(comptime Original: type) type {
    var orig = @TypeInfo(Original);
    inline for (orig.fields) |*field| {
            if (mem.indexOf(u8, field.name, "OFFSET_") != 0)
        field.offset = parseTag(mem.substring(u8, 0, mem.indexOf(u8, field.name, "OFFSET_")), usize);
    }
    return @Type(orig);
}

[mb64]

I have another use case for opting into more well-defined layout:

fn Rc(comptime T: type) type {
    return struct {
        strong_count: usize,
        weak_count: usize,
        value: T,

        fn weak(self: *@This()) *Weak(T) {
            self.weak_count += 1;
            return @ptrCast(*Weak(T), self);
        }
    };
}

fn Weak(comptime T: type) type {
    return struct {
        strong_count: usize,
        weak_count: usize,
        value: T,

        fn upgrade(self: *@This()) ?*Rc(T) {
            if (self.strong_count == 0) return null;
            self.strong_count += 1;
            return @ptrCast(*Rc(T), self);
        }
    };
}

Currently, this wouldn't work, since Rc(T) and Weak(T) aren't guaranteed to have the same layout. Some method for guaranteeing deterministic layout would be necessary.

I don't think there's a good way to do this is Zig right now, but I'd love to be corrected. extern annotations get partway there, but require T to be extern as well.

[daurnimator]

Currently, this wouldn't work, since Rc(T) and Weak(T) aren't guaranteed to have the same layout. Some method for guaranteeing deterministic layout would be necessary.

I don't think there's a good way to do this is Zig right now, but I'd love to be corrected. extern annotations get partway there, but require T to be extern as well.

packed should work for this use case; though you then need to reintroduce alignment.

[mb64]

I agree that packed struct should work, though in a strictly normative sense: it ought to work but currently doesn't.

const T = struct { x: i32 };
const P = packed struct { t: T align(@alignOf(T)) };

Produces the message

./t.zig:2:27: error: non-packed, non-extern struct 'T' not allowed in packed struct; no guaranteed in-memory representation
const P = packed struct { t: T align(@alignOf(T)) };
                          ^

[mrakh]

Just to add that I believe that this proposal would be the best way to address the issues presented in #6349, #985 and #1214 - any construction of nested anonymous structures could be easily converted to a memblock with translate-c while faithfully maintaining the field semantics and preserving the struct ABI.