alignmentForField

fn alignmentForField(
    t: *Translator,
    record_decl: aro.Type.Record,
    head_field_alignment: ?c_uint,
    field_index: usize,
) ?c_uint

File

Code

fn alignmentForField(
    t: *Translator,
    record_decl: aro.Type.Record,
    head_field_alignment: ?c_uint,
    field_index: usize,
) ?c_uint {
    const fields = record_decl.fields;
    assert(fields.len != 0);
    const field = fields[field_index];

    const bits_per_byte = 8;
    const parent_ptr_alignment_bits = record_decl.layout.?.pointer_alignment_bits;
    const parent_ptr_alignment = parent_ptr_alignment_bits / bits_per_byte;

    // bitfields aren't supported yet. Until support is added, records with bitfields
    // should be demoted to opaque, and this function shouldn't be called for them.
    if (field.bit_width != .null) {
        @panic("TODO: add bitfield support for records");
    }

    const field_offset_bits: u64 = field.layout.offset_bits;
    const field_size_bits: u64 = field.layout.size_bits;

    // Fields with zero width always have an alignment of 1
    if (field_size_bits == 0) {
        return 1;
    }

    // Fields with 0 offset inherit the parent's pointer alignment.
    if (field_offset_bits == 0) {
        return head_field_alignment;
    }

    // Records have a natural alignment when used as a field, and their size is
    // a multiple of this alignment value. For all other types, the natural alignment
    // is their size.
    const field_natural_alignment_bits: u64 = bits_per_byte * field.qt.alignof(t.comp);
    const rem_bits = field_offset_bits % field_natural_alignment_bits;

    // If there's a remainder, then the alignment is smaller than the field's
    // natural alignment
    if (rem_bits > 0) {
        const rem_alignment = rem_bits / bits_per_byte;
        if (rem_alignment > 0 and std.math.isPowerOfTwo(rem_alignment)) {
            const actual_alignment = @min(rem_alignment, parent_ptr_alignment);
            return @as(c_uint, @truncate(actual_alignment));
        } else {
            return 1;
        }
    }

    // A field may have an offset which positions it to be naturally aligned, but the
    // parent's pointer alignment determines if this is actually true, so we take the minimum
    // value.
    // For example, a float field (4 bytes wide) with a 4 byte offset is positioned to have natural
    // alignment, but if the parent pointer alignment is 2, then the actual alignment of the
    // float is 2.
    const field_natural_alignment: u64 = field_natural_alignment_bits / bits_per_byte;
    const offset_alignment = field_offset_bits / bits_per_byte;
    const possible_alignment = @min(parent_ptr_alignment, offset_alignment);
    if (possible_alignment == field_natural_alignment) {
        return null;
    } else if (possible_alignment < field_natural_alignment) {
        if (std.math.isPowerOfTwo(possible_alignment)) {
            return possible_alignment;
        } else {
            return 1;
        }
    } else { // possible_alignment > field_natural_alignment
        // Here, the field is positioned be at a higher alignment than it's natural alignment. This means we
        // need to determine whether it's a specified alignment. We can determine that from the padding preceding
        // the field.
        const padding_from_prev_field: u64 = blk: {
            if (field_offset_bits != 0) {
                const previous_field = fields[field_index - 1];
                break :blk (field_offset_bits - previous_field.layout.offset_bits) - previous_field.layout.size_bits;
            } else {
                break :blk 0;
            }
        };
        if (padding_from_prev_field < field_natural_alignment_bits) {
            return null;
        } else {
            return possible_alignment;
        }
    }
}