Assertion failure when a struct contains a struct which contains bit fields spanning [17, 25) or [33, 57) bits. about ldc HOT 2 OPEN

Thx for the report. Converted to D, compilable via -preview=bitfields:

struct BitField {
    uint _0 : 1;
    uint _1 : 1;
    uint _2 : 1;
    uint _3 : 1;
    uint _4 : 1;
    uint _5 : 1;
    uint _6 : 1;
    uint _7 : 1;
    uint _8 : 1;
    uint _9 : 1;
    uint _10 : 1;
    uint _11 : 1;
    uint _12 : 1;
    uint _13 : 1;
    uint _14 : 1;
    uint _15 : 1;
    uint _16 : 1;
}

struct Foo {
    BitField bf;
}

pragma(msg, BitField.sizeof, ", ", BitField.alignof); // 4, 4
pragma(msg, Foo.sizeof, ", ", Foo.alignof); // 4, 4

Compiling via -vv shows that the BitField IR type is an un-packed struct:

* * * * Building struct type test.BitField @ test.d(1)
* * * * * final struct type: %test.BitField = type { i24, [1 x i8] }

so that LLVM is indeed free to add some implicit padding. There might be a missing assertion checking the final aggregate size (IR vs. AST) too, catching this shouldn't require using the type as field of another aggregate.

from ldc.

Oh wait, the type-alloc size of a packed <{ i24, [1 x i8] }> is apparently 5 bytes too! Edit: Well, it is 8 if unpacked.

from ldc.