[llvm-dev] Constant unions are incredibly hard to work with - do they need to be?

[Christoffer Lernö via llvm-dev]

I just cleaned up another bug in my constant initializer code. Basically I forgot that when I have an extremely simple thing like this:

struct Baz
{
    char c;
    union
    {
        int a;
        double b;
    };
    int z;
};

And I initialize it like this:

struct Baz baz = { .a = 3, .z = 4 };

In this case I get  the ”wrong” automatic offset for a, so I need to add some extra padding.

This is not the only thing I’ve overlooked in this code. I likely have quite a few more that I need to handle.

But this begs the question: why isn’t LLVM offer a better interface for this, at least the case of creating a constant union value.

Clang has a lot of code just trying to do this well. Code that every other frontend that has unions must duplicate. Would it be complex to push this into LLVM?