Michael137 wrote:
> If I'm understanding correctly, the way this currently works is that you do
> normal field layout, then if you discover that the actual offset of a field
> is less than the offset normal field layout would produce, you assume the
> struct is packed. This misses cases where a struct is packed, but the packing
> doesn't affect the offset of any of the fields. But as you note, this can't
> be fixed without adjusting the overall architecture.
>
> There's an issue with the current implementation: it skips fields which
> actually are packed, I think. Consider the following:
>
> ```
> struct Empty {};
> struct __attribute((packed)) S {
> [[no_unique_address]] Empty a,b,c,d;
> char x;
> int y;
> };
> S s;
> ```
>
> In this case, the field "y" is both overlapping, and at a packed offset.
> Really, you don't want to check for overlap; you want to ignore empty fields.
> (Non-empty fields can't overlap.)
Good point, we would still infer packedness in this example, but for the wrong
reasons.
Skipping empty fields does seem like a better heuristic here
https://github.com/llvm/llvm-project/pull/97443
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