c-rbtree: fix struct alignment of CRBTree on m68k architectures

On m68k, 32bit integer are aligned to only 2 bytes. This breaks
assumptions and a static assertion of c-rbtree.

Explicitly require that the first field is aligned to at least 4 bytes.
This fixes the build and ensures that all valid pointers to a CRBTree have
the lowest two bits unset (so they can be used for storing 2 additional flags).

Use a union instead of aligning __parent_and_flags itself. That is
because alignas() cannot lower the natural alignment, so if we would
want to align __parent_and_flags, we could only do

  alignas(sizeof(unsigned long) > 4 ? sizeof(unsigned long) : 4)

That would not be correct if "long" is 8 bytes long but had a natural
alignment of only 4. The union allows us to specify an alignment
of at least 4, but otherwise follow natural alignment.

https://github.com/c-util/c-rbtree/commit/10d973a9e6718516a6839c8b87a3e630c5391d53

0 files changed, 0 insertions, 0 deletions