We sometimes store pointers to `CRBTree` in `CRBNode*` variables, so we
must make sure CRBTree has matching alignment guarantees. We already
check for that with static-assertions.
This commit aligns CRBTree with CRBNode for 2-byte aligned machines.
While at it, add a short comment explaining what the unions are for.
Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>
c795b7657f
(cherry picked from commit 1250fcc2b1)
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.
10d973a9e6
(cherry picked from commit 143130066b)
There are some Debian-supported architectures where `max_align_t` is
only aligned to 4-bytes. This is unfortunate and breaks our assumptions.
While glibc-malloc still guarantees 8 / 16 bytes alignment, this is not
necessarily guaranteed by the C standard (and alternative allocators
will deviate (see jemalloc, for instance)).
Fortunately, we only need 2 flags, so a 4-byte alignment is more than
enough.
Reported-by: Thomas Haller
Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>
https://github.com/c-util/c-rbtree/pull/4
(cherry picked from commit 1554936e33)
