While this was not the point of the comment, one thing missing is that oldsize a...

While this was not the point of the comment, one thing missing is that oldsize and newsize should be rounded for alignment purposes:

    struct my_allocator {
        struct hamt_allocator parent;
        size_t used;
        union {
            char buffer[8192];
            max_align_t _align;
        };
    };
    
    static inline size_t align_size(size_t value)
    {
        size_t alignment = _Alignof(max_align_t);
    
        // if (value % _Alignof(max_align_t) == 0) {
        //     return value;
        // } else {
        //     return ((value / alignment) + 1) * alignment;
        // }
    
        return value + (alignment - 1) & ~(alignment - 1);
    }

Even better, the allocator function should get the alignment so don't waste bytes unnecessarily.