size_sum
dst = xmalloc(size_sum(dirlen, prefixlen + suffixlen + 1));
linksize = size_sum(lenslash, targetsize);
dotdotetcsize = size_sum(size_product(dotdots, 3), taillen + 1);
ptrdiff_t lo_bits = alignment - 1, sum = size_sum(size, lo_bits);