[Clang] Add __builtin_counted_by_ref builtin (#114495)

The __builtin_counted_by_ref builtin is used on a flexible array
pointer and returns a pointer to the "counted_by" attribute's COUNT
argument, which is a field in the same non-anonymous struct as the
flexible array member. This is useful for automatically setting the
count field without needing the programmer's intervention. Otherwise
it's possible to get this anti-pattern:
    
      ptr = alloc(<ty>, ..., COUNT);
      ptr->FAM[9] = 42; /* <<< Sanitizer will complain */
      ptr->count = COUNT;
    
To prevent this anti-pattern, the user can create an allocator that
automatically performs the assignment:
    
      #define alloc(TY, FAM, COUNT) ({ \
          TY __p = alloc(get_size(TY, COUNT));             \
          if (__builtin_counted_by_ref(__p->FAM))          \
              *__builtin_counted_by_ref(__p->FAM) = COUNT; \
          __p;                                             \
      })

The builtin's behavior is heavily dependent upon the "counted_by"
attribute existing. It's main utility is during allocation to avoid
the above anti-pattern. If the flexible array member doesn't have that
attribute, the builtin becomes a no-op. Therefore, if the flexible
array member has a "count" field not referenced by "counted_by", it
must be set explicitly after the allocation as this builtin will
return a "nullptr" and the assignment will most likely be elided.

---------

Co-authored-by: Bill Wendling <isanbard@gmail.com>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

0 files changed, 0 insertions, 0 deletions