[Bug c/116983] New: counted_by not used to identify singleton pointers

[kees at outflux dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116983

            Bug ID: 116983
           Summary: counted_by not used to identify singleton pointers
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kees at outflux dot net
  Target Milestone: ---

When counted_by is present in a structure, it means that the object must be a
singleton.

For example:

struct counted {
    int counter;
    int array[] __attribute__((counted_by(counter)));
};

struct notcounted {
    int counter;
    int array[];
};

void __attribute__((noinline)) emit_length(size_t length)
{
    printf("%zu\n", length);
}

// This correctly cannot know size of p object, and returns SIZE_MAX
void objsize_notcounted(struct notcounted *p)
{
    emit_length(__builtin_dynamic_object_size(p, 1));
} 

// This must be operating on a singleton, therefor the
// return must be:
// max(sizeof(*p),
//     sizeof(*p) + offsetof(typeof(*p), array) * p->counter)
void objsize_counted(struct counted *p)
{
    emit_length(__builtin_dynamic_object_size(p, 1));
}