kees wrote:
> Consider this example. It tries to illustrate why putting `__counted_by()` on
> a pointer to a structs containing flexible array members doesn't make sense.
>
> ```c
> struct HasFAM {
> int count;
> char buffer[] __counted_by(count); // This is OK
> };
>
> struct BufferOfFAMS {
> int count;
> struct HasFAM* fams __counted_by(count); // This is invalid
> };
> ```
Agreed: structs with flexible array members must be considered to be
singletons. This property is actually important for being able to have
`__builtin_dynamic_object_size()` work on pointers to flexible array structs.
i.e.:
```
size_t func(struct foo *p)
{
return__builtin_dynamic_object_size(p, 0);
}
```
This must always return `SIZE_MAX` for fixed-size arrays since the pointer may
be in the middle of a larger array of `struct foo`s, but if it is a struct with
a flexible array marked with `counted_by`, then we know deterministically what
the size is, since it must be a single complete object.
https://github.com/llvm/llvm-project/pull/90786
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