[clang] [Clang] Fix __builtin_dynamic_object_size off by 4 (PR #111015)

[Bill Wendling via cfe-commits]

bwendling wrote:

> > This is why I believe the gcc behavior is correct. When it knows the size given to `malloc` it uses that. When it doesn't know that it simply returns INT_MAX. When you ask gcc for the `__bdos` of the FAM it will use the `count` to calculate the size.
> 
> (nit: `SIZE_MAX`, not `INT_MAX`)
> 
> AIUI, GCC considers this to be a bug in the face of `counted_by`. The reason GCC returns "unknown" (`SIZE_MAX`) in the case of a pointer like that is because (prior to `counted_by`) if the pointer wasn't to local storage, it could no know if it was a singleton or not. i.e. it may be pointing into a larger array of objects, so it cannot know the size. (This is most clear for `char *` variables, for example.)

[snip]

> This shows how GCC will adjust the `__bdos` when it is certain of the object being a singleton, but it still missing the "counted_by requires a singleton" logic. (See also `-Wflexible-array-member-not-at-end` in GCC). It also shows that Clang's `__bdos` can be lied to.

For what it's worth, GCC's return value isn't wrong, according to them. They want to return *something* when `__bdos` is called, even if it includes end padding. This includes when the second argument is `(arg2 & 1) == 1`, where they'll return the size from the struct field it's pointing at to the end of the struct (which I whole-heartedly despise and think is a major bug, but I'm in the minority).

https://github.com/llvm/llvm-project/pull/111015