[Bug tree-optimization/107952] tree-object-size: inconsistent size for flexible arrays nested in structs

["qing.zhao at oracle dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107952

--- Comment #10 from Qing Zhao <qing.zhao at oracle dot com> ---
> --- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
> 
> GCC handles for example
> 
> struct A { char data[1]; };
> struct B { int n; struct A a; };
> 
> as if the a.data[] array is a flex-array.

Okay. Then the maximum size of __builtin_object_size for it should be -1,
right?

>  It also handles
> 
> struct C { int n; struct A a; int x; };
> 
> as if a.data[] can be up to 4 elements large (we allow an array to extend
> flexibly to padding - but only if it is trailing).

A little confused here:  
        when the structure with a trailing flexible-array member is a middle
field of 
        an outer structure, GCC extension will treat it as a flexible-array
too? But the
        maximum size of this flexible-array will be bounded by the size of the
padding
        of that field? 
Is the above understanding correct?

>  I see that's not
> consistently handled though.
> 
> I think the [] syntax should follow the C standard as what testcases are
> accepted/rejected by the frontend and any extensions there should be
> documented

Agreed, usually where these extension should be documented?

> (those are separate from what the former array_at_struct_end_p
> allowed semantically

So, your mean to leave such extension out of “array_at_struct_end_p” (the
current “array_ref_flexible_size_p”)? 
Handle them separately instead?