[Bug c/108896] provide "element_count" attribute to give more context to __builtin_dynamic_object_size() and -fsanitize=bounds

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896

--- Comment #23 from qinzhao at gcc dot gnu.org ---
(In reply to Martin Uecker from comment #13)
> 
> VLAs and VM types exist since C99 and were made optional in C11.
> The minimal change we adopted to make support for VM types 
> (but not VLAs) mandatory again was:
> 
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2778.pdf

So, this will be official in C23? i.e, VM types will be mandatory, but VLA will
be optional (and later might be deprecated?)

> 
> I think using PLACEHOLDER_EXPR that are insert into the size
> expression and then replaced later by the struct being accessed, 
> e.g.
> 
> struct foo {
>  int len;
>  char buf[PLACEHOLDER_EXPR.len]
> };
> 
> and then later when we have
> 
> struct foo x;
> 
> x->buf
> 
> we would replace in the size of the type for x->buf the placeholder
> with x itself.

I see. Yes, this will resolve the implementation difficulty for filling the
size of the FAM field when the size is the previous declared field in the same
structure. 


> Yes, this was what I wanted to do...  My main use case is not flexible
> array members but VM types in struct:
> 
> struct foo {
>   int len;
>   char (*buf)[.len];
> };
> 
> 
> This has less issues because the size of the struct then does not depend
> on the length.

a little confused here:
what's the definition of VM type? it's size will not depend on the ".len" ?