[Bug tree-optimization/65724] __builtin_object_size difference for C and C++

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65724

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to xur from comment #2)
> Do you mean the result of 0 in g++ is intentional?

Yes.

> But I'm not quite understand the relation with _FORTIFY_SOURCE=2. This
> macro does not seem to be check in tree-objsz pass.
> In other words, if I unset _FORTIFY_SOURCE or set it 1, I still get the
> same result.

With -D_FORTIFY_SOURCE=1, __builtin_object_size (ptr, 1) is never used in the C
library headers, only __builtin_object_size (ptr, 0) for everything.
In that case you are allowed to cross field boundaries not just with memcpy and
the like, but also with strcpy etc.
-D_FORTIFY_SOURCE=2 is a mode that rejects some valid C programs, simply
imposes additional security related restrictions that str* etc. functions can't
cross field boundaries, %n in *scanf family can't be used if the string literal
isn't read only etc.