Warning about variable optimized away?

[Ian Lance Taylor]

On Tue, Aug 5, 2014 at 4:54 AM, Dmitry Antipov <dmantipov@yandex.ru> wrote:
>
> it it possible to get a kind of diagnostics if the variable
> is totally optimized away?  For example, in:
>
> void foo (struct some_type *obj) {
>   ... some code where 'obj' is not used ...
>   bar (obj->some_member);
>   ... some code where 'obj' is not used again ...
>   baz (obj->some_member);
> }
>
> 'obj' is likely to be optimized away so only 'obj->some_member' really
> exists (in a register or stack location).  Getting diagnostics
> or preserving 'obj' may be important if there is a GC which scans
> C stack and registers conservatively - if there is no direct reference
> to 'obj', it's likely to be reclaimed and so 'obj->some_member'
> becomes garbage.

This question would be more appropriate on gcc-help@gcc.gnu.org.  The
gcc@gcc.gnu.org list is for discussion about the development of GCC.
Please take any followups to gcc-help.  Thanks.

There are no diagnostics for a variable being optimized away.  That is
a normal occurrence.

If obj->some_member is loaded into a register, then it doesn't matter
whether obj is collected.  If obj->some_member is not loaded into a
register, then there must be some pointer into obj, and a conservative
GC will not collect the memory.

Ian