aliasing question

[Diego Novillo]

On Tue, 2004-09-07 at 06:51, Jan Hubicka wrote:

>   #   V_MUST_DEF <value_49>;
>   value = T.4862_48;
> 
> and alias1 dump:
> 
>   #   value_229 = V_MAY_DEF <value_19>;
>   value = T.4862_48;
> 
> Why the MUST_DEF is getting converted to MAY_DEF in such a obvious case
> for MUST_DEF?
> 
Before aliasing, 'value' did not belong to any alias set, but after
aliasing it is.  If we didn't do that, suppose that:

*p, alias tag TMT.1, alias set of TMT.1 = { value }

     1. p = (cond) ? &value : foo ();
     2. *p = 3;
     3. value = T.4862;
     4. return *p;

If you put a V_MUST_DEF in statement #3, you will consider '*p = 3'
dead.  Perhaps, we could put a V_MUST_DEF if there were more than one
element in TMT.1's alias set.  Not sure how much would that buy, but it
may be an interesting experiment.


Diego.