[lto] set alias info, poorly

[Kenneth Zadeck]

Toon Moene wrote:
> Kenneth Zadeck wrote:
>
>> If you want to ask a question about two types, the first part of
>> that check will to see if the types come from the same front end.  If
>> they do, then this issue can be resolved by asking some language
>> dependent code that will be located in the middle end of the compiler. 
>
> Well, that will mean that it'll be a fascinating piece of "language
> dependent code [...] located in the middle end of the compiler".
>
> The Fortran rules are basically:  If you don't *tell* the compiler
> (hence the front end) that two items alias, they don't.
>
> I think this means that LTO *with* Fortran-sensible alias analysis
> will only work if the Fortran front end actually determines alias
> equivalence sets and passes that down to the middle end, which then
> (in this magically "language dependent code") has to do something
> intelligent with it ...
And then we will need to "merge" that info as we bring in the code for
other fortran modules. 
i am not happy about going down this path, but the loss of that info is
not a good choice either. 

Consider the java case where two pointers cannot alias unless the type
of one pointer is derived from the other.  This means that most pointers
do not alias rather than the c/c++ case where most pointers can possibly
alias.

>
> Hmmmm, magic ...
>