Is there a way to look for a tree by its UID?

[Erick Ochoa]

On 03/09/2020 12:19, Richard Biener wrote:
> On Thu, Sep 3, 2020 at 10:58 AM Jakub Jelinek via Gcc <gcc@gcc.gnu.org> wrote:
>>
>> On Thu, Sep 03, 2020 at 10:22:52AM +0200, Erick Ochoa wrote:
>>> So, I am just wondering is there an interface where I could do something
>>> like:
>>>
>>> ```
>>>      // vars is the field in pt_solution of type bitmap
>>>      EXECUTE_IF_SET_IN_BITMAP (vars, 0, uid, bi)
>>>      {
>>>         // uid is set
>>>         tree pointed_to = get_tree_with_uid(uid);
>>>      }
>>> ```
>>
>> There is not.
> 
> And there cannot be since the solution includes UIDs of
> decls that are "fake" and thus never really existed.

Hi Richard and Jakub,

thanks, I was looking for why get_tree_with_uid might be a somewhat bad 
idea.

I am thinking about representing an alias set similarly to the 
pt_solution. Instead of having bits set in position of points-to 
variables UIDs, I was thinking about having bits set in position of 
may-alias variables' UIDs. I think an interface similar to the one I 
described can provide a good mechanism of jumping to different aliases, 
but I do agree that HEAP variables and shadow_variables (and perhaps 
other fake variables) might not be a good idea to keep in the interface 
to avoid jumping to trees which do not represent something in gimple.

Richard, you mentioned in another e-mail that I might want to provide 
the alias-sets from IPA-PTA to another pass in a similar way to 
ipa_escape_pt. I think using a structure similar to pt_solution for 
may-alias solution is a good idea. Again, the bitmap to aliasing 
variables in UIDs. However, I think for this solution to be general I 
need several of these structs not just one. Ideally one per candidate 
alias-set, at most one per variable.

> 
> I think you need to first get a set of candidates you want to
> transform (to limit the work done below), then use the
> internal points-to solutions and compute alias sets for this
> set plus the points-to solution this alias-set aliases. >
> You can then keep the candidate -> alias-set ID -> points-to
> relation (thus candidates should not be "all variables" for
> efficiency reasons).

I think I can use a relatively simple heuristic: start by looking at 
malloc statements and obtain the alias-sets for variables which hold 
malloc's return values. This should address most efficiency concerns.

So, I'm thinking about the following:

* obtain variables which hold the result of malloc. These are the 
initial candidates.
* for initial candidates compute alias-sets as bitmaps where only "real" 
decl UIDs are set. Compute this just before the end of IPA-PTA.
* for each alias_set:
     for each alias:
       map[alias->decl] = alias_set
* Use this map and the alias-sets bitmaps in pass just after IPA-PTA.
* Potentially use something similar to get_tree_with_uid but that is 
only valid for trees which are keys in the map.

Does this sound reasonable?

> 
> Richard.
> 
>>          Jakub
>>