On Wed, Sep 2, 2020 at 10:04 AM Erick Ochoa
<erick.oc...@theobroma-systems.com> wrote:
>
> Hello,
>
> I am trying to find out all pointers which alias a pointer and place
> them in a set.
>
> I am using `ptr_derefs_may_alias_p` to find out if two pointers may
> point to the same memory location. I think this yields conservative
> results (i.e., when it cannot be proven that to pointers may alias, it
> will give the result "true"). This is what I need.
>
> To collect all pointers which alias pointer "p", this means is that I
> have to collect all pointer variables and all pointer expressions in the
> program and then call `ptr_derefs_may_alias_p` O(n) times to find the
> set of pointers which alias pointer "p". If I have to do this for all
> pointers in the program, that would mean O(n^2).
>
> This also implies that in order for my sets to be correct I need to
> collect all pointer variables and all pointer expressions. I think that
> a better idea would be to have a list of all bitmap solutions and for
> every variable with a bitmap if position j is set, assigned to alias set
> j. In pseudocode:
>
> compute_alias_sets(bitmap set)
> {
> // create array of sets of same length as varmap
> // call it alias_map
> for (i = 0; i < varmap.length (); i++)
> {
> bitmap set = get_varinfo(i)->solution;
> EXECUTE_IF_SET_IN_BITMAP (set, 0, j, bi)
> {
> varinfo_t v = get_varinfo (j);
> alias_map[j].insert(v);
> }
> }
> }
>
> I think this is a better implementation. I don't need to worry about
> collecting all pointer expressions and I think varmap also contains
> declarations. (I think IPA-PTA already has "collected" all pointer
> expressions/declarations in varmap and created constraint variables for
> them.) *Would there be an issue with exporting varmap and delaying its
> deletion to this pass I'm working on?* This pass is intended to be run
> immediately after IPA-PTA. After this, I think I would still need to do
> some refining on the "alias_map" since (I think) there are more
> constraint variables than gimple variables in the partition, but the
> information I look for should be able to be derived from here.
I think you should add the code to compute your "alias sets" to
IPA-PTA if struct-reorg is enabled and export those like
we "export" the IPA escaped set (ipa_escaped_pt) to later
passes.
I agree that you want to work on the points-to solutions rather
than start from pointers and expressions in GIMPLE.
Richard.
> Thanks!
