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.
Thanks!
