Andrew Pinski wrote, On Friday 12 October 2012 10:29 AM:
Here's an example:
main()
{
int **p;
int *a, *d;
int w, x;
a = &w;
f1(a);
p = &a;
a = &x;
f2(p);
d = a;
return *d;
}
It is clear that d can only point to x and can never point to w.
I think you are wrong there.
int *a1;
void f1(int *a)
{
a1 = a;
}
void f2(int **p)
{
*p = a1;
}
That will change a to &w after f2 is called. So it looks like your
aliasing analysis does not take into account escaping like it should.
This is the whole point of marking a as escaped. Maybe I missed
something here though but d can point w with my functions for f1 and
f2.
Ah, you caught me there, but I think I can escape, at least in this situation
:-)
The call to f1 is not central to the point I am making; I had included it only
to ensure
that the assignment a=&w doesn't get eliminated by dead code elimination. Since
you
decided to hold the address of a into a1 through function f1, let me eliminate
the
call to f1 and make the assignment a=&w live in some other way. Here's the
changed code:
main()
{
int **p;
int *a, *d;
int w, x;
d = &x;
a = &w;
if (f1())
{
p = &a;
a = &x;
f2(p);
d = a;
}
return *d + *a;
}
Now when f2 is called, a definitely does not point to w. Hence d should not
point to w.
And yet, the dump shows that d continue to point to w.
In any case, your point about escaping variables strengthens my point about
inappropriateness
of SSA for pointer analysis, although not through this example.
Uday.
