[LLVMdev] PROPOSAL: struct-access-path aware TBAA

[Shuxin Yang]

On 03/12/2013 07:21 PM, Krzysztof Parzyszek wrote:
> On 3/12/2013 12:13 PM, Manman Ren wrote:
> >
> > Given
> >    struct A {
> >      int x;
> >      int y;
> >    };
> >    struct B {
> >      A a;
> >      int z;
> >    };
> >    struct C {
> >      B b1;
> >      B b2;
> >    };
> >    struct D {
> >      C c;
> >    };
> >
> > with struct-access-path aware TBAA, C::b1.a.x does not alias with 
> D::c.b2.a.x.
> > without it, the 2 scalar accesses can alias since both have int type.
>
> I browsed the 2012 standard for a while and I didn't see anything that 
> would make this illegal:
>
> char *p = malloc(enough_bytes);
> intptr_t x = reinterpret_cast<intptr_t>(p);
> x += offsetof(C, b2);
> D &vd = *reinterpret_cast<D*>(p);
> C &vc = *reinterpret_cast<C*>(x);
> vd.c.b2.a.x = 1;    // ..accessing the same
> int t = vc.b1.a.x;  // ..storage
>
> I don't think that the path through the type structure is really 
> sufficient.
>
> -Krzysztof
>

Not 100% sure your point here, but I think you are likely arguing TBAA 
is not sufficiently safe.
True for this case. However, in this case, TBAA is not supposed to kick 
in to disambiguate these
two access because the base/offset/size rule is supposed to give a 
definite answer if these two access
*definitely* alias or *definitely* not alias.

Note that, to make base/offset/size more effective, analyzer need to 
track the base along the UD chains
as far as possible. In this case, the "bases" for the memory are "p", 
instead of &vd and &vc, respectively.