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

[Krzysztof Parzyszek]

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

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