Prazek marked 10 inline comments as done.
Prazek added inline comments.
================
Comment at: clang/lib/CodeGen/CGExprScalar.cpp:1647
+ }
+ }
+
----------------
rsmith wrote:
> Prazek wrote:
> > rjmccall wrote:
> > > Prazek wrote:
> > > > rjmccall wrote:
> > > > > Incidentally, how do you protect against code like this?
> > > > >
> > > > > A *ptr;
> > > > > reinterpret_cast<B *&>(ptr) = new B();
> > > > > ptr->foo();
> > > > >
> > > > > Presumably there needs to be a launder/strip here, but I guess it
> > > > > would have to be introduced by the middle-end when forwarding the
> > > > > store? The way I've written this is an aliasing violation, but (1) I
> > > > > assume your pass isn't disabled whenever strict-aliasing is disabled
> > > > > and (2) you can do this with a memcpy and still pretty reliably
> > > > > expect that LLVM will be able to eventually forward the store.
> > > > Can you add more info on what is A and B so I can make sure I
> > > > understand it correctly?
> > > > Is the prefix of the layout the same for both, but they are not in the
> > > > hierarchy?
> > > >
> > > > I haven't thought about the strict aliasing. I think the only sane way
> > > > would be to require strict aliasing for the strict vtable pointers.
> > > It's whatever case you're worried about such that you have to launder
> > > member accesses and bitcasts.
> > >
> > > And like I mentioned, relying on strict aliasing isn't enough because you
> > > can do it legally with memcpy. Maybe it's okay to consider it UB? I'm
> > > not sure about that.
> > AFAIK reinterpreting one class as another is UB if they are not in
> > hierarchy (especially calling virtual function on reinterpreted class), not
> > sure if strict aliasing should allow it anyway (if it would be a hand
> > written custom vptr then it should be ok with strict aliasing turned off,
> > but with vptr I don't think it is legal).
> > @rsmith Can you comment on that?
> OK, here's how I think about what we're doing here:
>
> We view the IR-level pointer value for a pointer to a dynamic class type as
> being a fat pointer, containing the actual pointer value and also a tag
> indicating the dynamic type of the object (only notionally, though -- the
> actual bit representation of the pointer doesn't include the extra
> information, but we don't ever emit IR that inspects the bit representation
> of the fat pointer to avoid exposing that fact). In that model, if you try to
> type pun between a pointer to a dynamic class type and a pointer to a
> non-dynamic-class type, that can't be expected to work because the (notional)
> value is different, much as type punning between a derived and base class
> pointer wouldn't work for a pointer to something other than a base class at
> offset zero.
>
> I think @rjmccall's example is OK, because both `A` and `B` would need to be
> dynamic class types in a hierarchy to work, and that means we'd be using the
> same notional pointer representation. A slight variation of that example:
>
> ```
> struct A {};
> struct B : A { virtual void f(); };
> struct C : B { void f(); } c;
> A *p = &c;
> B *q;
> memcpy(&q, &p, sizeof(B*)); // or q = std::bit_cast<B*>(p);
> q->f();
> ```
>
> ... would be UB, because the representation of an `A*` and a `B*` are
> different (a `B*` contains a tag and an `A*` does not).
Does this answer satisfy you John? Can I push it to trunk?
Repository:
rL LLVM
https://reviews.llvm.org/D47299
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