[cfe-dev] C++11 and enhacned devirtualization

[Piotr Padlewski]

Because standard say, that when You put new object into previous object
(f.e. by placement new) the lifetime of an object has ended, and using
previous pointer to it leads to UB.

see c++11 standard, 3.8 Object lifetime



On Thu, Jul 16, 2015 at 10:26 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Piotr Padlewski" <prazek at google.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "Rafael Espíndola" <rafael.espindola at gmail.com>, "David Blaikie" <
> dblaikie at gmail.com>, "Richard Smith"
> > <richard at metafoo.co.uk>, "cfe-dev at cs.uiuc.edu Developers" <
> cfe-dev at cs.uiuc.edu>
> > Sent: Thursday, July 16, 2015 12:19:40 PM
> > Subject: Re: [cfe-dev] C++11 and enhacned devirtualization
> >
> >
> > It can, but it will be UB, so we don't have to worry about it.
>
> Why is it UB? Or when it the placement new changing the vtable not UB?
>
>  -Hal
>
> >
> >
> > On Thu, Jul 16, 2015 at 10:16 AM, Hal Finkel < hfinkel at anl.gov >
> > wrote:
> >
> >
> > ----- Original Message -----
> > > From: "Rafael Espíndola" < rafael.espindola at gmail.com >
> > > To: "Piotr Padlewski" < prazek at google.com >
> > > Cc: "David Blaikie" < dblaikie at gmail.com >, "Hal Finkel" <
> > > hfinkel at anl.gov >, "Richard Smith" < richard at metafoo.co.uk >,
> > > " cfe-dev at cs.uiuc.edu Developers" < cfe-dev at cs.uiuc.edu >
> > > Sent: Thursday, July 16, 2015 12:14:29 PM
> > > Subject: Re: [cfe-dev] C++11 and enhacned devirtualization
> > >
> > > On 16 July 2015 at 10:02, Piotr Padlewski < prazek at google.com >
> > > wrote:
> > > > The proposal of our solution will be published next week. Brief
> > > > introduction:
> > > >
> > > > "Basic idea is to mark vtable loads with !invariant , and also
> > > > put
> > > > assume()
> > > > after calling constructor to show how vtable will look like.
> > > > Function
> > > > @llvm.invariant.barrier that breaks the invariant will be needed
> > > > to
> > > > cope
> > > > with placement new and other cases where the vptr is legitimately
> > > > permitted
> > > > to change."
> > >
> > >
> > > Nice, this should also avoid loading the vtable pointer multiple
> > > times in
> > >
> > > struct C {
> > > virtual void foo();
> > > };
> > > void g();
> > > void f(C *x) {
> > > x->foo();
> > > g();
> >
> > Not if g() might placement-new something else on top of x. The
> > problem is that if we need to make conservative assumptions about
> > whether or not g() might contain a '@llvm.invariant.barrier', we
> > must assume it might.
> >
> > -Hal
> >
> > > x->foo();
> > > }
> > >
> > > Cheers,
> > > Rafael
> >
> >
> > >
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> >
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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