Re: Proposed resolution to aliasing issue.

[Mark Mitchell <mark at codesourcery dot com>]

Nathan Sidwell wrote:
> Mark Mitchell wrote:
>
>
> > Will the UK committee open a DR for this?  Or, would you care to send
> > mail to Steve Adamczyk about it?
>
> this can be done.  I shall wait until the minutes have been written up.

Excellent.

> > > The observation was made that if A is non-POD, one cannot play offsetof
> > > tricks to get from A::a to A::b, so the optimization is safe on non-PODs.
> > > (Of course one would have to prove the address of 'v' did not escape,
> > > so I guess the ctor and dtor would need to be trivial or visible.)
> >
> >
> > I argued last week that this was not in fact true, in that you can do:
> >
> >  ptrdiff_t x = &v.b - &v.a;
> >
> > and then use that instead of "offsetof (Foo, b) - offsetof (Foo, a)".
>
>
> Does not '&v.b - &v.a' cause the address to 'escape', and therefore
> lock down the structure?  If it doesn't, how hard would it be to
> make that DTRT?

It wouldn't look like escape to (at least some compilers') optimizers 
if, say, the front end folded it to a constant.  So, I'm not sure how to 
express what constitutes escape.

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304