Re: Strenghten assumption about dynamic type changes (placement new)

[Richard Biener <richard dot guenther at gmail dot com>]

On Sat, Jul 19, 2014 at 5:44 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> On 07/18/2014 11:03 AM, Jan Hubicka wrote:
>> >I really only care about types containing virtual table pointers to not change,
>> >so non-PODs are out of game.  Current propagation is built around assumption that
>> >once polymorphic type is constructed on a given location it won't change to
>> >completely different type, only possibly repetitively construct & destruct.
>> >This is based on our earlier conversation where the outcome was that changing
>> >non-POD variable by placement new to different type is not defined.
>>
>> For a variable, yes.  If I have a char array buffer (possibly
>> wrapped in a class, e.g. std::aligned_storage), it is OK to
>> construct one non-POD object, destroy it, then construct one of a
>> different type in the same space, just like if we have two automatic
>> variables in different blocks that happen to occupy the same
>> location on the stack.  Again, this really needs to be specified
>> better in the standard.
>
> To support this safely I really think we will need to mark placement new
> in the gimple code via builtin for a short while.
>
> As discussed today. I may make a difference in between objects allocated in
> char buffers and objects allocated via normal new and propagate only across
> the second ones, but that seems a bit slipperly, too.
>>
>> >Anything weaker will probably need some cooperation from the frontend - I
>> >suppose best tie we have is the fact that you can't use 'a' to call foo after
>> >changing object. If placement news was marked for some time by a builtin, we
>> >could effectively thread the re-allocated objects as a new memory locations.
>>
>> My concern about treating them as different memory locations is
>> danger of code reordering causing the lifetimes of the old and new
>> objects to overlap.
>
> I still do not see how this can work with aliasing rules - if the two objects
> allocated do not overlap, we will freely overlap the old and new objects.
> But adding extra builtin that will (for some time) keep the two pointer distinct
> should not make it any worse.

As discussen during the Cauldron keeping some builtin doesn't help because
you are not forced to access the newly created object via the pointer returned
by the placement new.  That is,

  template <T>
 struct Storage {
     char x[sizeof(T)];
    Storage() { new (x) T; }
    T& get() { return reinterpret_cast <T&> (x); }
};

is valid (and used in this way in Boost - with a type different from 'char'
to force bigger alignment).

Richard.

> Honza
>>
>> >Where I find current wording of DR1116?
>>
>> http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1116
>>
>> Jason