[Bug middle-end/70526] [5/6 Regression] GCC 6 miscompiles Firefox JIT compiler

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70526

--- Comment #26 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 3 May 2016, redi at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70526
> 
> --- Comment #25 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> std::aligned_storage has no equivalent of the addr() member function that would
> need adjusting, it's just the union U part of the AlignedStorage2 type in
> comment 23, where sizeof(T) is a template parameter, rather than T.
> 
> __gnu_cxx::__aligned_buffer<T> and __gnu_cxx::__aligned_membuf<T> are similar
> to the class template discussed above, but if I understand the issue correctly,
> they don't need adjusting because we don't misuse them.
> 
> We create objects in them with placement new, and we don't attempt to use the
> aggregate copy (because that would obviously do the wrong thing). In fact I
> don't think we ever try to copy those objects. The closest we get to copying
> them would do something like:
> 
> void f(__gnu_cxx::__aligned_buffer<X> buf)
> {
>   __gnu_cxx::__aligned_buffer<X> buf2;
>   X* x = ::new (buf2.addr()) X(*buf.ptr());
>   // use x
>   x->~X();
> }
> 
> i.e. we create copies of the stored object using placement new, but don't copy
> the aligned buffer objects themselves.
> 
> 
> (In reply to Richard Biener from comment #20)
> > (In reply to Jan de Mooij from comment #17)
> > > So IIUC it's wrong to store to |typed| in the constructor, by doing this:
> > > 
> > > *typed.addr() = reg;
> > > 
> > > Why is that? Any suggestions on how to fix this on our end?
> > 
> > No, it's valid to do that.
> 
> It's only valid because Register is a trivial type. Otherwise you'd be
> assigning to an object which was never constructed, i.e. it's lifetime never
> started.
> 
> In the general case the safe way to construct that object is with placement
> new:
> 
>   ::new ((void*)typed.addr()) Register(reg);
> 
> This begins the lifetime of a Register object at the desired location.

std::aligned_storage<T> has the requirement of a POD type T.  So I
wondered about whether the intent was to make copying it by plain
assignment possible.  Using std::memcpy() on the object would always
work at least as that is documented for POD types.