[Bug libstdc++/94295] use __builtin_operator_new and __builtin_operator_delete when available

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94295

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Smith from comment #2)
> (In reply to Marc Glisse from comment #1)
> > (In reply to Richard Smith from comment #0)
> > > The C++ language rules do not permit optimization (eg, deletion) of direct
> > > calls to 'operator new' and 'operator delete'.
> > 
> > I thought that was considered a bug?
> 
> No, it's intentional: if the user directly calls '::operator new(42)' and
> they've replaced that function, the replacement function is guaranteed to be
> called. In this regard, 'operator new' is just a regular function with a
> funny name.
> 
> To be clear, the implicit call to 'operator new' produced by, say, 'new int'
> *is* optimizable, but a direct explicit call to 'operator new(sizeof(int))'
> is not.
> 
> > Gcc does optimize those, like it does malloc/free...
> 
> That sounds like non-conforming behavior.


PR 23383 is where part of the discussion was done.

In fact GCC implements the optimization without the builtin:
https://gcc.gnu.org/legacy-ml/gcc-patches/2019-07/msg00136.html