https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349
--- Comment #39 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 16 Jun 2020, andrew2085 at gmail dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349 > > --- Comment #38 from Andrew Downing <andrew2085 at gmail dot com> --- > > int *p; > > int x; > > if (<condition>) > > p = &x; > > else > > p = malloc (4); > > memcpy (p, q, 4); > > > > there is a single memcpy call and the standard says that both the dynamic > > type transfers (from q) and that it does not (to x). > > I would say just that, that it both does and doesn't transfer the effective > type. Meaning that you need to be conservative during optimization and > consider > p to alias both int and whatever type q is. > > > Note the C++ standard makes the placement new optional. Do you say that > > your example is incorrect with the placement new elided? > > I'm not sure what you mean about the first part about it being optional. It Somewhere the C++ standard says (or said in some "old" version) that the lifetime of an object ends when "... or the storage is re-used". Likewise lifetime of an object starts "when storage with the proper alignment and size ... is obtained". Back in time when I designed the way GCC currently works to satisfy placement new and friends I concluded the safe thing to do is to treat every memory write as changing the dynamic type of the memory location (because we have to assume it is re-use of storage). Thus for types without a non-trivial ctor/dtor you do not need to use placement new. So take your example and remove the placement new. Does that change its semantics?
