On 28/09/16 13:14 +0200, Richard Biener wrote:
On Wed, Sep 28, 2016 at 12:57 PM, Jonathan Wakely <jwak...@redhat.com> wrote:
std::function::swap does swap(_M_functor, x._M_functor) which
exchanges the underlying bytes of the two _Any_data PODs using that
type's implicit assignment operator. However, unlike using placement
new to construct an object in the storage, simply memcpying the bytes
doesn't change the effective type of the storage, so alias analysis
decides it can't point to whatever we've tried to store in there.
To clarify -- the implicit assingment operator for PODs (including unions)
simply expands to an aggregate assignment which is subject to TBAA
rules and thus in this case instantiates an effective type of _Any_data.
Using memcpy would have worked as memcpy _does_ transfer
the effective type of the storage. It wasn't points-to deciding things
here but TBAA given automatic storage X with effective type T
read via an lvalue of type _Any_data.
This attribute tells the middle-end to assume anything those bytes
could contain any type of object, which is exactly what we want.
It tells the middle-end that accessing storage via a _pointer_ to such
marked type may access storage with a dynamic type that is not
compatible with the type.
Details ;)
But important ones, thanks.
