Re: [Xen-devel] [PATCH 4/4] xen/public: arm: rework the macro set_xen_guest_handle_raw

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Ian Campbell writes ("Re: [Xen-devel] [PATCH 4/4] xen/public: arm: rework the 
macro set_xen_guest_handle_raw"):
> The writer via one is the guest and reader via the other is the hypervisor,
> so no matter what they are certainly different compilation units, even in
> the face of whole program optimisations.

The question of them being different `compilation units' (YM
translation units) is irrelevant I think.

> The concerning issue is that if the compiler can observe you writing to
> both halves of the union then it can either omit the first write or dive
> off into deep undefined behaviour territory.

If the compiler can see you write to p, it is allowed to assume that
all subsequent readers will read the object as typeof(p).  Reading
typeof(p) does not read the padding.  Therefore the compiler is
allowed to `prove' that the padding is a dead store, and remove the
write to the padding.

This applies even if the compiler can't see the code which is doing
the reading.

Ian.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel