[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 4/4] xen/public: arm: rework the macro set_xen_guest_handle_raw
On Wed, 2015-11-04 at 15:46 +0000, Ian Jackson wrote: > 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).ÂÂReading > typeof(p) does not read the padding.ÂÂTherefore 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. Ah, yes :-( Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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