[cfe-commits] PATCH: Large re-working of bitfield IR-gen, and a fix for PR13691

[John McCall]

On Sep 11, 2012, at 6:09 PM, Richard Smith wrote:
> On Tue, Sep 11, 2012 at 6:04 PM, John McCall <rjmccall at apple.com> wrote:
> On Sep 11, 2012, at 4:41 PM, Richard Smith wrote:
> > On Tue, Sep 11, 2012 at 11:33 AM, John McCall <rjmccall at apple.com> wrote:
> > On Sep 11, 2012, at 11:07 AM, Chandler Carruth wrote:
> > > On Fri, Aug 31, 2012 at 1:01 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> > >> Another nasty case I just thought of:
> > >>
> > >> struct x { int i : 24; };
> > >> struct y { int i : 24; char j; };
> > >> union z {
> > >>   struct x x;
> > >>   struct y y;
> > >> };
> > >> union z a;
> > >> void f(int i) {
> > >>   a.x.i = i;
> > >> }
> > >> void g(char j) {
> > >>   a.y.j = j
> > >> }
> > >>
> > >> The two writes are to separate memory locations. :)
> > >
> > > Wait, hold on... I'm deeply confused. Maybe because I don't know how C11 unions work?
> > >
> > > With C++11, only one side of the union is allowed to be active, and so I don't think they are separate memory locations?
> >
> > I agree that this isn't a problem, but the analysis is a bit more complicated;
> > it hinges on the fact that it's okay to *read* from an inactive union member
> > under the common-prefix exception, but it's not okay to *write* to it.  The
> > same analysis applies in both standards:
> >
> > Is this still OK if the extra union member is volatile? Chandler and I have discussed this, and it's far from clear that it would be. (In particular, we can conceive of a seemingly-reasonable case where the union sits on an MMIO port, and only the fourth byte has volatile semantics.)
> 
> I see no reason why making the member volatile changes anything.
> Other members in the union can be assumed not to exist, because the
> active union member *must* be the one we're assigning to — indeed,
> in C such an assignment is how you change the active union member.
> 
> I'm talking about the load-widening case, not the store-widening. Given:
> 
> union U {
>   struct X { int a : 24; volatile char b; } x;
>   struct Y { int c : 24; } y;
> };
> 
> ... if x is the active member, and we load y.c, can we really load from the storage location containing x.b?

Yeah, Eli pointed out that that was what you probably meant.

I think the answer is "yes", because:
  - the standards' guarantees about volatile behavior are associated with
    accesses to volatile-typed objects, not to accesses to non-volatile-typed
    objects, and even then are basically implementation-defined; and
  - my Common Sense Hammer gives me incredible powers to ignore
    people with really stupid test cases involving unions that partially
    overlap MMIO ports.

John.
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