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

[Richard Smith]

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?
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