Re: [PATCH] Fix constexpr evaluation of comparisons involving pointer-to-members

[Jason Merrill <jason at redhat dot com>]

On 02/04/2016 12:24 PM, Patrick Palka wrote:
> On Thu, Feb 4, 2016 at 11:57 AM, Jason Merrill <jason@redhat.com> wrote:
> > On 02/04/2016 10:32 AM, Patrick Palka wrote:
> > > On Thu, Feb 4, 2016 at 9:24 AM, Jason Merrill <jason@redhat.com> wrote:
> > > > On 02/03/2016 12:51 PM, Patrick Palka wrote:
> > > > > +           && (integer_minus_onep (lhs)
> > > > > +           || integer_minus_onep (rhs)))
> > > >
> > > >
> > > >
> > > > Let's use null_member_pointer_value_p here.
> > >
> > >
> > > Done.
> > >
> > > > Please add pointers to member functions to the testcase.
> > >
> > >
> > > This does not currently work properly because comparisons between
> > > distinct pointers to (non-virtual) member functions eventually perform
> > > a comparison between two distinct FUNCTION_DECLs, and fold_binary_loc
> > > currently does not fold away such comparisons. So any static_asserts
> > > that perform comparisons between distinct function pointers or between
> > > distinct pointers to (non-virtual) member functions fail with
> > > "non-constant condition for static assertion".  (Comparisons between
> > > pointers to distinct virtual member functions _do_ work because in
> > > that case we are ultimately just comparing integer offsets, not
> > > FUNCTION_DECLs.  And comparisons between equivalent pointers trivially
> > > work.)
> > >
> > > I think the problem may lie in symtab_node::equal_address_to, which
> > > apparently returns -1, instead of 0, for two obviously distinct
> > > FUNCTION_DECLs.
> >
> >
> > Right, because they might be defined as aliases elsewhere.  But it looks
> > like it should work if you define f and g.
>
> That makes sense.  But even when I define f and g, the comparisons
> don't get folded away:
>
> #define SA(x) static_assert ((x), #x)
>
> void f () { }
> void g () { }
>
> struct X { int a, b; void f (); void g (); };
>
> void X::f () { }
> void X::g () { }
>
> void
> foo ()
> {
>    SA (&f != &g);
>    SA (&X::f != &X::g);
> }
>
> bool bar = &f != &g;
>
> Although the static_asserts fail, the initialization of the variable
> bar _does_ get folded to 1 during gimplification.  When evaluating the
> static_asserts, symtab_node::equal_address_to again returns -1 but
> during gimplification the same call to symtab_node::equal_address_to
> returns 0.  So probably the symbols are not fully defined yet in the
> symbol table at the point when we are evaluating the static_asserts.

Aha.  That seems wrong, but certainly not something to fix now.  The 
patch is OK.

Jason