On Thu, May 26, 2022 at 09:26:16AM -0400, Jason Merrill wrote:
> On 5/25/22 16:25, Marek Polacek wrote:
> > On Wed, May 18, 2022 at 09:43:47AM -0400, Jason Merrill wrote:
> > > On 5/16/22 13:06, Marek Polacek wrote:
> > > > dynamic_cast can legally return nullptr, so I don't think it's helpful
> > > > for -Waddress to warn for
> > > >
> > > > if (dynamic_cast<A*>(&ref))
> > > > // ...
> > > >
> > > > More generally, it's likely not useful to warn for the artificial
> > > > POINTER_PLUS_EXPRs created by build_base_path.
> > >
> > > Normally the POINTER_PLUS_EXPR is guarded by if (nonnull). But
> > > build_base_path isn't adding that guard in this case because the operand
> > > is
> > > known to be a reference, which cannot be null
> > > (http://eel.is/c++draft/dcl.ref#5). So a warning is indicated for this
> > > testcase, though it would be good to give a more informative one
> > > ("comparing
> > > address of reference to null").
> >
> > Ah, got it. How about this patch instead? Thanks,
>
> Do we also warn about plain &ref == nullptr?
Yeah, with different wording:
p.C:5:12: warning: the compiler can assume that the address of ‘ref’ will never
be NULL [-Waddress]
5 | if (&ref == nullptr)
| ~~~~~^~~~~~~~~~
warn/Walways-true-3.C tests this.
> > -- >8 --
> > This patch improves the diagnostic for -Waddress when it warns for
> >
> > if (dynamic_cast<A*>(&ref))
> > // ...
> >
> > where 'ref' is a reference, which cannot be null. In particular, it
> > changes
> > warning: comparing the result of pointer addition '(((A*)ref) +
> > ((sizetype)(*(long int*)((& ref)->B::_vptr.B + -24))))' and NULL
> > to
> > warning: comparing address of reference 'ref' to null
> >
> > PR c++/105569
> >
> > gcc/cp/ChangeLog:
> >
> > * typeck.cc (warn_for_null_address): Improve the warning when
> > the POINTER_PLUS_EXPR's base is of reference type.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/warn/Waddress-9.C: New test.
> > ---
> > gcc/cp/typeck.cc | 12 ++++++++--
> > gcc/testsuite/g++.dg/warn/Waddress-9.C | 31 ++++++++++++++++++++++++++
> > 2 files changed, 41 insertions(+), 2 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/warn/Waddress-9.C
> >
> > diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> > index 385cdf4d733..0837f2484ba 100644
> > --- a/gcc/cp/typeck.cc
> > +++ b/gcc/cp/typeck.cc
> > @@ -4757,8 +4757,16 @@ warn_for_null_address (location_t location, tree op,
> > tsubst_flags_t complain)
> > tree off = TREE_OPERAND (cop, 1);
> > if (!integer_zerop (off)
> > && !warning_suppressed_p (cop, OPT_Waddress))
> > - warning_at (location, OPT_Waddress, "comparing the result of pointer "
> > - "addition %qE and NULL", cop);
> > + {
> > + tree base = TREE_OPERAND (cop, 0);
> > + STRIP_NOPS (base);
> > + if (TYPE_REF_P (TREE_TYPE (base)))
> > + warning_at (location, OPT_Waddress, "comparing address of "
> > + "reference %qE and NULL", base);
> > + else
> > + warning_at (location, OPT_Waddress, "comparing the result of "
> > + "pointer addition %qE and NULL", cop);
> > + }
> > return;
> > }
> > else if (CONVERT_EXPR_P (op)
> > diff --git a/gcc/testsuite/g++.dg/warn/Waddress-9.C
> > b/gcc/testsuite/g++.dg/warn/Waddress-9.C
> > new file mode 100644
> > index 00000000000..a3654ff1f91
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/warn/Waddress-9.C
> > @@ -0,0 +1,31 @@
> > +// PR c++/105569
> > +// { dg-do compile { target c++11 } }
> > +// { dg-options -Waddress }
> > +
> > +class A {};
> > +
> > +class B : public virtual A {};
> > +
> > +class C : public A {};
> > +
> > +int main() {
> > + B* object = new B();
> > + B &ref = *object;
> > +
> > + bool b = nullptr == dynamic_cast<A*>(&ref); // { dg-warning "comparing
> > address of reference .ref. and NULL" }
> > + bool b4 = nullptr == static_cast<A*>(&ref); // { dg-warning "comparing
> > address of reference .ref. and NULL" }
> > + if (dynamic_cast<A*>(&ref)) // { dg-warning "comparing address of
> > reference .ref. and NULL" }
> > + {
> > + }
> > + if (static_cast<A*>(&ref)) // { dg-warning "comparing address of
> > reference .ref. and NULL" }
> > + {
> > + }
> > +
> > + auto ptr = dynamic_cast<A*>(&ref);
> > + bool b2 = ptr == nullptr;
> > +
> > + C* cobject = new C();
> > + C &cref = *cobject;
> > +
> > + bool b3 = nullptr == dynamic_cast<A*>(&cref);
> > +}
> >
> > base-commit: 34970d08c6297e12f3f9117b6ac19fb2de522e24
>
Marek
