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Re: V6 [PATCH] C/C++: Add -Waddress-of-packed-member


On Fri, Dec 14, 2018 at 11:48 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Fri, Dec 14, 2018 at 2:10 PM Jason Merrill <jason@redhat.com> wrote:
> >
> > On 12/13/18 6:56 PM, H.J. Lu wrote:
> > > On Thu, Dec 13, 2018 at 12:50 PM Jason Merrill <jason@redhat.com> wrote:
> > >>
> > >> On 9/25/18 11:46 AM, H.J. Lu wrote:
> > >>> On Fri, Aug 31, 2018 at 2:04 PM, Jason Merrill <jason@redhat.com> wrote:
> > >>>> On 07/23/2018 05:24 PM, H.J. Lu wrote:
> > >>>>>
> > >>>>> On Mon, Jun 18, 2018 at 12:26 PM, Joseph Myers <joseph@codesourcery.com>
> > >>>>> wrote:
> > >>>>>>
> > >>>>>> On Mon, 18 Jun 2018, Jason Merrill wrote:
> > >>>>>>
> > >>>>>>> On Mon, Jun 18, 2018 at 11:59 AM, Joseph Myers <joseph@codesourcery.com>
> > >>>>>>> wrote:
> > >>>>>>>>
> > >>>>>>>> On Mon, 18 Jun 2018, Jason Merrill wrote:
> > >>>>>>>>
> > >>>>>>>>>> +  if (TREE_CODE (rhs) == COND_EXPR)
> > >>>>>>>>>> +    {
> > >>>>>>>>>> +      /* Check the THEN path first.  */
> > >>>>>>>>>> +      tree op1 = TREE_OPERAND (rhs, 1);
> > >>>>>>>>>> +      context = check_address_of_packed_member (type, op1);
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>> This should handle the GNU extension of re-using operand 0 if operand
> > >>>>>>>>> 1 is omitted.
> > >>>>>>>>
> > >>>>>>>>
> > >>>>>>>> Doesn't that just use a SAVE_EXPR?
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> Hmm, I suppose it does, but many places in the compiler seem to expect
> > >>>>>>> that it produces a COND_EXPR with TREE_OPERAND 1 as NULL_TREE.
> > >>>>>>
> > >>>>>>
> > >>>>>> Maybe that's used somewhere inside the C++ front end.  For C a SAVE_EXPR
> > >>>>>> is produced directly.
> > >>>>>
> > >>>>>
> > >>>>> Here is the updated patch.  Changes from the last one:
> > >>>>>
> > >>>>> 1. Handle COMPOUND_EXPR.
> > >>>>> 2. Fixed typos in comments.
> > >>>>> 3. Combined warn_for_pointer_of_packed_member and
> > >>>>> warn_for_address_of_packed_member into
> > >>>>> warn_for_address_or_pointer_of_packed_member.
> > >>>>
> > >>>>
> > >>>>> c.i:4:33: warning: converting a packed ‘struct C *’ pointer increases the
> > >>>>> alignment of ‘long int *’ pointer from 1 to 8 [-Waddress-of-packed-member]
> > >>>>
> > >>>>
> > >>>> I think this would read better as
> > >>>>
> > >>>> c.i:4:33: warning: converting a packed ‘struct C *’ pointer (alignment 1) to
> > >>>> ‘long int *’ (alignment 8) may result in an unaligned pointer value
> > >>>> [-Waddress-of-packed-member]
> > >>>
> > >>> Fixed.
> > >>>
> > >>>>> +      while (TREE_CODE (base) == ARRAY_REF)
> > >>>>> +       base = TREE_OPERAND (base, 0);
> > >>>>> +      if (TREE_CODE (base) != COMPONENT_REF)
> > >>>>> +       return NULL_TREE;
> > >>>>
> > >>>>
> > >>>> Are you deliberately not handling the other handled_component_p cases? If
> > >>>> so, there should be a comment.
> > >>>
> > >>> I changed it to
> > >>>
> > >>>        while (handled_component_p (base))
> > >>>           {
> > >>>             enum tree_code code = TREE_CODE (base);
> > >>>             if (code == COMPONENT_REF)
> > >>>               break;
> > >>>             switch (code)
> > >>>               {
> > >>>               case ARRAY_REF:
> > >>>                 base = TREE_OPERAND (base, 0);
> > >>>                 break;
> > >>>               default:
> > >>>                 /* FIXME: Can it ever happen?  */
> > >>>                 gcc_unreachable ();
> > >>>                 break;
> > >>>               }
> > >>>           }
> > >>>
> > >>> Is there a testcase to trigger this ICE? I couldn't find one.
> > >>
> > >> You can take the address of an element of complex:
> > >>
> > >>     __complex int i;
> > >>     int *p = &__real(i);
> > >>
> > >> You may get VIEW_CONVERT_EXPR with location wrappers.
> > >
> > > Fixed.  I replaced gcc_unreachable with return NULL_TREE;
> >
> > Then we're back to my earlier question: are you deliberately not
> > handling the other cases?  Why not look through them as well?  What if
> > e.g. the operand of __real is a packed field?
> >
>
> Here is the updated patch with
>
> diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
> index 615134cfdac..f105742598e 100644
> --- a/gcc/c-family/c-warn.c
> +++ b/gcc/c-family/c-warn.c
> @@ -2669,6 +2669,9 @@ check_address_of_packed_member (tree type, tree rhs)
>     switch (code)
>       {
>       case ARRAY_REF:
> +     case REALPART_EXPR:
> +     case IMAGPART_EXPR:
> +     case VIEW_CONVERT_EXPR:
>         base = TREE_OPERAND (base, 0);
>         break;
>       default:

don't we have handled_component_p () for this?  (you're still
missing BIT_FIELD_REF which might be used for vector
element accesses)

>
> Now I got
>
> [hjl@gnu-cfl-1 pr51628-6]$ cat foo.i
> struct A { __complex int i; };
> struct B { struct A a; };
> struct C { struct B b __attribute__ ((packed)); };
>
> extern struct C *p;
>
> int*
> foo1 (void)
> {
>   return &__real(p->b.a.i);
> }
> int*
> foo2 (void)
> {
>   return &__imag(p->b.a.i);
> }
> [hjl@gnu-cfl-1 pr51628-6]$ make foo.s
> /export/build/gnu/tools-build/gcc-debug/build-x86_64-linux/gcc/xgcc
> -B/export/build/gnu/tools-build/gcc-debug/build-x86_64-linux/gcc/ -O2
> -S foo.i
> foo.i: In function ‘foo1’:
> foo.i:10:10: warning: taking address of packed member of ‘struct C’
> may result in an unaligned pointer value [-Waddress-of-packed-member]
>    10 |   return &__real(p->b.a.i);
>       |          ^~~~~~~~~~~~~~~~~
> foo.i: In function ‘foo2’:
> foo.i:15:10: warning: taking address of packed member of ‘struct C’
> may result in an unaligned pointer value [-Waddress-of-packed-member]
>    15 |   return &__imag(p->b.a.i);
>       |          ^~~~~~~~~~~~~~~~~
> [hjl@gnu-cfl-1 pr51628-6]$
>
> OK for trunk?
>
> --
> H.J.


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