[ovs-dev] Compiler warnings with gcc 6.1 (On Fedora 24)

[Numan Siddique]

On Mon, Jul 4, 2016 at 10:47 AM, Ben Pfaff <blp at ovn.org> wrote:

> On Mon, Jul 04, 2016 at 10:17:24AM +0530, Numan Siddique wrote:
> > I am seeing the below compiler warnings with GCC version 6.1 on Fedora
> 24.
> >
> > $gcc --version
> > gcc (GCC) 6.1.1 20160621 (Red Hat 6.1.1-3)
> >
> > libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -I ../include -I
> ./include
> > -I ../lib -I ./lib -Wstrict-prototypes -Wall -Wextra -Wno-sign-compare
> > -Wpointer-arith -Wswitch-enum -Wunused-parameter -Wbad-function-cast
> > -Wcast-align -Wmissing-prototypes -Wmissing-field-initializers
> > -fno-strict-aliasing -Werror -g -O2 -MT lib/classifier.lo -MD -MP -MF
> > lib/.deps/classifier.Tpo -c ../lib/classifier.c -o lib/classifier.o
> > ../lib/classifier.c: In function ‘cls_match_alloc’:
> > ../lib/classifier.c:96:53: error: self-comparison always evaluates to
> true
> > [-Werror=tautological-compare]
> >      *CONST_CAST(const struct cls_rule **, &cls_match->cls_rule) = rule;
>
> I think that this and similar warnings are coming from the definition of
> BUILD_ASSERT_TYPE in util.h.  It does compare a pointer to a cast
> version of itself, intentionally, to provoke an error if their types are
> sufficiently different:
>
>     #define BUILD_ASSERT_TYPE(POINTER, TYPE) \
>         ((void) sizeof ((int) ((POINTER) == (TYPE) (POINTER))))
>
> I'm not sure how to best suppress this particular warning, especially
> since I don't have a copy of GCC 6.x here.  It might be possible to do
> it by making the expression even more obvious:
>
> diff --git a/include/openvswitch/util.h b/include/openvswitch/util.h
> index 363fa39..2835def 100644
> --- a/include/openvswitch/util.h
> +++ b/include/openvswitch/util.h
> @@ -70,7 +70,7 @@ OVS_NO_RETURN void ovs_assert_failure(const char *,
> const char *, const char *);
>   * bool" warning from "sparse" (see
>   * http://permalink.gmane.org/gmane.comp.parsers.sparse/2967). */
>  #define BUILD_ASSERT_TYPE(POINTER, TYPE) \
> -    ((void) sizeof ((int) ((POINTER) == (TYPE) (POINTER))))
> +    ((void) sizeof ((int) (1 || (POINTER) == (TYPE) (POINTER))))
>
>
​Thanks Ben for the reply. I tried this and it didn't work.

​


>  /* Casts 'pointer' to 'type' and issues a compiler warning if the cast
> changes
>   * anything other than an outermost "const" or "volatile" qualifier.
>
> Maybe a build assertion on __builtin_types_compatible_p() would work,
> but I couldn't quickly figure out a good way.
>

​I think not many would be using gcc 6.1. So I guess, it's not urgent.