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))))
/* 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.
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