On Wed, Sep 30, 2020 at 10:28:33AM +0100, Lorenz Bauer wrote:
> On Tue, 29 Sep 2020 at 16:48, Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
>
> ...
>
> > There was a warning. I noticed it while applying and fixed it up.
> > Lorenz, please upgrade your compiler. This is not the first time such
> > warning has been missed.
>
> I tried reproducing this on latest bpf-next (b0efc216f577997) with gcc
> 9.3.0 by removing the initialization of duration:
>
> make: Entering directory '/home/lorenz/dev/bpf-next/tools/testing/selftests/bpf'
> TEST-OBJ [test_progs] sockmap_basic.test.o
> TEST-HDR [test_progs] tests.h
> EXT-OBJ [test_progs] test_progs.o
> EXT-OBJ [test_progs] cgroup_helpers.o
> EXT-OBJ [test_progs] trace_helpers.o
> EXT-OBJ [test_progs] network_helpers.o
> EXT-OBJ [test_progs] testing_helpers.o
> BINARY test_progs
> make: Leaving directory '/home/lorenz/dev/bpf-next/tools/testing/selftests/bpf'
>
> So, gcc doesn't issue a warning. Jakub did the following little experiment:
>
> jkbs@toad ~/tmp $ cat warning.c
> #include <stdio.h>
>
> int main(void)
> {
> int duration;
>
> fprintf(stdout, "%d", duration);
>
> return 0;
> }
> jkbs@toad ~/tmp $ gcc -Wall -o /dev/null warning.c
> warning.c: In function ‘main’:
> warning.c:7:2: warning: ‘duration’ is used uninitialized in this
> function [-Wuninitialized]
> 7 | fprintf(stdout, "%d", duration);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> The simple case seems to work. However, adding the macro breaks things:
>
> jkbs@toad ~/tmp $ cat warning.c
> #include <stdio.h>
>
> #define _CHECK(duration) \
> ({ \
> fprintf(stdout, "%d", duration); \
> })
> #define CHECK() _CHECK(duration)
>
> int main(void)
> {
> int duration;
>
> CHECK();
>
> return 0;
> }
> jkbs@toad ~/tmp $ gcc -Wall -o /dev/null warning.c
> jkbs@toad ~/tmp $
That's very interesting. Thanks for the pointers.
I'm using gcc version 9.1.1 20190605 (Red Hat 9.1.1-2)
and I saw this warning while compiling selftests,
but I don't see it with above warning.c example.
clang warns correctly in both cases.
> Maybe this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501 ? The
> problem is still there on gcc 10. Compiling test_progs with clang does
> issue a warning FWIW, but it seems like other things break when doing
> that.
That gcc bug has been opened since transition to ssa. That was a huge
transition for gcc. But I think the bug number is not correct. It points to a
different issue. I've checked -fdump-tree-uninit-all dump with and without
macro. They're identical. The tree-ssa-uninit pass suppose to warn, but it
doesn't. I wish I had more time to dig into it. A bit of debugging in
gcc/tree-ssa-uninit.c can probably uncover the root cause.
