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@gmail.com 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.