On Thu, Oct 1, 2020 at 12:25 AM Alexei Starovoitov alexei.starovoitov@gmail.com wrote:
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.
I think this might be the same problem I fixed for libbpf with [0]. Turns out, GCC explicitly calls out (somewhere in their docs) that uninitialized variable warnings work only when compiled in optimized mode, because some internal data structures used to detect this are only maintained in optimized mode build.
Laurenz, can you try compiling your example with -O2?
[0] https://patchwork.ozlabs.org/project/netdev/patch/20200929220604.833631-2-an...
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.