[Bug middle-end/82203] New: [4.9/5/6/7/8 regression] missing -Wmaybe-uninitialized warning with tree-vrp

[arnd at linaro dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82203

            Bug ID: 82203
           Summary: [4.9/5/6/7/8 regression] missing -Wmaybe-uninitialized
                    warning with tree-vrp
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arnd at linaro dot org
  Target Milestone: ---

I've looked at warnings I get in the Linux kernel with older compilers but not
with more modern versions. Unsurprisingly, most are false positives, but I
found three files that show a particular pattern that should be warned about:

$ gcc-8.0.0 -Wall -c test.c -O2 # no warning
$ gcc-8.0.0 -Wall -c test.c -O2 -fno-tree-vrp # warning
test.c:12:9: warning: 'err' may be used uninitialized in this function
[-Wmaybe-uninitialized]
$ gcc-4.8 -Wall -c test2.c -O2 # warning
test.c:12:2: warning: ‘err’ may be used uninitialized in this function
[-Wmaybe-uninitialized]

The original warnings all show up with gcc-4.6 and earlier, but not with
gcc-4.7+. The reduced test case also produces a warning with 4.7 and 4.8 but
not with 4.9 through 8.0 unless -ftree-vrp is disabled:

8<---
int f(void);
int mlx5_fpga_mem_read_i2c(int count)
{
        int i, err;

        for (i = 0; i < count; i++) {
                err = f();
                if (err)
                        break;
        }

        return err;
}
----->8

There are a lot of similar reports that are all marked duplicates of pr18501,
but none of them have the particular loop/break pattern that I found in the
three kernel files, so I'm opening another one to be sure.

If the underlying problem is the same, please mark this one as duplicate as
well.