[Bug tree-optimization/107839] New: spurious "may be used uninitialized" warning while all uses are under "if (c)"

[vincent-gcc at vinc17 dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107839

            Bug ID: 107839
           Summary: spurious "may be used uninitialized" warning while all
                    uses are under "if (c)"
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider

int f (int);
void g (int c)
{
  int v;
  if (c)
    v = f(0);
  while (1)
    if (c)
      f(v + v);
}

$ gcc-test -O -Wmaybe-uninitialized -c tst2.c
tst2.c: In function ‘g’:
tst2.c:4:7: warning: ‘v’ may be used uninitialized [-Wmaybe-uninitialized]
    4 |   int v;
      |       ^

All uses of v are under "if (c)", so the warning is incorrect. Note that
replacing "v + v" by "v" makes the warning disappear.

This occurs with GCC 8.4.0 and above up to at least 13.0.0 20220906
(experimental) from the master branch. No warnings with GCC 6.5.0 and below.

Note to myself (to check once this bug is fixed): this testcase is derived from
tmd/binary32/hrcases.c (warning on variable t0).