[Bug tree-optimization/80147] New: missing maybe-uninitialized warning on variable with no side effects

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80147

            Bug ID: 80147
           Summary: missing maybe-uninitialized warning on variable with
                    no side effects
           Product: gcc
           Version: 6.3.0
            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: ---

On the following test, GCC 6.3.0 with -Wuninitialize and -Wmaybe-uninitialized
(both implied by -Wall) warns as expected in the 4 cases without optimization,
but if -O is used, GCC misses the warning for j1.

----------------------------------------
void f0 (int i);
static void f1 (int i) { }
static void f2 (int i) { f0 (i); }

void g (int b)
{
  int i1, i2, j1, j2;
  f1 (i1);
  f2 (i2);
  if (b)
    {
      f1 (j1);
      f2 (j2);
    }
}
----------------------------------------

The following is OK:

zira% gcc -Wall -c tst.c
tst.c: In function ‘g’:
tst.c:8:3: warning: ‘i1’ is used uninitialized in this function
[-Wuninitialize]
   f1 (i1);
   ^~~~~~~
tst.c:9:3: warning: ‘i2’ is used uninitialized in this function
[-Wuninitialize]
   f2 (i2);
   ^~~~~~~
tst.c:12:7: warning: ‘j1’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
       f1 (j1);
       ^~~~~~~
tst.c:13:7: warning: ‘j2’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
       f2 (j2);
       ^~~~~~~

But the warning for j1 is missing if -O is added:

zira% gcc -Wall -c tst.c -O
tst.c: In function ‘g’:
tst.c:8:3: warning: ‘i1’ is used uninitialized in this function
[-Wuninitialize]
   f1 (i1);
   ^~~~~~~
tst.c:9:3: warning: ‘i2’ is used uninitialized in this function
[-Wuninitialize]
   f2 (i2);
   ^~~~~~~
tst.c:3:26: warning: ‘j2’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
 static void f2 (int i) { f0 (i); }
                          ^~~~~~
tst.c:7:19: note: ‘j2’ was declared here
   int i1, i2, j1, j2;
                   ^~