[Bug tree-optimization/81809] New: missing -Wuninitialized due to alias analysis limitation

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81809

            Bug ID: 81809
           Summary: missing -Wuninitialized due to alias analysis
                    limitation
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

This almost certainly a side-effect of the limitations of the flow-insensitive
alias analysis pointed out in bug 23384 but I wanted to make a record of the
impact the limitation has beyond optimization.

Both functions below read the uninitialized variable b.  The uninitialized read
is diagnosed in g() but not in h() because the address of the variable is
considered to have escaped even though it doesn't happen until after it has
been read.

$ cat a.c && gcc -O2 -S -Wall -Wextra -Wpedantic a.c
void f (const void*);

int g (void)
{
  int a, b;
  f (&a);

  int i = b;   // -Wuninitialized (good)

  return i;
}

int h (void)
{
  int a, b;
  f (&a);

  int i = b;   // missing -Wuninitialized (bug)

  f (&b);      // &b escapes here

  return i;
}

a.c: In function ‘g’:
a.c:8:7: warning: ‘b’ is used uninitialized in this function [-Wuninitialized]
   int i = b;   // -Wuninitialized (good)
       ^