[Bug analyzer/94754] New: -fanalyzer false positive due to it ignoring previous if

["colomar.6.4.3 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94754

            Bug ID: 94754
           Summary: -fanalyzer false positive due to it ignoring previous
                    if
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: colomar.6.4.3 at gmail dot com
  Target Milestone: ---

The analyzer follows branches that are incompatible (sometimes).

Code to reproduce the bug:

[[gnu::nonnull]]
static
void    init_x(int cond, int **x, int *y)
{

        if (!cond)
                return;
        *x = y;
}

int     foo(int cond)
{
        int     *x;
        int     y = 7;

        if (cond < 2)
                return  -1;

        /* cond >= 2 != 0, so it will initialize x */
        init_x(cond, &x, &y);

        return  *x;
}

$ gcc-10 -c false_positive.c -o foo -fanalyzer
In function ‘foo’:
false_positive.c:22:9: warning: use of uninitialized value ‘x’ [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
   22 |  return *x;
      |         ^~
  ‘foo’: events 1-4
    |
    |   11 | int foo(int cond)
    |      |     ^~~
    |      |     |
    |      |     (1) entry to ‘foo’
    |......
    |   16 |  if (cond < 2)
    |      |     ~
    |      |     |
    |      |     (2) following ‘false’ branch (when ‘cond > 1’)...
    |......
    |   20 |  init_x(cond, &x, &y);
    |      |  ~~~~~~~~~~~~~~~~~~~~
    |      |  |
    |      |  (3) ...to here
    |      |  (4) calling ‘init_x’ from ‘foo’
    |
    +--> ‘init_x’: events 5-7
           |
           |    3 | void init_x(int cond, int **x, int *y)
           |      |      ^~~~~~
           |      |      |
           |      |      (5) entry to ‘init_x’
           |......
           |    6 |  if (!cond)
           |      |     ~ 
           |      |     |
           |      |     (6) following ‘true’ branch (when ‘cond == 0’)...
!!! cond == 0, but previously it assumed cond > 1 !!!
           |    7 |   return;
           |      |   ~~~~~~
           |      |   |
           |      |   (7) ...to here
           |
    <------+
    |
  ‘foo’: events 8-9
    |
    |   20 |  init_x(cond, &x, &y);
    |      |  ^~~~~~~~~~~~~~~~~~~~
    |      |  |
    |      |  (8) returning to ‘foo’ from ‘init_x’
    |   21 | 
    |   22 |  return *x;
    |      |         ~~
    |      |         |
    |      |         (9) use of uninitialized value ‘x’ here
    |
$

___________________________________________________

But.

 - If I copy&paste (manual inline) `init_x` code inside `foo`, the warning goes
away.
 - If I use pointers instead of double pointers (`void init_x(int cond, int *x,
int y)`), the warning goes away.