[Bug middle-end/103720] New: bogus warning from -Wuninitialized + -ftrivial-auto-var-init + O2

[Bug middle-end/103720] New: bogus warning from -Wuninitialized + -ftrivial-auto-var-init + O2

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103720

            Bug ID: 103720
           Summary: bogus warning from -Wuninitialized +
                    -ftrivial-auto-var-init + O2
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qinzhao at gcc dot gnu.org
  Target Milestone: ---

There are multiple bogus warnings from -Wuninitialized +
-ftrivial-auto-var-init when building linux kernel the the latest upstream gcc.

One of them is from building fs/proc/array.c. 
and I used Creduce to reduce into the following small testing case:

[opc@qinzhao-ol8u3-x86 tem]$ cat t.c
typedef struct {
  unsigned long a[4];
} b;
int e, g;
long f;
int *h();
int *i(j) {
  int *a = h(j);
  return a;
}
void k(b *j) {
  for (e = 1; e <= 4; ++e) {
    b *b = j;
    b->a[0] = f;
  }
}
int l() {
  b c;
  unsigned long d;
  i(&d);
  k(&c);
  g = c.a[0];
  return 0;
}
[opc@qinzhao-ol8u3-x86 tem]$ sh t
/home/opc/Install/latest-d/bin/gcc -O2 -Wall -std=gnu89 -Werror
-Wno-pointer-sign -ftrivial-auto-var-init=zero t.c -S
t.c: In function ‘l’:
t.c:19:17: error: ‘c.a[0]’ is used uninitialized [-Werror=uninitialized]
   19 |   unsigned long d;
      |                 ^
t.c:18:5: note: ‘c.a[0]’ was declared here
   18 |   b c;
      |     ^
cc1: all warnings being treated as errors

[Bug middle-end/103720] bogus warning from -Wuninitialized + -ftrivial-auto-var-init + O2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103720

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-12-14
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=102608

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, 

Dom transformed:
  c$a$0_6 = .DEFERRED_INIT (8, 2, 0);
  _1 = .DEFERRED_INIT (8, 2, 0);

into:
  c$a$0_6 = .DEFERRED_INIT (8, 2, 0);
  _1 = c$a$0_6;

Which it should not be done. FRE had did the same thing but it was fixed, see
PR 102608.

[Bug tree-optimization/103720] bogus warning from -Wuninitialized + -ftrivial-auto-var-init + O2

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103720

--- Comment #2 from qinzhao at gcc dot gnu.org ---
(In reply to Andrew Pinski from comment #1)
> 
> Dom transformed:
>   c$a$0_6 = .DEFERRED_INIT (8, 2, 0);
>   _1 = .DEFERRED_INIT (8, 2, 0);
> 
> into:
>   c$a$0_6 = .DEFERRED_INIT (8, 2, 0);
>   _1 = c$a$0_6;
> 
> Which it should not be done. FRE had did the same thing but it was fixed,
> see PR 102608.

Yes, looks like that the current definition of .DEFERRED_INIT might trigger
more other similar potential issues. 
it's might be better to change the 3rd parameter from the current IS_VLA to a
string that represent the name of the original variable. then different
variables will map to a different .DEFERRED_INIT, as a result, all such
optimization will not happen anymore.

this can be combined with the patch to fix -Wuninitialized +
-ftrivial-auto-var-init for address taken variable issue as:

https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586491.html

As I just checked, that patch did resolve this issue.

[Bug tree-optimization/103720] bogus warning from -Wuninitialized + -ftrivial-auto-var-init + O2

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103720

--- Comment #3 from qinzhao at gcc dot gnu.org ---
this is fixed with the following commit in gcc12.
https://gcc.gnu.org/pipermail/gcc-cvs/2022-January/359118.html

[Bug tree-optimization/103720] bogus warning from -Wuninitialized + -ftrivial-auto-var-init + O2

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103720

qinzhao at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #4 from qinzhao at gcc dot gnu.org ---
fixed in GCC12