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From: "slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110228] [13/14 Regression] llvm-16 miscompiled due
 to an maybe uninitialized variable
Date: Tue, 04 Jul 2023 13:36:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110228

--- Comment #32 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #29)
> I can't reproduce the original failure on trunk or with GCC 13.1, but the
> testcase from comment#16 is now fixed.  I also cannot reproduce the testc=
ase
> from comment#21 before the fix.
>=20
> Can people double-check this is now fixed, maybe also on the branch when =
this
> fix is cherry-picked?

Test suite on llvm-16 still fails the same single 'LLVM ::
ExecutionEngine/JITLink/X86/MachO_weak_references.s' test for me.=20

gcc was built from 2c12ccf800fc7890925402d30a02f0fa9e2627cc.

The following test still fails for me:

$ cat bug.c
static unsigned a[4] =3D {1,1,1,1};

__attribute__((noipa))
static void bug(unsigned * p, unsigned *t, int n) {
     unsigned LookupFlags;

    for(int i =3D 0; i < n; i++) {
      unsigned v =3D t[i];
      if      (v =3D=3D 0) LookupFlags =3D 0;
      else if (v =3D=3D 1) LookupFlags =3D 1;
      *p =3D LookupFlags;
    }
}

int main() {
    unsigned r =3D 42;
    bug(&r, a, 4);
    if (r !=3D 1) __builtin_abort();
    return 0;
}

$ gcc bug.c -o bug -O1 -Wall && ./bug
bug.c: In function 'bug':
bug.c:10:15: warning: 'LookupFlags' may be used uninitialized
[-Wmaybe-uninitialized]
   10 |       else if (v =3D=3D 1) LookupFlags =3D 1;
      |               ^
bug.c:5:15: note: 'LookupFlags' was declared here
    5 |      unsigned LookupFlags;
      |               ^~~~~~~~~~~
Aborted (core dumped)

bug.c.255t.optimized still turns stores into '|=3D' against uninit value:


__attribute__((noipa, noinline, noclone, no_icf))
void bug (unsigned int * p, unsigned int * t, int n)
{
  unsigned long ivtmp.8;
  unsigned int v;
  unsigned int LookupFlags;
  _Bool _16;
  unsigned int _17;
  unsigned int _18;
  void * _19;
  unsigned long _20;
  unsigned long _26;
  unsigned long _28;

  <bb 2> [local count: 118111600]:
  if (n_10(D) > 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 7>; [11.00%]

  <bb 3> [local count: 105119324]:
  ivtmp.8_21 =3D (unsigned long) t_11(D);
  _20 =3D (unsigned long) n_10(D);
  _26 =3D _20 * 4;
  _28 =3D ivtmp.8_21 + _26;

  <bb 4> [local count: 955630225]:
  # LookupFlags_22 =3D PHI <LookupFlags_4(6), LookupFlags_8(D)(3)>
  # ivtmp.8_6 =3D PHI <ivtmp.8_5(6), ivtmp.8_21(3)>
  _19 =3D (void *) ivtmp.8_6;
  v_12 =3D MEM[(unsigned int *)_19];
  if (v_12 =3D=3D 0)
    goto <bb 6>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 5> [local count: 477815112]:
  _16 =3D v_12 =3D=3D 1;
  _17 =3D (unsigned int) _16;
  _18 =3D _17 | LookupFlags_22;

  <bb 6> [local count: 955630225]:
  # LookupFlags_4 =3D PHI <v_12(4), _18(5)>
  *p_13(D) =3D LookupFlags_4;
  ivtmp.8_5 =3D ivtmp.8_6 + 4;
  if (ivtmp.8_5 !=3D _28)
    goto <bb 4>; [89.00%]
  else
    goto <bb 7>; [11.00%]

  <bb 7> [local count: 118111600]:
  return;

}=