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From: "haoxintu at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/103237] New: wrong code with -ftree-vectorize
 at -O1 on x86_64-linux-gnu
Date: Sun, 14 Nov 2021 17:38:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103237

            Bug ID: 103237
           Summary: wrong code with -ftree-vectorize at -O1 on
                    x86_64-linux-gnu
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

Hi all.

This seems a long latent bug existed after GCC-10.1.

$cat small.c
#include <stdint.h>
#include <stdio.h>
struct S0 {
  uint16_t a;
};
union U1 {
  struct S0 b;
};
int32_t g1[1] =3D {0};
uint32_t g2[][10][1] =3D {
    {}, {}, {}, {}, {}, {}, {{}, {}, {}, {}, {}, {}, {4294967295UL}}};
union U1 g3;
static int32_t func_1() {
  int32_t *l_1 =3D &g1[0];
  uint32_t l_2 ;
  for (g3.b.a =3D (27); (g3.b.a !=3D 45);
       g3.b.a =3D g3.b.a + 9) {
    for (l_2 =3D 0; (l_2 <=3D 3); l_2 +=3D 1) {
      uint32_t *l_3 =3D &g2[6][6][0];
      *l_1 =3D *l_3 ^=3D ((0 =3D=3D 0) << 0);
    }
  }
  return 0;
}
int main() {
  func_1();
  for (int i =3D 0; i < 1; i++)
    printf("%d\n", g1[i]);
  return 0;
}


$gcc -O2 -ftree-vectorize small.c ; ./a.out
-1

$gcc -O1 -ftree-vectorize small.c ; ./a.out
0

$gcc -O1 small.c ; ./a.out
-1

$gcc -v
Using built-in specs.
COLLECT_GCC=3Dgcc
COLLECT_LTO_WRAPPER=3D/home/haoxin/haoxin-data/compilers/gcc/build/libexec/=
gcc/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:../configure
--prefix=3D/home/haoxin/haoxin-data/compilers/gcc/build/ --enable-bootstrap
--enable-checking=3Drelease --enable-multilib --enable-languages=3Dc,c++,lto
--no-create --no-recursion
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20211114 (experimental) (GCC)=20

Can also check here: https://godbolt.org/z/ncar1aWYM

Thanks,
Haoxin=