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From: "vsevolod.livinskij at frtk dot ru" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/95248] New: GCC produces incorrect code with
 -O3 for loops
Date: Wed, 20 May 2020 21:27:06 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95248

            Bug ID: 95248
           Summary: GCC produces incorrect code with -O3 for loops
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vsevolod.livinskij at frtk dot ru
  Target Milestone: ---

GCC produces incorrect code with -O3 for loops

Error:
>$ gcc -O0 driver.cpp func.cpp && ./a.out
1
>$ gcc -O3 driver.cpp func.cpp && ./a.out
0

Reproducer:
>$ cat driver.cpp
#include <stdio.h>

int var_2 =3D -2013646301;
int var_3 =3D -1126567434;
unsigned int var_12 =3D 1;
unsigned int var_19;
unsigned int arr_25 [24] [21] [15] [17] [15] ;

void test();

int main() {
    test();
    printf("%u\n", var_19);
}

>$ cat func.cpp
extern int var_2;
extern int var_3;
extern unsigned int var_12;
extern unsigned int var_19;
extern unsigned int arr_25 [24] [21] [15] [17] [15] ;

void test() {
  for (int a =3D 0; a < 3; a =3D 42)
    for (int b =3D 0; b < 20; b++)
      for (int c =3D 0; c < 4; c =3D 4)
        for (int d =3D 0; d < 6; d +=3D 4)
          for (int e =3D 0; e < 4; e +=3D 2) {
            arr_25[a][b][c][d][e] =3D var_2 || var_3;
            var_19 =3D var_12;
          }
}

GCC version:
gcc version 11.0.0 (ed63c387aa0bc1846082524455a6ff1fcec40f9d)=