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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/95396] New: GCC produces incorrect code with
 -O3 for loops
Date: Thu, 28 May 2020 20:44:27 +0000
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X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: vsevolod.livinskij at frtk dot ru
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95396

            Bug ID: 95396
           Summary: GCC produces incorrect code with -O3 for loops
           Product: gcc
           Version: 11.0
            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: ---

Error:
>$ g++ -O0 small.cpp && ./a.out=20
42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 0 0 0=20
>$ g++ -O3 small.cpp && ./a.out=20
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 42 0 0 0=20

Reproducer:
>$ cat small.cpp=20
#include <stdio.h>

unsigned short arr_26 [20];
signed char arr_25 [2000];

unsigned char b;

int c(int d, int f) { return d > f ? d : f; }

void test() {=20
    for (char e =3D c(b, 241) + 15; e < 17; e +=3D 1)
        arr_26[e] =3D 42;
}

int main() {
    test();
    for (size_t i_0 =3D 0; i_0 < 20; ++i_0)
        printf("%d ", arr_26 [i_0]);
    printf("\n");
}

GCC version:
11.0.0 (1852a26b925970f64f8d31518ba732fe9c3ade23)=