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From: "pterjan at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/96326] New: Incorrect loop optimization at -O3
Date: Mon, 27 Jul 2020 01:27:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96326

            Bug ID: 96326
           Summary: Incorrect loop optimization at -O3
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pterjan at gmail dot com
  Target Milestone: ---

Found in https://github.com/Sereal/Sereal/issues/229

#include <string.h>
#include <stdio.h>

int main() {
        char buf[128];
        char *src =3D buf;
        char *op =3D buf + 3;
        int len =3D 64;
        strcpy(buf, "abc");
        while (op - src < 8) {
                *(unsigned long*)op =3D *(const unsigned long*)src;
                len -=3D op - src;
                op +=3D op - src;
        }
        while (len > 0) {
                *(unsigned long*)op =3D *(const unsigned long*)src;
                src +=3D 8;
                op +=3D 8;
                len -=3D 8;
        }
        printf("%ld\n", strlen(buf));
}

$ gcc -O2 -Wall t.c; ./a.out=20
68

$ gcc -O3 -Wall t.c; ./a.out=20
24

Adding a printf in the second loop also fixes it.=